chapter 1 soils and plant nutrition - clemson...

Soil and Plant Nutrition ◆ 1

Bob Polomski, Joey Williamson, Ph.D.,and Brian J. Callahan, Ed.D.

Chapter 1

Soils and Plant Nutrition

Learning Objectives

◆ Identify the different soil components and their relative proportions in soils.

◆ Know the three types of soil particles that affect soil texture.

◆ Be familiar with soil organisms and their importance in maintaining soil productivity.

◆ Know the importance of soil organic matter and its effect on soil structure.

◆ Know the role of colloids, CEC, cations, and anions and how they affect plant nutrition and soil fertility.

◆ Understand why pH is such an important factor in nutrient uptake, and how changing pH affects different soil properties.

◆ Identify the essential elements required for plant growth and the different inorganic and organic fertilizer sources that supply them.

◆ Know how to calculate the amount of fertilizer needed for a given area for lawns, vegetable gardens, and flower gardens.

◆ Be able to discuss the advantages and disadvantages of slow-release, conventional, and organic fertilizer sources.

◆ Be able to explain how to take a soil test.


Soils and Plant Nutrition

IntroductionThe foundation of any garden or landscape is

the soil. In fact, soil is one of the three primary environmental factors involved in plant growth. Sunlight (heat and light) and air (carbon dioxide and oxygen) are the other two. This chapter will introduce you to the wonderful world of soil and, hopefully, will give you an appreciation of the many dynamic processes that occur beneath our feet.

Soil ComponentsSome folks pick up a handful of soil and call it

“dirt”--a term that impugns its importance and com-plexity. Examine soil closely and you’ll find that it is comprised of minerals derived from weathered rocks, air, water, and decaying organic matter. A healthy soil harbors a treasure trove of living organisms, such as earthworms, fungi, and bacteria. Altogether, soil supplies our plants with minerals (nutrients), air, moisture, and support. Depending on the soil, the various components that are present can render the soil fertile and productive or inhospitable for plant growth.

Soil minerals and organic matter make up the solid part of the soil, and air and water occupy the pore spaces between the solid particles. Pores are the spaces between soil particles. Macropores (great-er than .03 mm. diameter) facilitate air and water movement, and provide room for the roots to grow. Micropores (less than .03 mm in diameter) store wa-ter. Plant roots grow through the same channels as soil air and water, so a soil with good pore structure has a balance of macropores and micropores that en-sures adequate air and water movement.

An ideal soil for plant growth is about equally divided between solid materials and pore space on

a volume basis (Figure 1.1). Solid material is about 45% mineral matter, 5% organic matter, 25% wa-ter, and 25% air. The pore space is equally divided between water-filled and air-filled pores. These proportions vary from time to time and from place to place. Generally, air and water will occupy about 50% of the pore space and bear a direct reciprocal “give-and-take” relationship with each other. As water moves in the soil, it displaces air and fills the pores. However, air returns when the water drains from the soil, evaporates, or is absorbed by plants.

Physical and Chemical Properties of SoilA soil’s physical and chemical properties affect

plant growth and soil management. Some impor-tant physical and chemical properties of soil are mineral content, texture, structure, porosity, cation exchange capacity, and organic matter content.

Soil mineralsSoils consist of particles of varying sizes and

shapes. They are divided into three categories based

Four major components of soil

Figure 1.1


on their size: sand, silt, and clay. Sand grains can be seen with the naked eye. Silt particles need to be magnified to be seen (10X hand lens). Clay par-ticles must be viewed with an electron microscope.

The U.S. Department of Agriculture (USDA) has established standards for the size limits of sand, silt, and clay particles (Table 1.1). Note that for a fixed amount of soil, the total surface area increases as particle size decreases. The surface of soil par-ticles is important because that’s where much of the water and nutrient reserves are held and where many important chemical reactions occur. Since many physical and chemical reactions in the soil occur at the surfaces of mineral particles, you can expect that clay minerals and their large surface area would play a major role in the reactions that take place in the soil.

Key points from this table for Master Gardeners:

• The largest sand particles are 1,000 times larger than the largest clay par-ticles.

• The smallest sand particle is 50 times larger than the largest clay particle.

• Silt particles are intermediate in size.• Clay has 10,000 times more surface area

per gram than silt and 10,000 times more surface area per gram than me-dium size sand.

Soil mineral particles contain ele-ments important to plant nutrition, such as potassium, calcium, sodium, iron, and magnesium. These elements are confined within the crystalline structure of the par-ticle. Sand and silt are composed mainly of primary minerals such as quartz, feldspar, mica, hornblende, and augite. As the pri-

mary minerals weather into secondary minerals, the elements that were bound within their crystalline structures are released, and the plant mineral nutri-ents become available for absorption by plant roots.

Clay is composed largely of secondary minerals, which are the weathering products of the primary minerals-quartz, feldspar, mica, hornblende, and augite--of which sand and silt are largely composed.

Soil textureThe relative proportions of sand, silt, and clay

minerals in a given soil is called soil texture. It is one of the most important physical properties of soils that affect plant growth. Soil texture deter-

Soil type Particle diameter (mm.) Surface area of 1 gram (sq. cm.)Very coarse sand 2.0-1.0 11Coarse sand 1.0-0.5 23Medium sand 0.5-0.25 45Fine sand 0.25-0.10 91Very fine sand 0.10-0.05 227Silt 0.05-0.002 454Clay < 0.002 8,000,000

Size and surface area of soil mineral particles (USDA system).

Table 1.1

Figure 1.2

Credit: California Master Gardener Handbook. 2002.Dennis R. Pittenger, Ed. University of Calif. Agric. and Natural Resources, Pub. 3382. Copyrighted by the Regents of the University of California and used with permission.


Texture Aeration/ Ease of Water Nutrient-holding Water retention Ease of Porosity Infiltration capacity Digging

Loam Fair Fair Poor Fair FairClay Poor Poor Poor Good PoorSilt Good Fair Very poor Fair FairSand Excellent Excellent Very poor Very poor Good

The effect of texture on various soil properties

A laboratory can accurately determine the texture or particle size distribution of a soil, but you can use the “feel” method to get a decent approximation. Take some moist soil and rub it between your thumb and forefinger.

(D) Clay, a fine-textured soil, will form a strong ribbon. You can form a ribbon longer than one inch.

(A) Loamy sand, a coarse-textured soil, will form a cast when squeezed in your hand; however, it cannnot be mold-ed into a ribbon.

(B) Loam, a medium-tex-tured soil, can be formed into a short ribbon. The ribbon will split and break away when it’s about one-half inch long.

(C) Clay loam is a medium-textured soil. You can form a fairly strong ribbon, but it will break away when it’s about 3/4 inch long.

A

B

C

D

mines tilth or fitness as a growing medium as well as its nutrient- and water-holding capacities. It is described visually in a textural triangle, which shows the names of the 12 basic soil textural classes and the percentage of sand, silt, and clay associated with them (Figure 1.2). For example, a soil high in sand content (85 to 100%) and containing small amounts of silt and clay is classified as “sand.” When the silt or clay content increases, the soil becomes either

a “loamy sand” or a “sandy loam” soil. A sandy, coarse-textured soil is often called a light soil.

Soils composed mostly of clays (40 to 100%) are “clay.” These fine-textured soils are often referred to as heavy soils.

When a soil contains equal parts of sand, silt, and clay, it is called a “loam.”

Although organic matter has a significant impact on a soil’s physical properties, it is not con-sidered in defining a soil’s textural class. Soil is conventionally defined as particles smaller than 2 mm. in diameter, although some soils may contain coarser materials such as gravel, pebbles, and stones.

The 12 textural classes can be grouped into 3 general texture categories :

• coarse (sandy soils): sand, loamy sand• medium (loamy soils--moderately coarse, me-

dium, moderately fine): sandy loam, loam, silt loam, silt, clay loam, sandy clay loam, silty clay loam.

• fine (clay soils): clay, sandy clay, and silty clay.The textural classes of a soil affect the nutrient-

and water-holding capacity of a soil along with other soil properties (Table 1.2). As the particle sizes get smaller as in fine-textured clay soils, the spaces get smaller and retain more water and nutrients than coarse-textured soils with a high percentage of sand. However, the larger pore spaces cause water and nu-trients to drain rapidly (Figure 1.3).

Silt has mineral particles whose size falls be-tween clays and sands. Water percolates or moves slowly through the pores, but the water-holding ca-pacity of silt is greater than sand.

Fine-textured soils high in clay or silt can be easily compacted or packed closely together, thereby reducing pore space and hampering air and water movement through the soil.

The best garden soils are loam, sandy loam, and silt loam soil that contain about 5 to 10 percent organic matter. They have the ideal combination of sand, silt, and clay that retain adequate water but

Table 1.2

Credit: California Master Gardener Handbook. 2002.Dennis R. Pittenger, Ed. University of Calif. Agric. and Natural Resources, Pub. 3382. Copyrighted by the Regents of the University of Cali-fornia and used with permission.


also permit infiltration and percolation.

Soil structureExcept for sand grains, soil particles do not oc-

cur as individual particles but clump together into aggregates or granules. Soil structure refers to the arrangement and organization of these aggregates. Plants require air, water, and nutrients from the soil, and the air-water relationship depends largely on soil structure. A soil with good structure will have good water infiltration, drainage, aeration, and overall tilth.

The easiest way of improving soil structure is with regular additions of organic matter, which binds the soil particles together. In sandy soils that have very little organic matter and clay, little, if any, aggregation occurs. Adding organic matter will improve soil structure as well as increase water retention and improve fertility in these coarse-textured soils.

SoilProfile/SoilHorizonsWhen you dig into some undisturbed South Carolina

soils, it changes in color and texture, with depth. These layers of soil are called “soil horizons.” The upper horizon is called the “A” horizon or topsoil. This is the zone of maximum biological activity. Eighty percent of the roots are found here, along with most of the soil’s bacteria, fungi, and other microorganisms. This is where you’ll find earthworms, insects, nematodes, moles, crickets, and similar living organisms and where most nutrient recycling occurs as dead leaves, twigs, roots, and other organisms decompose. the topsoil can be an inch thick or a foot thick depending on the location. In most gardens we say the topsoil is the thickness that the soil is tilled, about 6 to 8 inches.

As rainfall trickles downward in a well-drained soil, it carries with it dissolved minerals and the smallest soil particles called clay. They accumulate or “leach” into the “B” horizon or subsoil. This subsoil can be a reserve of moisture and can provide some nutrients for plants when the topsoil is too dry.

Beneath the B horizon is the “C” horizon or parent material. This is the stuff from which the topsoil and sub-soil formed.

Soils around homes, townhouses, and apartment complexes that have been disturbed by construction may not show the neat soil profile and distinct horizons depicted in the illustration on the right. The best place to see soil horizons and the of natural soil formation are at road cuts or other excavation sites.--Bob Lippert, Ph.D. Ex-

Penetration of equal amounts of water in furrows consist-ing of three soil types. Water depth is highest in the sand while water in the clay soil moved the least distance.

Figure 1.3

Credit: California Master Gardener Handbook. 2002.Dennis R. Pittenger, Ed. University of Calif. Agric. and Natural Resources, Pub. 3382. Copyrighted by the Regents of the University of California and used with permission.

Credit: California Master Gardener Handbook. 2002.Dennis R. Pittenger, Ed. University of Calif. Agric. and Natural Re-sources, Pub. 3382. Copyrighted by the Regents of the University of California and used with permission.


To avoid damaging soil structure, never dig or cultivate when the soil is too wet or extremely dry soil. If the soil sticks to your shovel—the soil is too wet. Postpone digging until it dries out.

Soil ChemistrySoil Colloids and Ions

As soils are formed during the weathering pro-cess, some minerals and organic matter are broken down into extremely small particles. Chemical changes further reduce these particles until they cannot be seen without magnification. The very smallest are called colloids. The most chemically reactive portion are inorganic and organic col-loids. The inorganic colloids are present almost exclusively as clay minerals; the organic colloids occur as humus--organic matter that has undergone extensive decomposition and is resistant to further degradation.

On a weight basis, organic or humus colloids have a higher nutrient- and water-holding capacity than clay; however, clay is generally present in larg-er amounts and its total contribution to the chemi-cal and physical properties will usually be equal to humus.

Mineral clay colloids are platelike in structure and crystalline in nature. The amount and kind of clay in the soil affects the retention of plant nutri-ents in soil.

