basic soil and plant fertility

8/18/2019 Basic Soil and Plant Fertility

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330 MONTEVUE LANE • FREDERICK, MD 21702-8200 • (301) 694-1594 • FAX: (301) 694-1588

OPERATING A PROFITABLESMALL FARM

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Local Governments • U.S. Department of Agriculture Cooperating

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Agriculture and Natural Resources, Symons Hall, College Park, MD 20742.

FACT SH EET 4Basics of Soi l a n d Plan t Fer t i l i t y

Terry E. PooleExtension agent, Agricultural Science

Frederick County, MD

Farm operators need to develop an under-standing of the basics of soil physiology andthe factors that affect plant fertility. This isessential if they are to have successful cropproduction enterprises. This basic knowledge

is also important to livestock producers depen-dent on pastures as a primary part of their live-stock feeding program. Pasture grass andlegume species respond to the same basic soiland plant fertility principles as other farmcrops. Attention to the basics are necessary forproductive pastures and crops.

I. Wh at is soil?

Soil can be defined as a natural body, synthe-sized from a variable mixture of broken andweathered minerals, decaying organic matter,water, and air. In addition to air, water, andnutrients, soil also provides mechanical sup-port to growing plants.

Mineral soils consist of four major components;minerals, organic matter, water, and air. Theapproximate volume composition of a typicalsilt loam soil in optimum condition for plantgrowth would have the solid space made up of 45 percent mineral and 5 percent organic mat-ter, and at optimum moisture for plant growth,would have roughly 25 percent water and 25percent air. The water and air would be con-tained within the pore spaces of the soil. It is

important to note that, in this typical silt loamsoil, 50 percent of the volume is pore spaces.

II. Soil Texture

A very important physical property of soil issoil texture. This concerns the size of mineral

particles, specifically the relative proportion ofvarious size groups in a given soil. This proper-ty helps to determine the nutrient supplyingability of soil solids and the supply of waterand air that support plant life.

Soil texture is separated into three soil sepa-rates based on particle size. These are sand , silt ,and clay .

Silt and clay soils impart a fine texture andslow water and air movement. They also havehigh water holding capacity due to the higherpercentage of pore spaces. These are referred toas heavy soils, with clay being the heavier ofthe two.

Clay separates are also the primary availableplant nutrient holding mechanisms in the soil.

Shaped like a plate, clay colloids or platelets actlike micro-magnets capturing and holding plantnutrients, which remain until “kicked off” by anoverload of another element, absorbed by aplant root, eaten by a soil macro- or micro-organism, or adsorbed into the soil chemically.

Sandy to gravelly soils are referred to as lightersoils. Water moves through these soils morerapidly than the heavier soils and they havelower water holding capacities.

Soil textural names are how we refer to and iden-

tify our soils. Sands are soils where sand separatesmake up 70 percent or more by weight. Claysoils have at least 40 percent clay and may havenames like sandy clay, or silty clay.

Loamy soils possess the desirable qualities ofsand and clay without exhibiting the undesir-


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able characteristics of extreme looseness, lowwater holding capacity, and slow water and airmovement. Most soils of agricultural impor-tance are loams. Some are called loams, how-ever, in most cases, the quantity of sand, silt,or clay present modifies the name. Someexamples would be clay loam, sandy loam, siltloam, and silty clay loam.

III. Soil Com paction

Fine textured soils are more easily compactedthan lighter soils, especially when they arewet. Compaction reduces pore spaces thathold air and water. Plant growth in compactedsoils will be significantly reduced. Operatingequipment on wet soils can create problems ina field for an entire season or longer.

IV. Soil Dep th

Sometimes a soil is referred to as being deep,or shallow. Soil depth can be defined as thatdepth of soil material favorable for plant rootpenetration. Deep, well-drained soils of desir-able texture and structure are favorable forplant growth. Shallow, poorly drained soils arevery restrictive to plant growth.

V. Field Slope

Land topography largely determines theamount of drainage, runoff, and erosion. Thesteeper the land, the more management isrequired. The ease with which surface soilserode, along with the percent slope, are deter-

mining factors in a soil’s potential productivi-ty. Most of the soils in this region are highlyerodible.

VI. Soil Organic Fraction

A good, loamy soil contains about one-half pore space (air and water) and one-half solidmaterials. Of this one-half solid material, 90percent is composed of minerals (bits of rock). The remaining 10 percent is the organic frac-tion. The influence of this small part of thesoil on the soil’s ability to support plantgrowth is significant.

The soil’s organic fraction is dynamic and isalways undergoing a process of change. Theorganic fraction consists of living organisms,plant and animal residues, and plant roots. The total weight of living organisms in thetop 6 inches of an acre of soil can range from5,000 to 20,000 pounds.

The soil organic matter is a part of the soilorganic fraction. It consists of plant and ani-mal residues in various stages of decay.

Adequate levels benefit soil in many ways:

1- improve physical condition2- increase water infiltration3- improve soil tilth4- decrease erosion losses5- supply plant nutrients6- retain available plant nutrients.

VII. Essential Plan t Nutrients There are sixteen identified elements that areessential to plant growth. Three of these ele-ments are obtained mostly from air and water;they are carbon (C), hydrogen (H), and oxygen(O). The other thirteen essential elements comefrom the soil solids and are the elements we tendto focus more on in plant fertility management.

The thirteen essential plant nutrients havebeen divided into three categories based on theamount of element required for plant growth.

1- Primary Nutrientsa) Nitrogen (N)b) Phosphorus (P)c) Potassium (K)

2- Secondary Nutrientsa) Calcium (Ca)b) Magnesium (Mg)c) Sulphur (S)

3- Micro-nutrientsa) Iron (Fe)b) Magnesium (Mg)c) Boron (B)d) Molybdenum (Mo)e) Copper (Cu)f) Zinc (Zn)g) Chlorine (Cl)

Primary nutrients are required by plants inhigh amounts, while secondary nutrients arerequired in lesser amounts. M icro-nutrients areneeded only in small amounts. Whether anutrient is primary, secondary, or a micro-nutrient it is essential to plant growth. A defi-ciency in any one of the essential nutrientswill restrict plant growth.

A typical expression of the primary nutrientscan be found when purchasing a commercialfertilizer. A fertilizer product will have threenumbers to identify the type, or percentage offertilizer contained in the bag. Some exampleswould be 10-6-4, 5-10-5, and 20-20-20. Thesenumbers refer to the percentage of N, P, and Kin the fertilizer.

Fertilizer numbers also reflect the ratio of theseelements to each other. For example, 10-6-4 is


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a 2:1:1 ratio, 5-10-5 is a 1:2:1 ratio, and 20-20-20 is a 1:1:1 ratio fertilizer. The ratio of ele-mental nutrients in fertilizer is important formaintaining or correcting the balance of P andK in the soil. A fertilizer with a higher ratio of P or K can be applied to satisfy that need with-out adding to the other element if it was notneeded.

Fertilizers of comparable ratio can be substitut-ed for each other by compensating for the dif-ference in material concentration. Differencesin price or availability of a recommended fer-tilizer may require the use of an alternative fer-tilizer product. Using a substitute fertilizerwith a comparable ratio will be consistentwith the soil test recommendation.

Example : Soil test recommends 5-10-5 at a rate of1,000 lbs./