Colloids are charged particles. They have net negative (-) charges on their surfaces, which means that they can attract and hold positively (+) charged particles. Colloids behave like magnets having positive and negative poles (Figure 1.4). Clay minerals repel other negatively charged par-ticles, just as the same poles of a magnet repel one other.

Mineral nutrients occur in the soil as elements or group of elements with electrical charges; they are called ions. Ions with negative charges, such as nitrate, phosphate, and sulfate are called anions. Positively charged ions are called cations, and in-clude hydrogen, ammonium, potassium, magnesium, calcium, iron, and aluminum. Most mineral nutri-ents are taken up or absorbed by plants as anions or cations. They can be written in ionic form as shown in Table 1.3.

The attraction of ions or molecules to the sur-faces of a solid, such as soil particles, is called ad-sorption. The adsorption of plant nutrient cations

Negatively charged clay particle behave like magnets. Positively charged ions (cations) in the soil solution are attracted to the negatively charged clay particle. Anions (negatively charged ions) are repelled and move with water.

Figure 1.4

onto the surfaces of clay minerals balances the nega-tive charges on their surfaces. Absorption refers to the active or passive movement of ions or water into plants roots.

The negatively charged colloids attract and hold cations like a magnet holding small pieces of metal; they repel negatively charged plant nutrient ions, such as nitrate and sulfate. This characteristic of colloids explains why nitrate-nitrogen (NO3

–) is more easily leached or lost from the soil than

Nutrients, their chemical symbols, and ionic forms

Nutrient Ionic Form

Ammonium NH4+

Nitrate NO3–

Phosphate PO43–

Potassium K+

Sulfate SO42–

Calcium Ca2+

Magnesium Mg2+

Hydrogen H+

Table 1.3


tities of cations against potential loss through leach-ing, therefore they possess relatively high fertility levels. Sandy soils with a low CEC retain only small quantities of cations; thus, they have comparatively low fertility levels. CEC can be improved with ad-ditions of organic matter as discussed in the next section.

Soil Organic MatterThe organic fraction of the soil is the solid

phase that is derived from living organisms, unlike the mineral fraction which comes from nonliving rocks. Soil organic matter falls into two categories. One is a stable material called humus, which is somewhat resistant to further rapid decomposition. The other are organic materials, which range from fresh plant and animal residues to relatively stable humus. Organic matter benefits the soil in many ways: (1) improves its physical condition; (2) im-proves drainage in heavy clay soils and retains water and nutrients in sandy soils; (3) improves soil tilth; (4) decreases erosion losses; and (5) supplies plant nutrients. Organic matter serves as a storehouse for nitrogen and other essential elements required by plants, such as phosphorus, magnesium, calcium, sulfur, and the micronutrients. As organic residue decomposes, these elements become available to growing plants.

Some soils naturally contain very little organic matter. In our region, most soils are normally low in organic soil matter because warm temperatures and high rainfall speed-up decomposition. In order to maintain a reasonable amount of organic matter in garden soils, an annual application of organic matter is necessary. Commonly available sources of organic matter include compost, manures, leaf mold, saw dust, and cover crops.

Adding Organic Matter With Cover Crops

A cover crop is a crop planted in a garden to protect the soil from erosion and to improve the soil. Cover crops can be divided into two groups: legumes and nonlegumes (Table 1.4). Legumes are associated with “nitrogen-fixing” bacteria. These rhizobia bacteria reside in the plant roots and cap-ture or “fix” nitrogen from the air and make this nitrogen available to the legume. Nonlegume cover crops provide less nitrogen but more organic matter.

Within these two groups are both warm- and

ammonium-nitrogen (NH4+). Nitrate has a negative

charge, so it is not held in the soil. It remains as a free ion in soil solution in soil pores where it can be absorbed by roots or washed out by excess water.

Cation Exchange Capacity Cations replace one another on the surfaces of

clay minerals because of their relative attractions for the surfaces of the soil particles and because the soil maintains a chemical balance between the ions adsorbed onto the soil surfaces and those located in the soil solution. When cations are exchanged and released from the clay mineral into the soil solution, they become available for absorption by plant roots.

The amount or quantity of cations that can be absorbed or held by the soil is called the cation ex-change capacity (CEC), and is a measure of the fer-tility and productivity of a soil. There is a constant exchange of cations between the clay minerals and the soil solution and between the soil solution and plants. Because clay minerals are so active in these nutrient exchanges, they largely dictate the chemi-cal and physical properties of a given soil and how well plants will grow in that soil.

Soils differ in their capacities to hold exchange-able potassium and other cations. The CEC de-pends on the amounts and kinds of clay and organic matter present. Because clay and humus particles are so small, they have a large surface area and greatly influence the physical and chemical proper-ties of the soil. For example, a high-clay soil can hold more exchangeable cations than a low-clay soil. Also, the CEC increases greatly as soil organic matter content increases.

The CEC of a soil is expressed in terms of mil-liequivalents (milligram equivalents) per 100 grams of soil and is written as meq/100 g. It shows the relative CEC of clay and organic matter. CEC val-ues of clays range from 3 to 150 meq/100 g, depend-ing on the kinds of clay minerals present. Organic matter ranges from 100 to 300 meq/100 g. So, the kind and amount of clay and organic matter content greatly influence the CEC of soils.

In the South where soils are highly weathered and organic matter levels are low, CEC values are very low. Typically the CEC of weathered clays in South Carolina are 2 to 10 meq/100 g. In other parts of the U.S. where less weathering has occurred and organic matter levels are usually higher, CEC values can be quite high.

Clay soils with high CEC can retain large quan-


Soil formation in South Carolina, like any other loca-tion, is a very complex process that involves physical, chemical, and biological processes. For the most part, the soils in the major geographical regions of the state are related, have similar characteristics, and can be managed in somewhat the same fashion within these regions.

Five major soil groups are associated with five land resource areas in South Carolina: Blue Ridge mountain soils (2% of the state), Piedmont soils (35%), Carolina Sandhill soils (11%), Upper Coastal Plains soils (14%), and the Lower Coastal Plains, including the Atlantic Coast Flatwoods (29%) and the Tidewater (9%) soils (Figure 1.5). Each of these areas has distinctive groups of soils. The soils of South Carolina are predominantly acidic and have a low nutrient-holding capacity and little organic matter. Most of the soils are loams of some type. A loam is a soil composed of various parts of sand, silt, and clay. However, in the southeast sand and clay pre-dominate. Silt is normally found where soils have been formed by river deposition.

The Blue Ridge, a part of the Appalachian Moun-tains, is one of the oldest mountain ranges in the world and is composed principally of granitic rocks. Piedmont and mountain soils in South Carolina are often referred to as “red clay” soils. Even though they are often red in color, many times they aren’t clay soils at all. The clayey soils are often subsoils exposed by poor logging and farming practices in the past. The parent materials of these soils were rich in iron, manganese, and aluminum. The presence of iron and manganese give these soils their colors as well as many of their chemical properties.

Piedmont soils are often heavier than soils in the lower part of the state; they are less porous, take longer to dry, and runoff occurs more readily. Due to their low po-rosity, these soils generally retain fertilizer better than the sandy soils of the Sandhills and Coastal Plain. However, the presence of iron, manganese, and aluminum is one of the determining factors for a process called phosphorus fixation. These elements are found in clays which are ac-tually breakdown products of various rocks. Usually the higher the clay content in a soil, the greater the potential for phosphorus fixation. This process makes the phos-phorus generally unavailable to plants. Close attention to phosphorus levels as well as pH is necessary to manage the fertility of these soils.

Humus, formed by the breakdown of plant debris, is oxidized rapidly due to the moderate temperatures, high precipitation rates, and porous nature of the soil. This means that levels of organic matter are generally low throughout the region.

The Sandhills characterizes the Midlands region. This is an area of deep sandy soils with even lower organ-ic matter levels and cation exchange capacities than the soils of the Piedmont and Coastal Plain. The advance and retreat of early seas led to the deposition of sandy soils, which were supplemented with eroded Piedmont clays. Over time wind and water erosion led to the creation of

the rolling, sandy hills that make up the Sandhills region. These soils are generally excessively drained, droughty, and infertile. The water holding capacity is very low, and irrigation is usually necessary to successfully raise crops. In many areas of the Sandhills, it is more than seven or eight feet before any clay-containing subsoil is reached.

The broad Atlantic Coastal Plain is composed of soils formed from marine sediments. These sediments were deposited on the bottom of the Atlantic Ocean during the millions of years it took for the seacoast to recede from the “fall line” to its present position. Being formed on the bottom of the ocean, most of the Coastal Plain is fairly level with some rolling areas and valleys associated with the larger rivers and their tributaries. Coastal Plain soils are usually sandy loams or loamy sands. They are ordi-narily of a fine texture and have higher clay content than the soils of the Sandhills. The clayey subsoils are much closer to the surface than in the deeper sandy soils of the Midlands. This can create drainage problems in some Coastal Plain soils.

Soils of the coastal marshes and beaches are usually very poorly drained and may have dark, grayish-brown, mucky sandy loam surfaces over a gray sandy loam sub-soil.

Coastal Plain soils are acidic, low in cation exchange capacity, and fairly low in organic matter content. Coast-al Plain soils that were formed under poorly drained con-ditions tend to be higher in organic matter than many of the other soils in South Carolina. They generally have a higher exchange capacity than soils in other areas of the state. The soils in coastal counties often have very high levels of phosphorus; therefore, soil testing is important to determine if fertilizers should contain additional phos-phorus. With proper soil testing, limestone application, and water management, Coastal Plain soils can be very productive.

Although South Carolina does not have the rich soils found in the Midwest and California valleys, their produc-tivity can be enhanced with proper management. Con-duct regular soil tests, apply recommended amendments, such as limestone or sulfur and other minerals, and add organic matter to improve soil tilth. One thing to remem-ber—although the type of soil cannot be changed, it can be managed to get the most out of it. --J. Powell Smith, Ph.D., an Associate Professor of Entomology and Exten-sion Entomologist, is a lifelong resident of South Carolina. This section was written during his tenure as a Lexington County Extension Agent.

Editor’s Note: To learn more about the soils in your county, visit the USDA National Resources Conservation Service Web Soil Survey site (http://soils.usda.gov/survey). For more information about natural resources conservation in South Carolina, visit (http://www.sc.nrcs.usda.gov/) or call your local USDA Service Center.

Soil Formation in South Carolina


cool-season species. Cool-season legumes include Austrian winter peas and vetch. Warm-season le-gumes include all of the southern peas and the com-mon beans. Cool-season nonlegumes include the cereals oats, wheat, rye and barley.

Cover crops improve the overall productivity of the soil. While the cover crop is growing, it helps prevent soil erosion and assists in weed control. The organic matter provided when a cover crop is plowed under improves soil structure and aeration, water and nutrient-holding capacity, and supplies a portion of the nutrient requirements for subsequent crops. The type of cover crop and the length of time it is growing determines how much organic matter and nutrients will be returned to the soil. A legume may provide more nitrogen but less total organic matter than a vigorously growing non-legume, such as a grain crop. As a group, legumes are more likely to harbor virus diseases and allow some soilborne diseases to survive than most nonlegumes. However, the advantages of the nutrition provided by legumes may more than offset this disadvantage.

Regular use of cover crops over a period of years slowly raises the organic matter content of the soil, increasing the activity of soil organisms such as earthworms and fungi in the soil. As these organ-

isms break down the organic materials, they help improve soil structure and tilth, making the soil a more favorable place for root development.

Organic Matter DecompositionandtheCarbon-to-Nitrogen Ratio

When organic matter is added to soil, it’s bro-ken down in the soil through decomposition by soil organisms, primarily bacteria and fungi. Various factors, such as moisture, temperature, and nitrogen availability determine the rate of decomposition through their effects on these organisms. Adequate water must be present, and warm temperatures will increase the rate at which the microbes work. The proper balance of carbon and nitrogen in the mate-rial is also needed for rapid decomposition.

Organic materials have a carbon to nitrogen ratio (C:N) that affects decomposition by microor-ganisms. The ratio of carbon to nitrogen (C:N) in stable soil organic matter varies from about 10:1 to 12:1, which means that the carbon level is about 10 to 12 times the nitrogen level. Organic matter prior to decomposition may have a C:N that can range from alfalfa (10:1) rotted manure (20:1) to straw (C:N = 80:1) or sawdust (C:N=500:1) (see Table 2.1 on p. 45). This ratio is important in decomposition

The four major land resource areas of South Carolina Figure 1.5

1 Mountains

2 Piedmont

3 Sandhills

4 Coastal Plain


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because when the C:N ratio exceeds 30:1 (wheat straw, mature corn stalks, saw dust), there is a small amount of N compared to C. So, the microbes will “rob” the nitrogen that’s present in the soil to make up for the low amount of nitrogen in the organic materials, which they eventually convert to humus. In essence, the microbes compete directly with the plants for inorganic nitrogen (ammonium [NH4

+] and [NO3

-]). As a result, the levels of nitrogen in the soil available to plants declines. This conver-sion of inorganic N ( NH4

+] and [NO3-]) in the soil

to organic nitrogen is called immobilization, which will be revisited in the “Nitrogen” section of this chapter.

As decomposition continues, the C:N ratio narrows and the energy supply decreases as the microbes die because of the reduced food supply as humus formation nears completion. The levels of inorganic nitrogen (NH4+) in the soil then increase as microorganisms (mostly bacteria) breakdown the proteins of predecessors–one group feeds on the next one down the line–until the material is decomposed. This process is called mineralization. In time, the soil becomes more fertile and more productive be-cause nitrogen availability to plants increases.

Since soil microorganisms compete effectively with plants for inorganic nitrogen, plants can be-come deficient in nitrogen when incorporating undecomposed organic residues that have a high carbon to nitrogen ratio. Therefore, it is best to add an inorganic nitrogen fertilizer to the soil prior to planting or during the growing season.

When organic matter with a high C:N is applied to the soil surface as a mulch, which means that it is not incorporated into the soil, nitrogen deficiency is not usually a problem.

When organic residues with a low C:N (less than 25:1) are incorporated into the soil, they de-compose rapidly; nitrogen levels will not be limiting; and the decomposition process will release nitrogen from organic matter in inorganic ammonium- and nitrate-nitrogen forms that garden plants can ab-sorb. A nitrate-depression period does not occur.

When the decayed plant residues are returned to the soil, the organic nitrogen that they contain can-not initially be used by growing plants. It must be converted by various microorganisms into inorganic forms the plant can use, namely ammonium and nitrate ions. This process is called mineralization,

which is discussed in the “Nitrogen” section.

HumusWhen soil organic matter resists further decom-

position, it is called humus. Like clay minerals, soil humus has negative charges on its surfaces that attract plant nutrient cations, such as ammonium, calcium, magnesium, hydrogen, and potassium. Hu-mus increases a soil’s cation exchange capacity and improves soil fertility because the adsorbed cations remain available in the root zone for absorption by plants. There is a constant exchange of nutrients between soil solids (clay minerals and humus) and the soil solution and between the soil solution and crop plants. Humus also aids in the formation of granular aggregates, thereby improving soil struc-ture. Humus tends to be brown or very dark brown, and soils rich in organic matter tend to take on this darker hue.

Leaf mold is made up of partially decayed leaves. It is a natural product of woodlands where leaves accumulate and decay on the forest floor. To make it at home, rake up fallen leaves and cor-ral them in a cylinder of snow fencing, chicken wire, or hardware cloth. Without any further at-tention the leaves will convert to leaf mold in a couple of years.

You can speed the process by shredding the leaves with a lawnmower or shredder prior to pil-ing them up. Since leaves are high in carbon, add-ing a nitrogen source will speed the composting. Half a cup of fertilizer with a 10% nitrogen con-tent per 20 gallons of packed leaves will speed the breakdown, as will turning the pile periodically to assure that it is evenly moist and well aerated.

Whether you allow slow-acting fungi to do the decomposition, or let fast-acting nitrogen-consum-ing bacteria break down the organic matter, there is no need to add any starter cultures. Occasion-ally add a shovelful of soil or compost to speed-up the process.

The leaf mold that results will be dark and crumbly. It can be applied to the soil surface as a mulch or incorporated into the soil where it will Improve the organic content, the moisture-hold-ing capacity, drainage, and soil tilth. --B. Polomski


Soil OrganismsMany groups of organisms live in the soil.

Earthworms and other soil-dwelling organisms con-tribute to soil fertility. Most of these creatures live near the soil surface in the root zone of crop plants. They range in size from microscopic (bacteria, nem-atodes, and fungi) to groups readily visible to the na-ked eye (earthworms and insect larvae). Although most are beneficial to crop plants, a few destruc-tive plant pathogens use garden crops as their food source. These disease-causing soil microorganisms

Symptoms appear in older leaves first (mobile elements):

NITROGEN - General pale green to yellow leaf color (chlorosis) followed by necrosis (tissue death) and leaf drop; overall growth stunted

PHOSPHORUS-Leaves develop blue-green or red-purple coloration, possibly yellowing; poor fruit or seed development; slow, stunted growth

POTASSIUM-Interveinal tip chlorosis, then necrosis; leaf margins may become brown and curl downward; small fruit or shriveled seeds; slow growth

MAGNESIUM-Marginal chlorosis (yellowing) with green “Christmas tree” pattern along midrib of leaf, usu-ally followed by interveinal chlorosis (yellowing); tips and margins may become brittle, curl upward, and die

MOLYBDENUM-General then marginal then interveinal chlorosis; leaf margins may curl or roll, then die; stunting and lack of vigor

Symptoms appear in youngest leaves first (immobile elements):

SULFUR - Light green to yellowish, interveinal area even lighter; small and spindly plants

IRON - Distinctive interveinal chlorosis—yellow or white—with green veins; leaf drop or death of entire limbs or plants if severe

MANGANESE - Smallest veins green, interveinal chlorosis beginning at margins, progressing to midribs, fol-lowed by interveinal necrotic spots; no sharp difference between veins and interveinal areas as with iron deficiency

ZINC - Very small, stiff, chlorotic or mottled leaves; decrease in stem length and shortened internodes causing rosetting

COPPER - Terminal wilting, chlorosis, rosetting and death; veins lighter than inter-veinal areas

Terminal buds die:

BORON - Lateral buds and root tips also die, or lateral growth has few leaves that are chlorotic or necrotic, small, brittle, thick and cupped downward

CALCIUM-Tips distorted or die back (“scorched”); young leaves chlorotic, hard, stiff, margins distorted; pre-mature shedding of blossoms and buds; weakened stems; water-soaked, discolored areas on fruits—blos-som-end rot of tomatoes, peppers, and melons

Symptoms of nutrientdeficiencies

are described in chapter 7, “Basic Plant Pathology.”Earthworms are segmented worms that are part

of the beneficial macrofauna visible to the naked eye. Night crawlers (Lumbricis terrestris) produce deep vertical underground burrows and feed on or-ganic matter mainly on the soil surface. As they ex-cavate their burrows, these worms consume mineral soil and litter. They excrete their fecal matter or casts in mounds on the soil surface.

The casts are chock-full of readily available nutrients, such as phosphates, potassium, nitrate-


The fungi, called mycorrhizal fungi, live inside and outside root cells and help them reach for water and nutrients by extending long threads called hyphae into the soil. The plant, in exchange, provides the fungi glucose and possibly other organic materials that they need to survive. Unfortunately, modern agricultural practices have reduced populations of arbuscular mycorrhizal (AM) fungi, the most com-mon type.

SoilpHSoil pH or soil reaction is an indication of the

relative acidity or alkalinity (basicity) of soil or growth media. The pH scale contains 14 divisions known as pH units (Figure 1.6). It goes from 0 to 14 with the value of 7 as the neutral point. Values below 7 constitute the acidic range of the scale, and values above 7 make up the alkaline range.

The reading expresses the degree of acidity or alkalinity in terms of pH values, very much like heat and cold are expressed in degrees.

The measurement scale is not a linear scale but a logarithmic scale. (Technically, the soil pH is the negative logarithm of the hydrogen ion concentra-tion.) That is, a soil with a pH of 7.5 is 10 times more alkaline than a soil with a pH of 6.5, and a soil with a pH of 4.5 is ten times more acid than a soil with a pH of 5.5.

The pH condition of soil is one of a number of environmental conditions that affects the quality of plant growth. A near neutral or slightly acid soil be-tween 6 and 7 is generally considered ideal for most plants. However, acidity does not retard the growth of all crops. Some crops need acid conditions to grow well because of specific nutrient availabilities. Irish potatoes, however, are grown at pH 5 to 5.5 to prevent damage from potato scab disease, since this disease increases with increasing pH.

The major impact that extremes in pH have on plant growth is related to the availability of plant nutrients and the soil concentration of plant-toxic minerals (Figure 1.7; Note: the thicker the bar, the more available the element is at that particular pH value). In highly acid soils (pH less than 5.5), alu-minum and manganese can concentrate and become highly available at toxic levels. Also, at low pH

nitrogen, and exchangeable calcium and magne-sium. Their burrowing activity and their mixing of soil also improves the growing environment for plants. Earthworms improve soil drainage, porosity, aeration, and structure; they also encourage thatch decomposition and stimulate microbial activity.

Beneficial saprophytic bacteria and fungi have important roles in the improvement of soil fertility and structure and the recycling of organic waste. Soil saprophytes are beneficial microorganisms (bacteria and fungi) that are known as decomposers and recyclers because they feed on decaying plant residues left in the soil after harvest, decompose them, and recycle them into beneficial products, such as humus. Saprophytes also release plant nu-trient elements in simple mineral forms--formerly bound up in complex organic molecules in soil or-ganic matter--that then become available for uptake by plants. Recycling nutrients and creating humus are two important functions of these microbes.

As they feed on organic residues, saprophytes churn the soil and excrete gummy substances that bind the soil particles into aggregates to improve soil structure.

Not all beneficial soil bacteria and fungi are saprophytes. Instead of feeding on decaying organic matter, other groups of beneficial fungi and bacteria form associations with plant roots. Some groups of soil bacteria and fungi can fix atmospheric nitrogen (N2), which means that they can convert nitrogen in the air into ammonia (NH3) or ammonium ions (NH4

+). Crop plants cannot use N2, but they can absorb NH4

+. For example, Rhizobia spp. bacteria that live in association with the roots of legume plants inhabit nodules in the roots and make ni-trogen available to the legume plants by fixing N2. Other species of beneficial bacteria convert NH4

+ into nitrite (NO2

-) and then to nitrate (NO3-)

which plants can absorb.

MycorrhizaeMore than 90 percent of all terrestrial plants, in-

cluding the crops and herbaceous and woody plants in your garden and landscape, form associations be-tween their roots and soil fungi. These root-fungal associations are called mycorrhizae (“fungus root”) and are beneficial to both the plant and the fungus.

Sand has often been touted as the perfect fix for improving drainage in clay soils. Unless you add it at the rate of at least 6 inches of sand per 8 inches of soil, your soil will be better suited for making bricks than growing toma-toes or azaleas.


values, nitrogen, phosphorous, potassium, calcium, and magnesium become tied up and unavailable. At pH values of 7 and above, phosphorus, iron, copper, zinc, boron, and manganese become less available.

Adding more fertilizer will not help. The soil pH will have to be corrected by mixing in the rec-ommended amount of ground agricultural limestone to raise the pH. If the soil pH is too alkaline, add elemental sulfur or aluminum sulfate to lower the soil pH. Follow soil test results to determine the re-quired amounts.

FactorsAffectingSoilpHThe degree of soil acidity or basicity is influ-

enced by the kinds of parent materials from which

Nitrogen

Phosphorus

Potassium and Sulfur

Calcium

Fe, Cu, Zn, Mn, Co

Molybdenum

Boron

Acid pH Neutral pH Alkaline pH

Effects of soil pH on nutrient availability

the soil was formed. Rainfall also affects soil pH. Water passing through the soil leaches basic nu-trients such as calcium and mag-nesium into drainage water. They are replaced by acidic elements such as hydrogen, iron, and alumi-num. For this reason, soils formed under high rainfall conditions are more acidic than those formed un-der arid conditions.

Fertilization, especially with ammonium-nitrogen-containing fertilizers, speeds up the rate at which acidity develops. The de-composition of organic matter also contributes to soil acidity. One of the first products formed during decomposition is the ammonium form of nitrogen. When the ammonium-nitrogen is converted to nitrate-nitrogen, hydrogen ions are released, which increases soil acidity.

By the application of certain materials to the soil, adjustments can be made in pH values. To make soils less acid, apply a mate-rial that contains some form of lime. Ground agricultural lime-stone is most frequently used. The finer the grind the more rapidly it becomes effective. Different soils will require a different amount of lime to adjust the reaction to the proper range. The texture of the soil, organic matter content, the crop to be grown, and soil type are

all factors to consider in adjusting pH. For example, soils low in organic matter require much less lime than soils high in organic matter to make the same pH change.

If the soil pH is too high, elemental sulfur or aluminum sulfate can be added to the soil to reduce alkalinity. Most horticultural plants require slightly to strongly acid soil. These species develop iron chlorosis when grown in soils in the alkaline range. Iron chlorosis is often confused with nitrogen defi-ciency because the symptoms (a definite yellowing of the leaves) are similar. Iron chlorosis can be cor-rected by reducing the soil pH. Applying chelated iron formulations to the soil or by spraying foliage

pH values

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Range of acidity Range of alkalinityNeutral

Figure 1.7

Magnesium

Figure 1.6


with solutions of iron chelate or iron (ferrous) sulfate is a temporary solution.

The term chelate comes from the Greek word for “claw.” Chelates are chemical claws that help hold metal ions, such as iron, in solution, so that the plant can absorb them. Different chemicals can act as chelates, from relatively simple natural chelates like citrate, to more complex manufactured chemicals. When a chelated metal is added to the soil, the nutrient held by the chelate will remain available to the plant.

Most nutrients do not require the addition of a chelate to help absorption. Only a few of the metals, such as iron, benefit from the ad-dition of chelates. The types of chelate used will depend on the nutrient needed and the soil pH.

HowLime Reduces Soil AcidityWhen soils become too acid, excess

acidity is neutralized by applying limestone. Limestone is added to gardens to increase the soil pH, improve the availability of nutrients, encourage biological activity, and improve soil structure. In garden jargon it’s called “sweet-ening” the soil.

The process and reactions by which lime re-duces soil acidity are very complex. The pH of a soil is an expression of the hydrogen ion (H+) activity. Lime reduces soil acidity (increases pH) by changing some of those hydrogen ions into water and other reaction products. For chemistry buffs, the reaction looks like this:1. CaCO3

_________________________> Ca2+ + CO32–

(lime) in solution (calcium (carbonate ion)

ion) 2. CO3

2– + H2O ___________> HCO3– + OH–

3. OH– + H+ <___________> H2OBasically, a Ca2+ ion from the lime replaces two H+ on the cation exchange complex. The H+ are com-bined with hydroxyl ions (OH–) to form water. As the H+ concentration--the source of soil acidity--decreases, the pH increases.

Remember, the reverse of the above process can also occur. An acid soil can become more acidic if a lime is not applied. As basic ions, such as calcium, magnesium, and potassium are removed by plants, they can be replaced by hydrogen ions. These basic ions can also be lost by leaching, again being re-

RangeofpHvaluesforseveralcommonsubstances.

Example pH Description

Lye (bleach) 13.0 Strongly alkalineHousehold ammonia 12.0Soap 9.3Antacid tablets 9.4Baking soda 8.0Seawater 7.9Human blood 7.3 Weakly alkaline

Pure water 7.0 Neutral

Fresh milk 6.7 Weakly acidRain 5.6 Sour milk 4.7Beer 4.4Coffee 4.2Orange juice 3.7Wine 3.5Vinegar 2.9Classic Coke® 2.5Lemon juice 2.4Stomach acid 2.0 Battery acid 0.5 Strongly acid

Credit: Adapted Southern Lawns, 2003.

➤

➤

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placed by hydrogen ions. The hydrogen ion activity will steadily increase, lowering soil pH if the soil is not limed properly.

Time of Application and Lime PlacementLime needs should be determined by soil test

results. Soil samples should be taken in the fall for the succeeding year’s garden. If test results indicate a need for limestone, it can be applied in the fall or winter months. Generally, for best results, lime-stone should be applied at least 2 to 3 months prior to planting.

The most important factor determining the ef-fectiveness of lime is placement. Maximum contact of lime with the soil is essential. Most common liming materials are only slightly soluble in water,

An acid is a substance that releases hydrogen ions (H+). When saturated with hydrogen ions, a soil behaves as a weak acid. The more hydrogen ions held on the exchange complex, the greater the soil’s acidity. Aluminum also acts as an acidic element and activates hydrogen ions. Basic ions, such as calcium and magnesium, make soil less acid (more alkaline) in reaction.


so distribution in the soil is a must for lime reaction. Even when properly mixed with the soil, lime will have little effect on pH if the soil is dry. Moisture is essential for lime-soil reaction to occur.

When applying large amounts of lime to clay soils, best mixing comes from applying part before tilling or plowing and the rest after. On sandy soils that can be tilled 4 to 6 inches deep, one applica-tion before tilling will do.

Selecting a Liming MaterialNeutralizing values of all liming materials are

determined by comparing them to the neutralizing value of pure calcium carbonate (CaCO3). Setting the neutralizing value of calcium carbonate at 100, a value for other materials can be assigned. This value is called the “relative neutralizing value” or “calcium carbonate equivalent.” The relative neu-tralizing values for several common liming materials are shown in Table 1.5.

The higher the neutralizing value, the less lime is required to effect a change in pH. For example, if burned lime had a neutralizing value of 160, 63 lbs. of the material would produce the same effect in pH

as 100 lbs. of calcium carbonate.

Calcitic limestone (CaCO3) and Dolomitic limestone (CaMg[CO3]2) – Deposits of high-grade calcitic and dolomitic limestone are widespread throughout the U.S. They are the most common sources of limestone. Their neutralizing values (CaCO3 equivalents) usually range from 85 per-cent to slightly more than 100 percent. Calcitic limestone or calcium carbonate contains about 40 percent calcium. Dolomitic limestone or calcium-magnesium carbonate contains 21 to 30 percent

LimingMaterial RelativeNeutralizing Value

Calcium Carbonate 100Dolomitic Lime 95-108Calcitic Lime 85-100Baked Oyster Shells 80-90Marl 50-90Burned Lime 150-175Hydrated Lime 120-135Basic Slag 50-70Gypsum None

Liming materials

Using Wood AshWood ashes are often used as a substitute

for lime. They are a rich source of potash (potas-sium). Wood ashes can be used to raise soil pH with twice as much ash applied as limestone for the same effect.

Ashes should not come into contact with germinated seedlings or plant root as they may cause root damage. Spread on a thin layer dur-ing the winter and incorporate into the soil in the spring. Check the soil pH yearly if you use wood ashes. Avoid using large amounts (no more than 20 pounds per 1,000 square feet) because toxicity problems may occur.

Also, the soil pH may be raised too high. Since wood ashes are low in calcium compared to lime-stone, supplemental gypsum (a source of calcium) will be needed if the calcium level indicated on the soil test report is medium or low. Add about 1 or 2 pounds of gypsum per 100 square feet to compen-sate for the lack of calcium. Never use coal ashes, which can contain plant toxic (phytotoxic) levels of sulfur and iron.--B.Polomski

Table 1.5

calcium, and 6 to 11 percent magnesium. Dolomitic limestone is often recommended when soil tests in-dicate deficient levels of magnesium.

Both of these liming materials can be purchased in a ground or pulverized form that consists of a fine dust. To make it easier to spread with a centrifugal-type lawn spreader, both types are also available in a pelleted form that dissolves when it rains.

Limestone is relatively insoluble in water (only 1 pound will dissolve in 500 gallons of water), so it’s best to thoroughly mix the limestone into the soil.

Apply it at anytime of year, but fall and winter are the most effective times because rain will help work it into the soil. When using calcitic or dolo-mitic limestone, expect an increase in pH to take from 4 to 6 months.

Calcium oxide (CaO) and calcium hydroxide (Ca(OH)2) – – For a more rapid change in pH, commercial growers use quicklime or burned lime (calcium oxide and/or magnesium oxide) as well as hydrated or slaked lime (calcium hydroxide). Both of these white powders change the pH rapidly when added to soil, the quicklime faster than hydrated. However, both are corrosive, difficult to handle, and have a high potential of burning or injuring plants.


most often associated with lawn and landscape fer-tilization. Plants use large quantities of nitrogen, and most mineral soils simply can’t supply enough to give satisfactory plant performance. Nitrogen is a constituent of all living cells and is a necessary part of all proteins, enzymes, and metabolic processes involved in the production and transfer of energy. Nitrogen is a structural part of chlorophyll, the green pigment of the plant that is responsible for photosynthesis. Other functions of nitrogen include stimulating rapid, vigorous growth, increasing seed and fruit yield, and improving the quality of leaf and forage crops. Proper use of nitrogen fertilizer in the landscape is vital for proper plant performance and for reducing environmental pollution. Soil nitrogen is present in three major forms: organic, inorganic, and elemental.

Organic nitrogen makes up about 5% of the soil’s organic matter (humus) by weight and about 98% of the total soil nitrogen. Although organic nitrogen is not avail-able to plants, soil organisms convert a portion of it each year to inorganic ammonium (NH4

+) through a process called miner-alization (Figure 1.8). The NH4

+ that is produced can be readily used by plants, converted to ni-trate, or slowly released back to the atmosphere as dinitrogen gas (N2).

Organic nitrogen fertilizers (for example, manures or sewage sludge) are popular for lawns and gardens because of their slow release and long-lasting properties.

Nitrogen can also be changed from inorganic nitrogen (ammonium and nitrate) to organic nitro-gen in a process called immobilization, which is the reverse of mineralization (Figure 1.8). This process occurs when fresh organic materials are added to the soil. As the soilborne microorganisms responsible for decomposing organic matter vigorously break down the plant residues, they need nitrogen to build protein for their body tissues. Unless the residues are relatively high in nitrogen, these organisms take up inorganic nitrogen from the soil to meet their nitrogen requirements, reducing its availability to plants. This is called immobilization as the mineral nitrogen in the soil is changed into organic nitrogen in microbial proteins, rendering them unavailable

Plant NutrientsThere are 17 nutrient elements that have been

proven to be essential for the growth and repro-duction of plants. Plants obtain the three most abundant – carbon, hydrogen, and oxygen – from water and the air, and about 94% of their dry tissue is composed of these three elements. The other 14 elements combined represent less than 6% of the plant’s dry matter. Yet, growth is frequently reduced or limited by a deficiency of one or more of these 14 essential elements, which may be supplied in fertil-izers (Table 1.6).

The 14 elements are often divided into three groups. The primary group of nutrients is composed of nitrogen (N), phosphorus (P) and potassium (K). In the secondary group are the nutrients sulfur (S), calcium (Ca), and magnesium (Mg). The primary

and secondary nutrients are often collectively re-ferred to as macronutrients. The essential elements in the third group are called micronutrients be-cause they are required in small (“micro”) amounts by plants. Although these elements are frequently referred to as minor or trace elements, the term micronutrient is preferred. Iron (Fe), copper (Cu), manganese (Mn), molybdenum (Mo), zinc (Zn), boron (B), and chlorine (Cl), and nickel (Ni) are all micronutrients. Understanding how these elements work can help make better choices among the many fertilizers and their claims.

Primary NutrientsNitrogen

Nitrogen is probably the nutrient whose defi-ciency most often limits plant growth. This is also the element that gives the visual green-up response

Macronutrients Micronutrients

From Air and Water From Soil From Soil

Carbon Nitrogen Iron Hydrogen Phosphorus Copper Oxygen Potassium Manganese Calcium Molybdenum Magnesium Zinc Sulfur Boron Chlorine Nickel

Essential elements required by plants.

Table 1.6


Mineralization

for plant growth. Eventually, much of this nitrogen is returned to the available form as the bacterial bodies decompose.

Nitrogen in fertilizers for agricultural crops is largely inorganic nitrogen consisting of three types: ammonium, nitrate, and urea (CO[NH2]2) (Table 1.7). Although urea is an organic nitrogen fertilizer, it is rapidly converted to the ammonium form with-in a short time after exposure to moist, aerated soil. Therefore, under most conditions urea acts more like inorganic, ammonium fertilizers than natural organic fertilizers.

In warm, moist soils with a pH above 6, the majority of ammonium nitrogen is converted to nitrate nitrogen by soil organisms rather quickly. Therefore, most nitrogen taken up by plants is in the nitrate form, although ammonium is taken up when present in the soil solution. The nitrate ion carries a negative charge which prevents its reten-tion by the negatively charged soil colloids. Since it is soluble and mobile, the nitrate ion is readily and easily available to plants and will give quick growth results.

Nitrogen must be singled out for further discus-sion because it is usually the most expensive com-ponent in a fertilizer bag, and the various nitrogen sources that are used behave very differently once applied.

For example, nitrate dissolves readily in water and moves freely in the soil solution, that is, the water that is in the spaces between the soil particles. Under heavy rainfall or irrigation, nitrate nitrogen can be lost from the plant root zone. The loss of nitrate by leaching is a common problem on coarse-textured, sandy soils of the sandhills and coastal plain regions. Leaching losses of fertilizer nitrogen are minimal when rates of application conform to recommendations consistent with the yield poten-tial for the crop and soil in question. Nitrate ni-trogen is also subject to denitrification, a process in

which the nitrate ion is reduced through several intermediate steps to a gaseous nitrogen oxide or to elemental nitrogen (N2).

Ammoniacal nitrogen also dissolves readily in water, but does not leach from the soil as rapidly as nitrate nitrogen. Soil bacteria usually convert it to the nitrate form in a few days or weeks. Ammoniacal

nitrogen has an acidifying effect as it converts to nitrate nitrogen and is, therefore, almost always found in acid-forming fertilizers such as “Azalea/Rhododendron Special Fertilizer.” Because most soils in South Carolina are already very acid, the acid-forming fertilizers are not usually the best forms to apply. Typically, a good slow-release fertilizer that does not make the soil more acid would be a better choice, unless the soil was overlimed.

Water-soluble organic nitrogen is supplied main-ly from urea. Water-soluble nitrogen changes to am-moniacal nitrogen within a few days of application. Urea is a fairly inexpensive form of nitrogen.

Water-insoluble nitrogen originally meant natural organic materials such as manure or dried blood. Natural materials such as these break down very slowly and yield their nitrogen over a long period of time. Today, however, many forms of water-insoluble nitrogen have been developed such as IBDU (isobutylidene diurea) and sulfur-coated urea. These materials are manufactured to release nitrogen slowly, so these are also included under the water-insoluble nitrogen category.

This form of nitrogen is the most expensive of the four discussed, but it will provide a continued

Source Percentage of N

Ammonium nitrate 33.5-34.0Ammonium sulfate 21Calcium nitrate 15Sodium nitrate 16Potassium nitrate 13Urea 45Animal tankage 9*Blood, dried 12*Fish meal 10*Manure variable

Nitrogen fertilizer sources and their N content

*Average value

Table 1.7

Organic NPool

(Proteins, etc.)

Inorganic NPool

Mineralization and immobilization of nitrogen

(ammonia and ammonium ions

Figure 1.8

Immobilization


Avoid applying too much fertilizer, which can pol-lute our rivers, streams, lakes and estuaries.

release of nitrogen over an extended period of time.Elemental nitrogen, found in a gaseous form

(N2) in the soil atmosphere, cannot be used by plants. It is of direct significance to plants only because it is involved in bacterial nitrogen fixation. For example, Rhizobium bacteria live in nodules on leguminous plant roots and convert atmospheric N2 into other nitrogen compounds that can be used by plants.

PhosphorusLike nitrogen, phosphorus (P) is an essential

part of the process of photosynthesis. Plants use the energy of sunlight, and phosphorus must be present in the active portions of the plant for this energy transfer to be made and for photosynthesis to occur. In young plants, phosphorus is most abundant in tis-sue at the growing point. It is readily translocated or moved about from older to younger tissue, and as plants mature, most of the element moves into the seeds and/or fruits. Phosphorus is responsible for such characteristics of plant growth as utilization of starch and sugar, cell nucleus formation, cell divi-sion and multiplication, fat and albumen formation, cell organization, and transfer of heredity.

The immediate source of phosphorus for plants is that which is dissolved in the soil solution. Plants absorb phosphorus as PO4

3-. A soil solution contain-ing only a few parts per million of phosphate ions is usually considered adequate for plant growth. Phos-phate ions are absorbed from the soil solution and used by plants; the soil solution is replenished from soil minerals, soil organic matter decomposition, or applied fertilizers.

The availability of phosphorous is related to soil pH. Phosphorous is most readily available at pH 6 to 7, but is not very mobile in the soil. There-fore, it should be worked into the soil to make it available for absorption by plant roots. Due to the importance of P for root development, all of the recommended phosphorous should be applied to gardens and mixed into the soil prior to planting or transplanting. When supplemental applications are needed, several sources are available (Table 1.8).

PotassiumPotassium (K), one of the big three macronu-

trients, is absorbed by plants in larger amounts than any other mineral element except nitrogen and, in some cases, calcium (Ca). Unlike nitrogen and phosphorus, potassium is not found in organic com-

Phosphorous fertilizer sources and their P content

Source Percentage of P2O5

Ordinary superphosphate 20Concentrated superphosphate 46Animal tankage 10*Bonemeal, steamed 22*Fish meal 5*Manure variable

* Average value

Table 1.8

bination with plant tissues. Potassium plays an es-sential role in the metabolic processes of plants and is required in adequate amounts in several reactions involving enzymes. Potassium is also essential in carbohydrate metabolism, a process by which energy is obtained from sugar. Evidence shows that potas-sium plays a role in raising the disease resistance and drought tolerance of many plant species.

Potassium is supplied to plants by soil miner-als, organic materials and inorganic fertilizer (Table 1.9). Potassium occurs in the soil solution as a mon-ovalent cation (K+). The CEC of the soil controls the retention of K+. In very sandy soils with a low CEC under high rainfall, potassium is subject to leaching losses.

Secondary NutrientsCalcium

An essential part of the wall structure of plant cells, calcium provides for normal transport and re-tention of other elements as well as strength in the plant. It is also thought to counteract the effect of alkali salts and organic acids within a plant. Cal-cium is absorbed as the cation Ca2+ and exists in a delicate balance with magnesium and potassium in the plant. Too much of any one of these elements may cause deficiencies of the other two.

Calcium (Ca) occurs in the soil solution as a divalent cation (Ca2+,) and is supplied to plants by soil minerals, organic materials, fertilizers, and lim-ing materials. There is a strong preference for Ca2+ on the cation exchange sites of most soils, and it is the predominant cation in most soils with a pH of 6


or higher. Because most calcium-deficient soils are acid, a good liming program can add calcium most efficiently. Both calcitic and dolomitic limestone are excellent sources of calcium. When soil pH is adequate and additional calcium is needed, gypsum (CaSO4•2H2O) can be used to supply the needed calcium without changing the soil pH. Normal su-perphosphate, which is 50 percent gypsum and, to a lesser extent triple superphosphate, can also add calcium to the soil.

When using calcium sources other than ground calcitic and dolomitic limestone, apply with cau-tion. Too much hydrated lime and burned lime can partially sterilize the soil.

MagnesiumMagnesium makes up a part of the chlorophyll

in all green plants and is essential for photosynthe-sis. It also helps activate many plant enzymes need-ed for growth. A relatively mobile element in the plant, magnesium is absorbed as the cation Mg2+ and can be readily translocated from older to younger plant parts in the event of a deficiency.

Soil minerals, organic material, fertilizers, and dolomitic limestone are sources of magnesium (Mg) for plants. Magnesium occurs as a divalent cation (Mg2+) and is held on the exchange sites like cal-cium (Ca2+) and potassium (K+).

The most common source of magnesium is do-lomitic limestone –an excellent material that offers both calcium and magnesium which neutralize soil acidity (Table 1.10).Sulfur

Sulfur is taken up by plants primarily in the form of SO4

2– and assembled into organic com-pounds. It is a constituent of the amino acids cys-

tine, cysteine and methionine and, hence, proteins that contain these amino acids. It is found in vita-mins, enzymes, and coenzymes. Sulfur is also pres-ent in glycosides which give the characteristic odors and flavors in mustard, onion, and garlic plants. It is required for nodulation and nitrogen fixation of legumes.

+Most organic materials such as tankage, bonemeal, etc., contain low amounts of K2O, usually between1 and 2%.

Potassium fertilizer sources and their K content

Source Percentage of K2O

Muriate of Potash 60-62Potassium nitrate 44Potassium sulfate 50-53Sulfate of Potash Magnesia 22*Manures variable*Organic fertilizers +

* Average value.

Table 1.9

In most soils, sulfur (S) is present primarily in the organic matter (for example, humus and crop residues), and becomes available upon their decom-position. Much like the nitrate ion, the sulfate ion remains in soil solution until it is taken up by the plant. In this form it is subject to leaching as well as microbial immobilization. In waterlogged soils, it may be reduced to elemental sulfur or other unavail-able forms.

Sulfur may be supplied to the soil from the at-mosphere in rainwater. It is also added in some fer-tilizers as an impurity, especially the lower grade fer-tilizers. The use of gypsum (CaSO4) also increases soil sulfur levels.

Sulfur is used to lower soil pH because it acidi-fies the soil. Finely ground sulfur should be broadcast and incorporated several weeks before planting the crop because soil microbes (Thiobacillus bacteria) are required to convert the sulfur to sulfuric acid.

Aluminum sulfate Al2(SO4)3 is also used to acidify soils. It changes the soil pH instantly as soon as it dissolves in the soil.

MicronutrientsEight of the 17 elements known to be essential

for plant growth are required in such small quanti-ties that they are called micronutrients. Micronu-trients have become of more widespread concern during the past 20 or so years.

Deficiencies in micronutrients are most likely to limit crop growth under the following condi-tions: (1) highly leached acid sandy soil; (2) muck

Common magnesium sources and their Mg content

Material Percentage of Mg

Dolomitic Limestone 6-11Magnesia 55Hydrated Magnesium Sulfate* 17Potassium-Magnesium Sulfate 11

*Epsom salts.

Table 1.10


ganese deficiency.

CopperCopper (Cu) is essential for growth and acti-

vates many enzymes. A deficiency interferes with protein synthesis and causes a buildup of soluble ni-trogen compounds. Excess quantities of copper may also induce iron deficiency.

ZincZinc (Zn) is essential for plant growth because

it controls the synthesis of indoleacetic acid, which dramatically regulates plant growth. Zinc is also ac-tive in many enzymatic reactions.

BoronBoron (B) primarily regulates the metabolism

of carbohydrates in plants. The need varies greatly with different plant species. Rates required for re-sponsive crops may cause serious damage to boron-sensitive crops. Boron deficiency may occur on both alkaline and acid soils but is more prevalent on the calcareous, alkaline soils.

MolybdenumMolybdenum (Mo) functions largely in the

enzyme systems of nitrogen fixation and nitrate

soils; (3) soils high in pH or lime content; and (4) soils that have been intensively cropped and heavily fertilized with macronutrients. Four of the micro-nutrients occur predominantly as cations in the soil solution. They are iron (Fe3+), copper (Cu2+), man-ganese (Mn2+), nickel (Ni2+), and zinc (Zn2+). Two occur predominantly as anions. These are molybde-num (MoO4

2-) and chloride (Cl-). Boron occurs as the neutral species H3BO3.

ManganeseManganese (Mn) is mainly absorbed by plants

in the form of the ion Mn2+. Manganese is an en-zyme-activator involved in plant growth processes. It’s also involved with iron in chlorophyll formation. High manganese concentration may induce iron de-ficiency in plants.

Manganese availability is closely related to the degree of soil acidity. Deficient plants are usually found on slightly acid or alkaline soils.

IronIron (Fe) is a constituent of many organic com-

pounds in plants. It is essential for the synthesis of chlorophyll, which gives rise to the green pigment of plants. Iron deficiency can be induced by high levels of manganese. High iron can also cause man-

Gypsum or hydrated calcium sulfate, has been used as a soil amendment for over 200 years. It supplies calcium without increasing the soil pH, unlike ground limestone (calcium carbonate), which causes an upward shift in soil pH. Its principal use is to restore the tilth and physical structure of sodic soils. These are soils that either naturally contain a high level of sodium, as in arid parts of the country, or that have been made sodic through irrigation or the application of large quantities of road salt or salt from hurricanes. An excessive accu-mulation of sodium causes the soil particles to deflocculate or disperse. These individual soil particles become tightly arranged and restrict pore space. As a result, sodic soils are poorly drained and, when wet, become slick and sticky. When they dry out, they become rock hard. Also, high levels of sodium preclude the uptake of other nutrients such as calcium and magnesium; in extreme cases, sodium levels can also be toxic to the plants.

The calcium that gypsum adds to the soil replaces the sodium, which leaches out as sodium sulfate. As a result, the individual soil particles start to flocculate to form larger particles. Organic matter stabilizes these ag-gregates to produce pores that improve root growth and facilitate air and water movement.

The acidic, fine-textured, compacted clay soils in the Piedmont may appear to have the same properties of sodic soils, especially if you have made the mistake of working them when they were wet. However, adding gypsum to clay will not substantially improve its workability.

What you should be adding to your clay, instead, is organic matter such as compost, leaf mold, or rotted manure. A soil test will determine whether you need to adjust your pH (or apply gypsum for its calcium). Al-though it’s not as easy to apply, adding organic matter is by far the best way to improve the growing environment both for your plants and for the soil fauna, which, in the long run, will contribute to the development of a highly productive soil.

Gypsum is ideally suited for crops that require calcium, but where an increase in soil pH would be undesir-able. These crops include peanuts, vegetables, and acid-loving ornamentals; an increase in soil pH may result in a deficiency of micronutrients such as iron, zinc, manganese, copper, and boron.--B. Polomski

The Truth About Gypsum....


reduction. Plants that can neither fix nitrogen nor incorporate nitrate into their metabolic systems be-cause of inadequate molybdenum become nitrogen-deficient.

ChlorineChlorine (Cl) is an enzyme activator for one

or more reactions in which water is split for pho-tosynthesis. Chlorine deficiency is not likely to be a problem in South Carolina, especially along the coast where the salt air from the ocean provides traces of chlorine in rainwater necessary for plant growth. The general use of muriate of potash in fer-tilizers is another common source of chlorine.

NickelNickel (Ni) has recently been recognized as an

essential plant nutrient since 1987. The American Association of Plant Food Control Officials ap-proved nickel as a micronutrient fertilizer in 2004. Nickel (Ni2+) is important in nitrogen metabolism, because it’s a component of the enzyme urease, which converts urea to ammonia in plant tissue.

“Mouse ear” disorder occurs in river birch and pecan trees as a result of a nickel deficiency; it can be corrected by a foliar or drench applications of nickel sulfate. Recently, a product containing nickel has been labeled for use against daylily rust (Puccinia hemerocallis).

Commonly Applied NutrientsThe most commonly applied nutrients are

nitrogen, phosphorus and potassium (N-P-K). Re-sponses to all three elements were fairly widespread in the past, and it became customary to apply the three together. Because of habit, all three are still applied even though there are now many situations, especially in home landscapes, where plants do not respond to phosphorus and potassium fertilization.

Other plant-essential nutrients used in fairly large quantities are calcium, magnesium and sulfur. However, fertilization with these nutrients is not usually necessary because the calcium and magne-sium contents of soil are generally sufficient. Also, large quantities of calcium and magnesium are sup-plied when acidic soil is limed with dolomite. Sulfur is usually present in sufficient quantities from the slow decomposition of soil organic matter, an impor-tant reason for not throwing out grass clippings and leaves.

Micronutrients are often used to sell fertilizers.

While there are instances of deficiencies of one or more micronutrients in specific plant-growth set-tings, most micronutrients in fertilizer are included as a preventative measure and do not give a plant response. Buy and apply only those nutrients that soil test results indicate will be needed by plants. If one plant species has a micronutrient deficiency, ap-ply the recommended rate of the deficient nutrient. Recycling organic matter by leaving grass clippings on the lawn and by mulching with tree leaves is an excellent means of providing micronutrients (as well as macronutrients) to growing plants.

FertilizersThe fertilizer grade, popularly referred to as the

analysis, refers to how much of an element there is in a formulation based on percentage by weight. All fertilizers are labeled with three numbers that indi-cate the guaranteed analysis, or the fertilizer grade. These three numbers give the percentage by weight of nitrogen (N), phosphate (P2O5), and potash (K2O). State regulations require that actual analysis values must be within certain limits of the labeled grade. Often, to simplify matters, these numbers are said to represent nitrogen, phosphorus, and potassi-um, or N-P-K. Remember that it is not N-P-K, but N-P2O5-K2O. For example, in a 100 lb. bag of fertil-izer labeled 10-10-10, there are 10 lbs. of N, 10 lbs. of P2O5, and 10 lbs. of K2O. To convert the P205 to actual phosphorous, multiply it by 0.44; to convert K2O to actual potassium, multiply it by 0.83.

In a 100 lb. bag of 10-10-10, the N, phosphorus, and potassium make up only 23 lbs. of the actual net weight of fertilizer. The nutrients actually are in a compound or material that contains other elements. For example, much of the potassium used in fertil-izers is in the form of potassium chloride. A fertil-izer may also contain nutrient sources other than N, P, and K; conditioners to keep the fertilizer in a granular, easily spread form; and lightweight absor-bents to give a slow-release of nutrients. Filler may be used to dilute concentrated fertilizer materials to make low analysis fertilizers. In some premium fertilizers limestone is added both as a filler and as a nutrient to neutralize the acidifying effects of some N sources.

Fertilizer ratio. The ratio describes the rela-tive proportions of nitrogen, phosphoric pentoxide (P2O5), and potassium oxide (K2O) in a fertilizer. For example, according to conventional fertilizer


standards, a 100 pound bag of a 16-4-8 fertilizer con-tains 16 percent or 16 pounds of nitrogen, 4 percent or 4 pounds of P2O5, and 8 percent or 8 pounds of K2O. Since P2O5 is really only 44% actual elemen-tal phosphorus and K2O is only 83 percent actual elemental potassium, a 100 pound bag of 16-4-8 actually contains 1.8 percent or 1.8 pounds of el-emental phosphorus, and 6.6 percent or 6.6 pounds of elemental potassium. Once the fertilizer is added to the soil, the oxide forms, P2O5, and K2O are no longer used when discussing these two nutrients. Soil test results express these nutrients as pounds of P and K per acre.

Complete fertilizer. A fertilizer is called “com-plete” when it contains each of the major plant nutrients: nitrogen, phosphorus, and potassium. Grades such as 10-10-10, 16-4-8, and 5-10-10 repre-sent complete fertilizers. If plants need only one of these nutrients, a complete fertilizer is not needed.

Balanced fertilizer. A fertilizer is called “bal-anced” only because it contains equal amounts of N, P2O5, and K2O. A 17-17-17 or 10-10-10 fertilizer is a balanced fertilizer.

Incomplete fertilizer. An incomplete fertilizer lacks one of the major plant nutrients. Examples of incomplete fertilizers are indicated in Table 1.11.

The blending of incomplete fertilizers is used to

make complete fertilizers. As an example, if 100 lb of urea (46-0-0) were combined with 100 lb of triple superphosphate (0-45-0) and 100 lb of muriate of potash (0-0-60), a fertilizer grade of 15-15-20 would result.

This tabulation thus indicates that 100 lbs. of urea fertilizer originally contains 46 lbs. of nitrogen; triple superphosphate has 45 lbs. of phosphorus; and muriate of potash has 60 lbs. of potash. When these three quantities are combined, each quantity is di-luted by the other by one-third, provided each bag has equal weight.

Although the fertilizer ratio indicates the pro-portion of nitrogen, phosphate, and potash con-tained in the product, the specific fertilizer ratio needed depends on the soil nutrient level as deter-mined by a soil test.

FertilizerLabelsFor many years there has been a model label law

that many states have adopted for the classification of fertilizers. The law also establishes minimum lev-els of nutrients allowable and provides specific label-ing requirements. To date, model label legislation has not met with total acceptance, so differences still exist from state to state as to what constitutes a fertilizer and what type of information appears on labels. Even so, the information contained on

Virtually all fertilizers are salts. When they dissolve in water in the soil they increase the salt concentration. As the salt concentration increases, so does the osmotic potential. In an ideal situation, water moves through osmosis from a low salt concentration in the soil solution and into a higher concentration in the root cells. When the concentration of salts in the soil solution exceeds the amount in the roots, or when the osmotic potential of the soil increases, water tends to move out of the root cells and into the surrounding soil solution.

The high salt concentration in the root zone inhibits water uptake by plant roots and can lead to yellowing leaves, leaf scorch (browning of the leaf margins), early leaf drop, dieback, and reduced growth.

A salt index was created to quantify the osmotic potential of fertilizers. The value of the salt index is a com-parison between the increase in osmotic potential of a fertilizer added to water and an equivalent weight of so-dium nitrate added to water, which has been given the salt index value of 100.

Fertilizers with a salt index above 100, such as ammonium nitrate (33.5-0-0) and potassium chloride (0-0-60), produce an osmotic potential greater than an equal weight of sodium nitrate. These fertilizers cause water to move out of root cells. A fertilizer with a high salt index is more soluble than one with a lower salt in-dex. As a result, it has a higher probability of causing salt damage.

Nitrogen and potassium salts have much higher salt indices than phosphorus salts. When applied close to or in touch with seeds, nitrogen and potassium can injure or kill the germinating seeds.

Minimize the burn potential by using fertilizers with a low salt index. To reduce the probability of salt dam-age to trees and shrubs, use a fertilizer with a salt index less than 50. Also, use a fertilizer that contains slow-release or controlled-release nitrogen. This kind of fertilizer reduces the chances of leaching and loss of nutrients as well as the chance of fertilizer burn from occurring. Determine if it’s slow-release by looking at the amount of “water insoluble nitrogen” or “slowly available nitrogen” on the label.

When using a fertilizer with a salt index greater than 20, work it into the soil before planting. Or apply it to the soil surface and then water it in to reduce the potential for salt injury caused by an accumulation of pockets of fertilizer.--B. Polomski

SaltIndexNumberonaFertilizerBag


fertilizer labels has been well-standardized, and the consumer is protected by state laws requiring manu-facturers to guarantee the claimed nutrients.

The law requires that the manufacturer guar-antee what is claimed on the label, so in some cases a fertilizer will contain secondary nutrients or micronutrients not listed on the label because the manufacturer does not want to guarantee the exact amounts. The gardener/consumer can rest assured that nutrients listed on the label are contained in the fertilizer.

Special-purposefertilizers

Fertilizer Nitrogen Phosphorus Potassium

Ammonium nitrate 34 0 0Ammonium sulfate 21 0 0Mono-ammonium phosphate 11 48 0Muriate of potash (potassium chloride) 0 0 60Potassium sulfate 0 0 52Superphosphate 0 20 0Triple superphosphate 0 45 0Urea 46 0 0Ureaformaldehyde 38 0 0

Examples of a few commonly available incomplete fertilizers.

Table 1.11 There are fertilizers pack-aged for certain uses or types of plants such as “Camellia Food,” “Rhododendron and Azalea Food,” or “Rose Food.” The ca-mellia and rhododendron/azalea fertilizers belong to an old es-tablished group, the “acid plant foods.” Some of the compounds in these fertilizers are chosen because they have an acidifying reaction, so they are especially beneficial to acid-loving plants growing in soil that is naturally neutral or alkaline. The other

fertilizers packaged for certain plants do not have as valid a background of research. Compare, for ex-ample, the fertilizer ratios of three different brands of rose foods, which may be quite different.

Slow-releasefertilizersPlants can take up fertilizers continuously, so

it is beneficial to provide them with a balance of nutrients throughout their growth. Perhaps the most efficient way to achieve this is to apply a slow-release fertilizer. Slow-release fertilizers contain one or more essential elements. These elements

Ideally, test your soil before you fertilize to determine the soil pH and level of nutrients present in the soil. It can be done at anytime of year. Soil testing allows you to customize your fertilizer and lime applica-tions to your plants’ needs. It will also help prevent problems with nutrient deficiencies (in the case of under-fertilization) or problems associated with over-fertilization, such as excessive vegetative growth, delayed maturity, salt burn, and wasted money. In addition, it can protect against any environmental hazards resulting from excessive fertilizer applications leading to leaching and runoff.For an accurate test, collect a representative sample, which means collecting 12 or more cores and combining them into one composite sample. The samples should include soil from the surface to a depth of 6 to 8 inches in all areas except for lawns where cores should be taken from a depth of only 3 to 6 inches. Use a soil auger, spade, shovel, or garden trowel. When using a spade or shovel, take a thin slice from the side of a “V”-shaped hole. Place the samples in a clean bucket and mix them thoroughly. Bring a minimum of 2 cups of soil per sample to your county Clemson Extension office or purchase a soil sampler mailer through Clemson University Public Service Publishing (www.clemson.edu/psapublishing/). Included with the mailer are a soil sample bag and a “Soil Analysis” form. The Agricultural Service Laboratory (www.clemson.edu/agsrvlb/) at Clemson analyzes the soil. At the end of this chapter (Figure 1.9) is an example of a soil test report with accompanying recommendations and comments.

Soil Testing


are released or made available to the plant over an extended period. Slow-release fertilizers are less susceptible to leaching and are preferred on sandy soils, which tend to leach. They are a good choice for areas where the potential of runoff is very high, such as slopes, compacted soil, or sparsely covered areas; since the nutrients are released slowly, the potential for runoff and water contamination is less.

Slow-release fertilizers can be categorized by the way in which the fertilizer is released. The three major types of nutri-ent release are: (1) materials that dissolve slowly, such as granite meal, greensand, and rock phos-phate; (2) materials from which nitrogen is released by microor-ganisms; and (3) granular mate-rials with coatings made of resin or sulfur that control the rate of nutrient release from the granules into the soil.

Sulfur-coated urea is a slow-release fertilizer with a covering of sulfur around each urea particle. Different thicknesses of sulfur control the rate of release of nitrogen, which becomes more rapid as temperature increases. Watering does not affect its release rate. Sulfur-coated urea applied to the soil surface releases more slowly than it does if it is incorporated into the soil. This material generally costs less than other slow-release fertilizers, and it supplies the essential element sulfur.

When fertilizer products coated with multiple layers of resin come into contact with water, water is absorbed, the layers swell and increase the pore size in the resin so that the dissolved fertilizer moves through the coating into the soil. The rate at which the fertilizer is released depends primarily on the coating thickness and temperature. There is often a large release of fertilizer during the first 2 or 3 days after application. Release-timing can vary depend-ing on the coating.

Slow-release fertilizers need not be applied as frequently as other fertilizers, and higher amounts can be applied without danger of burning (Table 1.12). Plants may use the nitrogen in slow-release fertilizers more efficiently than in other forms, be-

cause it is continually being released over a long pe-riod. These fertilizers are generally more expensive than other types. The real savings, however, is in time because they do not need to be applied as often as conventional fertilizers.

InorganicfertilizersVarious salts and minerals can serve as fertilizer

materials. Examples are ammonium sulfate, potas-sium nitrate, superphosphate, rock phosphate, potas-sium chloride, and potassium sulfate.

Synthetic organicfertilizersHuman-made organic materials used for fertil-

ization are termed synthetic organics. Examples are urea (a water-soluble nitrogen source made by react-ing ammonia and carbon dioxide under controlled conditions); ureaform (made by reacting urea and formaldehyde); and IBDU (isobutylidene diurea–made by reacting urea and isobutyraldehyde).

NaturalorganicfertilizersThe term “organic” is applied to fertilizers de-

rived solely from the remains or a by-product of a once-living organism. Various wastes and by-prod-ucts of the plant and animal processing industries can be used as fertilizers (Table 1.13). Cottonseed meal, blood meal, bone meal, hoof and horn meal,

Advantages Disadvantages

Slow release1. Fewer applications 1. Unit cost is high2. Low burn potential 2. Availability limited3. Release-rates vary depending 3. Release-rate governed by on fertilizer characteristics factors other than plant need4. Comparatively slow release-rate

Conventional1. Fast-acting 1. Greater burn potential2. Some are acid forming 2. Solidifies in the bag when wet3. Low cost 3. N leaches readily

Manures or sewage sludge1. Low burn potential 1. Salt could be a problem2. Relatively slow release 2. Bulky, difficult to handle3. Contains micronutrients 3. Odor4. Improves soil structure 4. Expensive per pound of actual nutrient 5. Weed seeds a problem 6. Heavy metals may be present in sewage sludge

Comparison of three types of fertilizers Table 1.12


and all manures are examples of organic fertilizers. When packaged as fertilizers these products show the fertilizer grades on the label. Some organic ma-terials, particularly composted manures and sludges, are sold as soil conditioners and do not have a nutri-ent guarantee although small amounts of nutrients are present. Most natural organics are higher in one of the three major nutrients and lower or zero in the other two, although you may find some fortified with nitrogen, phosphorus, or potash for a higher analy-sis. Many are low in all three.

In general, organic fertilizers release nutrients over a fairly long period; the potential drawback is that they may not release enough of their principal nutrient at a time to give the plant what it needs for best growth. Because organic fertilizers depend on soil organisms to break them down to release

nutrients, most of them are effective only when soil is moist and the soil temperature is warm enough for the soil organisms to be active. Microbial activity is also influenced by soil pH and aeration. Microbial activity decreases significantly below a pH of 6.

Cottonseed meal is a by-product of cotton manufacturing. As a fertilizer it is somewhat acid in reaction. Formulas vary slightly but generally con-tain 7% nitrogen, 3% phosphorus, and 2% potash. Nutrients in cottonseed meal are more readily avail-able to plants in warm soils, but there is little danger of burn. Cottonseed meal is frequently used for fer-tilizing acid-loving plants, such as azaleas, camellias, and rhododendrons.

Blood meal is dried, powdered blood collected from cattle slaughterhouses. It is a rich source of nitrogen—so rich, in fact, that it may do harm if

Organic Materials %N %P2O5 %K2O Availability Acidity

Fish scrap 5.0 3.4 0 slowly acid Fish meal 10.0 4.0 0 slowly acid Feather meal 11-15 0 0 slowly Guano, Peru 13.0 8.0 2.0 moderately acid Guano, bat 10.0 4.0 2.0 moderately acid Sewage sludge 2.6-2.0 1.0-2.5 0.0-0.4 slowly acid Dried blood 12.0 1.5 0.8 mod. slow acid Soybean meal 7.0 1.2 1.5 slowly very slightly acid Tankage, animal 6.0 3.0 0.1 slowly acid Tankage, garbage 2.5 1.5 1.5 very slowly alkaline Tobacco stems 1.5 0.5 5.0 slowly alkaline Seaweed 1.0 -- 4.0-10.0 slowly -- Kelp 1 0 2 slowly -- Crab shell fertilizer 2.5 3.0 0.5 slowly -- Bone meal 3.5 22.0 -- slowly alkaline Urea 45.0 -- -- quickly acid Castor pomace 5.0 1.8 1.1 slowly acid Wood ashes -- 2.0 4.0-10.0 quickly very alkaline Cocoa shell meal 2.5 1.0 2.5 slowly neutral Cotton seed meal 6.0 2.5 1.5 slowly acid Ground rock phosphate -- 33.0 -- very slowly alkaline Green sand -- 1.0 6.0 very slowly -- Basic slag -- 8.0 -- quickly alkaline Horn and hoof meal 12.0 2.0 -- -- -- Milorganite 6.0 2.5 -- -- -- Peat and muck 1.5-3.0 0.25-0.5 0.5-1.0 very slowly acid

Table 1.13

Average nutrient content of natural and organic fertilizer materials (percentage on a dry weight basis)

NOTE: Urea and calcium cyanamid are organic compounds; but, since they are synthetic, it is doubtful that most organic gardeners would consider them acceptable. Wood ashes should not be used if pH is 6.3 or above.


used in excess. The gardener must be careful not to exceed the recommended amount suggested on the label. In addition to nitrogen, blood meal supplies certain of the essential trace elements, including iron.

Fish emulsion, a well-rounded fertilizer, is a partially decomposed blend of finely pulverized fish. No matter how little is used, the odor is intense, but it dissipates within a day or two. Fish emulsion is high in nitrogen and is a source of several trace ele-ments. In the late spring, when garden plants have sprouted, an application of fish emulsion followed by a deep watering will boost the plant’s early growth spurt. Contrary to popular belief, too strong a solu-tion of fish emulsion can burn plants, particularly in containers.

Manure is a complete fertilizer, but low in the amounts of nutrients it can supply. Manures vary in nutrient content according to the animal source and what the animal has been eating, but a fertilizer ratio of 1:1:1 is typical. Manures are best used as soil conditioners instead of nutrient suppliers. Com-monly available manures include horse, cow, pig, chicken, and sheep. The actual nutrient content varies widely. The highest concentration of nutri-ents is found when manures are fresh. As manures are aged, leached, or composted, nutrient content is reduced.

Even though fresh manures have the highest amount of nutrients, most gardeners prefer to use composted forms to ensure a lesser amount of salts, thereby reducing the chance of burning plants. Fresh manure should not be used where it will con-tact tender plant roots. Typical rates of manure ap-plications vary from a moderate 70 lb. per 1000 sq ft to as much as one ton per 1,000 sq ft. Uncomposted manure should be applied to vegetable gardens at least 120 days before the crop is planted. However, they may contain numerous species of viable weed seeds.

Sewage sludge is a recycled product of municipal sewage treatment plants. Two forms are commonly available, activated and composted. Activated sludge has higher concentrations of nutrients (about 6-2-0) than composted sludge and is usually sold in a dry, granular form for use as a general purpose, long-last-ing, nonburning fertilizer. Sludge is applied in liquid form to agricultural land. Composted sludge is used primarily as a soil amendment and has a lower nutri-ent content (about 1:2:0).

There is some question about the long-term ef-

fect of using sewage sludge products in the garden, particularly around edible crops. Heavy metals, such as cadmium, are sometimes present in the sludge, and may build up in the soil. Possible negative ef-fects vary, not only with the origin of the sludge, but also with the characteristics of the soil where it is used. Sludge use should be guided by results of chemical analyses of the sludge in question.

Compared to synthetic fertilizer formulations, organic fertilizers contain relatively low concentra-tions of actual nutrients, but they perform other important functions the synthetic formulations do not. Some of these functions are increasing the soil’s organic matter content; improving the soil’s physi-cal structure; and increasing bacterial and fungal activity, particularly the mycorrhiza fungi that alone makes other nutrients more available to plants.

FertilizersCombinedWithPesticidesThe major reason for buying a fertilizer com-

bined with a pesticide is convenience. The prob-lem, however, is that the timing of a fertilizer appli-cation often does not coincide with the appearance of a disease, weed, or insect problem.

Fertilizer-herbicide combinations are available for both preemergence crabgrass control and broad-leaf weed control on lawn areas. The timing of the preemergence herbicide for best weed control may not correspond to the best time to fertilize the lawn. Always follow label recommended rates. Increasing rates to use more fertilizer could cause turf injury from excess herbicide. Compare costs of combina-tion products to those of individual products. De-pending on the formulations, savings may or may not be realized.

FertilizerFormulationsFertilizers come in many shapes and sizes. The

type or form the fertilizer comes in is called the for-mulation. Different formulations are made to facili-tate types of situations in which fertilizer is needed. All formulations must give the amount of nutrients and may tell how quickly a nutrient is available. Some of the formulations available to the hom-eowner include: water soluble powders, slow-release pellets, slow-release collars or spikes, liquids, tablets, and granular solids.

Liquid fertilizers come in a variety of different formulations, including complete formulas and special types that offer just one or two nutrients. All are made to be diluted with water; some are concentrated liquids themselves, others are powder or pellets.


Liquid fertilizers can be applied to the soil to be absorbed by roots or to the leaves, which is called fo-liar feeding. Foliar feeding is used for the following reasons: (1) insufficient fertilizer was applied before planting; (2) a quick growth response is wanted; (3) micronutrients (such as iron and zinc) are locked in the soil; or (4) the soil is too cold for plants to ex-tract or use the fertilizer applied to the soil.

Foliar-applied nutrients are absorbed and used by the plant quite rapidly. Absorption begins within minutes after application and, for most nutrients, is completed within 1 to 2 days. However, plants were not designed to take up nutrients through the leaves and fruit. Only a few nutrients, such as potassium, can easily enter the above-ground parts of the plant. Most are restricted from entering the leaves and fruits by waxes, oils, and hairs.

While this method can give relief from nutrient deficiency symptoms (generally only micronutrients, such as iron and zinc), it is temporary relief at best, only affecting the existing leaves, and only giving good results if applied in the spring. In addition, foliar application does not address the underlying cause of the deficiency, which is generally an im-balance of soil pH or nutrient availability. Once a foliar fertilizer is applied, measures should be under-taken to correct the real cause of the deficiency. If underlying causes are not remedied, then foliar ap-plications will constantly need repeating.

Foliar fertilizer applications are best used as a supplement to a soil-applied fertilizer program. It can be a way to supply micronutrients such as iron, manganese, zinc, and copper, when their uptake from the soil is restricted. While foliar fertilizers will temporarily correct micronutrient deficiencies, a soil test is recommended to determine the underly-ing cause of the deficiency.

The longer the nutrient solution is present in a fine liquid film on the leaf surface, the greater the chance of absorption. Therefore, when applying a foliar fertilizer do so on a cool, cloudy day or in the evening to improve its effectiveness.

PurchasingtheRightFertilizerWhen shopping for fertilizer, evaluate the con-

tents and cost. For quick results, look for fast-release fertilizers that contain the majority of the nitrogen in the nitrate, ammoniacal, and/or urea forms. For long-lasting results with a low potential for leaching, shop for a high percentage of water-insoluble nitro-gen, which will be more expensive than the readily

soluble forms. The best buy for routine lawn and garden maintenance is a combination of fast- and slow-release nitrogen.

Once you’ve decided on nitrogen sources, check for secondary plant nutrients, and buy the fertil-izer which offers these extras if they are necessary. Micronutrients often are used as an incentive to buy one fertilizer instead of another. Fear of micro-nutrient deficiencies and the feeling that somehow micronutrients make the fertilizer better are each motives exploited in fertilizer sales promotions. However, as a rule, lawns and gardens exposed to reasonable recycling of organic matter seldom need the small amounts of micronutrients contained in general fertilizer mixes. If a serious micronutrient deficiency does exist, the small quantities of micro-nutrients applied in popular fertilizer mixes would usually be insufficient to correct the problem. The situation might be compared to eating a vitamin-enriched breakfast cereal to cure a serious vitamin deficiency. There wouldn’t be enough vitamins in the cereal to correct such a deficiency.

If there are two or more fertilizers that fit your needs, but the prices vary, calculate the actual cost per pound of plant nutrients. Price per bag can be deceptive because bag weights differ (a 40 lb. bag contains 20% less material than a 50 lb. bag), and the concentration of nutrients in a bag can also dif-fer. One way of comparison-shopping for fertilizer is to compare the cost per pound of primary nutri-ents in the various products: (1) Add the nitrogen, phosphorus, and potassium percentages; (2) Mul-tiply this total by the net weight of the package to approximate the pounds of minerals contained; and (3) Divide the cost of the package by the pounds of N + P205 + K2O in the package to obtain the per-pound cost.

For example, a 50 lb. bag of 20-3-7 fertilizer contains 15 lbs. of nutrients (that is, 20% N + 3% P2O5+ 7% K2O multiplied by 50 lbs.). If the bag of fertilizer costs $6.00, the average cost of the nutri-ents is $0.40 per lb ($6/15 lbs.). While this method has shortcomings (for example, nutrients have dif-ferent values, nitrogen costs about twice what potas-sium does, and nutrients other than nitrogen, phos-phorus, and potassium aren’t considered), it allows quick comparisons between products. Remember that a higher analysis fertilizer such as a 16-4-8 will probably cost more, but will cover more area than a lower analysis, such as a 7-7-7 product.

When you’ve selected the right fertilizer, pur-


chase the amount you need based on the area to be fertil-ized. Computing any given area is rather straightforward. Examples 1 and 2 illustrate how to determine fertilizer amounts for a lawn and garden. Refer to page 35 to learn how to calcu-late the square footage of vari-ous areas.¶

Example 1.

Determinetheamountofammoniumsulfatea5,000-sq.ft.lawnneedsifthelawnrequiresonelb. of nitrogen per 1,000 sq. ft.

Lawn: 5,000 sq. ft.Fertilizer: Ammonium sulfate (21-0-0)Rate: 1 lb of nitrogen per 1,000 sq. ft.

• Ammonium sulfate is 21% nitrogen.• 21% is the same as 0.21 or 21/100.• This means for every 100 lbs. of fertilizer there are 21 lbs. of nitrogen.• We need 1 lb. of nitrogen for every 1,000 sq. ft. Using proportions we can calculate the amount of ammonium sulfate needed to get 1 lb. of N. “X” represents the unknown amount being calculated.

21 lbs. N = 1 lb. N 100 lbs. 21-0-0 = X lbs. 21-0-0

21 X = 100

X = 100 = 4.8 lbs. of 21-0-0 to deliver 1 lb. of N/1,000 sq. ft. 21• We are fertilizing 5,000 sq. ft. (5 multiplied by 1,000 sq ft). Five multiplied by 4.8 lb of fertilizer = 24 lbs. of fertilizer per 5,000 sq. ft. Total fertilizer needed = N application rate (lbs./1,000 sq. ft.) multiplied by lawn size (sq. ft.)

N content of fertilizer as a decimal 1,000 = 1 x 5,000 0.21 1,000 = 5 0.21 = 24 lbs. of 21-0-0 fertilizerA second approach is to determine the amount of N required for the entire area (5 lbs.), and then use proportions to calculate the amount of fertilizer needed. 5 = X where X = 24 lbs. of 21-0-0 fertilizer. 0.21 100

Example 2.

Determinehowmuch5-10-5needstobeappliedtoget2lb.ofphosphorusper1,000sqftinagarden that measures 20 by 10 ft.

Garden area: 20 multiplied by 10 = 200 sq. ft.

Fertilizer: 5-10-5 = 10% phosphorus

Rate: 2 lb. of phosphorus per 1,000 sq. ft.

Total phosphorus needed:

Step 1. 10 lb. P = 100 lb. of 5-10-5 X = 20 lb of 5-10-5 fertilizer per 1,000 sq. ft 2 lb. P X lb. of 5-10-5

Step 2. 20 lb. = 1,000 sq. ft. X lb. 200 sq. ft. 1,000 (X) = (200)(20) X = 4,000 = 4 lb. of 5-10-5 per 200 sq. ft. of garden area 1,000


Figure 1.9 Example of a typical soil test report from the Agricultural Service Laboratory (page 1 of 3).


Figure 1.9Example of a typical soil test report from the Agricultural Service Laboratory (page 2 of 3).


Figure 1.9Example of a typical soil test report from the Agricultural Service Laboratory (page 3 of 3).


References and Further Reading

TechnicalThe Nature and Properties of Soils. 8th ed. 1974. Ny-

cle C. Brady. Macmillan Publishing Co., NY.

Soil Fertility and Fertilizers. 1993. 5th ed. Samuel L. Tisdale, Werner L. Nelson, James D. Beaton, and John L. Havilin. Macmillan Publishing Co., NY.

Fundamentals of soil science. 1984. Henry D. Foth. John Wiley & Sons, NY.

Mineral Nutrition of Higher Plants. 2nd. ed. 2003. Horst Marschner, Academic Press, San Diego, CA.

Mineral Nutrition of Plants: Principles and Perspectives. 2nd ed. Emanuel Epstein and Arnold J. Bloom. Sinauer Assoc., Inc., Sunderland, MA.

Western Fertilizer Handbook. 9th ed. 2002. Albert E. Ludwick, ed. Interstate Publishers, Inc., Danville, IL.

A book for GardenersSecrets to great soil; A grower’s guide to composting,

mulching, and creating healthy, fertile soil for your garden and lawn. 1998. Elizabeth P. Stell, Storey

Communications, Inc., Pownal, VT.

Clemson Extension publicationNutrient Management for South Carolina; Based on

Soil Test Results, 2001. Agronomic and Horticul-tural Crops, Home Gardens, Turfgrasses, Plant Nutrients, Soil Testing and Analysis, Nutrient Waste Management, and SC Land Resource Infor-mation. Clemson University Coop. Ext. Serv. EC 476.

Southern Lawns; Best Management Practices for the Selection, Establishment, and Maintenance of South-ern Lawngrasses. 2003. L. B. McCarty, ed. Clemson Extension Circ. 707, Clemson University Public Service Publishing, Clemson, SC.

InternetBob Lippert’s Frequently Asked Questions Regard-

ing Soil Testing, Plant Analysis and Fertilizers (http://hubcap.clemson.edu/~blpprt/index.html)

Clemson Extension Home & Garden Information Center (http://hgic.clemson.edu)

Clemson University Fertilizer Inspection Program (http://fscs.clemson.edu/ins.htm)


Soil Composition1. List the four major components of soil and indicate their approximate percentages.

2. Describe why soil texture is important to gardeners.

3. What are the two components that comprise the pore space of soil?

4. What do the percentages of sand and clay influence or determine?

5. Describe the importance of organic matter in soil.

6. What is meant by soil texture?

Soil Chemistry7. What is CEC?8. What is the significance and impact of pH on soils?9. Define cation and anion.10. List 3 common mineral nutrient cations and 3 anions.11. Describe the pH scale, noting which numbers indicate acidic, neutral, and alkaline conditions.12. Describe the effect of pH on nutrient availability.13. Describe methods gardeners can use to raise or lower the pH.

Plant Nutrition14. Name 3 macronutrients and 3 micronutrients.15. Name the nutrients that are the most common ingredients of fertilizer.16. What three mineral nutrients are identified by the three numbers on a fertilizer container.17. Explain these different fertilizer terms: - complete - incomplete - balanced

SoilsandPlantNutritionReview


Squares, rectangles

Area = Length x Width

Length = 95 ft.Width = 50 ft.Area: 95 ft. x 50 ft. = 4,750 sq. ft.

Triangles

Area = 1/2 x Base x Height

Base = 35 ft.Height = 25 ft.Area: 0.5 x 35 ft. x 25 ft. = 437.5 sq. ft.

Circles

Area = π x (r)2 (π = 3.14)

radius (r) = 50 ft.Area: 3.14 x (50 ft. x 50 ft.) = 7,850 sq. ft.

Determiningthesquare footage of some familiar shapes.

Irregular Shapes

Divide the area into smaller sections of familar shapes, such as triangles A and D; rectangles B and C. Then use the following formula to determine the total area:

Area = Area A + Area B + Area C + Area D

A: 0.5 x 30 ft. x 80 ft. = 1,200 sq. ft.B: 45 ft. x 15 ft. = 675 sq. ft.C: 65 ft. x 45 ft. = 2,925 sq. ft.D: 0.5 x 10 ft. x 65 ft. = 325 sq. ft.

Area: 1,200 + 675 + 2,925 + 325 = 5,125 sq.ft.

radius = 50 ft.

Length = 95 ft.

Width = 50 ft.

Base = 35 ft.

Height = 25 ft.

B

CD

A

30 ft.15 ft.

45 ft.

65 ft.

10 ft.


Another approach is to treat the irregular shape like a rectangle. First, establish a line down the middle of the length of the property. Then, walk down this middle line and take measurements from both sides of the line. Areas with very irregular borders will require more side-to-side measurements to achieve a fairly accurate approximation of the width. The average of these side-to-side measurements can be used to determine the width of the shape. Finally, calculate the area as a rectangle.

+

A

C

E

D

FGH

I

B

A

J

A

C DE

FG

B

A CD

Irregular shapes

The area of this irregular shape can be divided into three common shapes: triangles (A and B), rectangle (C), and a circle (D). You can deter-mine the total area by adding up the areas of each shape:

B

HI

JK

Some irregular shapes can be converted into a circle. From the center measure the distance to the edge of the shape in 10 to 20 increments. Average these measurements to find the average radius for this circular shape. Then calculate the area using the formula for a circle:

Area = π x r2 (π = 3.14)

So, Area = 3.14 x Line A + B + C + D + E + F +G + H + I + J 10

Total Area = Area A + Area B + Area C + Area D

Area = Length x Width

Length = Line AB

Width = Line C + D + E + F + G + H + I + J + K 9


Howtomeasuretheareaofyourlandscape.

To apply the correct amount of fertilizer on your lawn, you need to know the total amount of lawn area that will be fertilized. Take a look at the following example.

If your lot is 260 feet deep and 160 feet wide*, multiply 260 x 160 to get a total area of 41,600 square feet. Subtract from this total area the square footage of the house, driveway, deck, patio, and any other areas that will not be fertilized. The remainder will be the area that can be fertilized.

Total Lot: Lot, 260 ft. x 160 ft. = 41,600 sq. ft.

Subtract: House, 100 ft. x 75 ft. = 7,500 sq. ft. Deck, 20 ft. x 20 ft. = 400 sq. ft. Patio, 30 ft. x 30 ft. = 900 sq. ft. Drive, 75 ft. x 10 ft. = 750 sq. ft. Garden, 40 ft. x 25 ft. = 1,000 sq. ft. Walkway area, 45 ft. x 25 ft. = 1,125 sq. ft.

Area to be subtracted: = 11,675 sq. ft.

Remainder(lawnareatobefertilized): =29,925sq.ft.

Six bags of 5,000 sq. ft. material will fertilize this lawn.

*An easy way to measure long distances is with your garden hose, as long as you know its length. For instance, assume your garden hose is 50 ft. long. You measure the area and find that it is 21/2 hose lengths long ((50’ ft. + 50 ft. + 25 ft.) by 2 hose lengths (50 ft. + 50 ft.) wide. This means that the area is 125 ft. x 100 ft. or 12,500 square feet.

G

P

D

H

Drive75 ft.

75 ft.

100 ft.

10 ft.

30 ft.

30 ft.

20 ft.

20 ft.

25 ft.

40 ft.

45 ft.

260 ft.

25 ft.

W160 ft.

chapter 1 soils and plant nutrition - clemson...

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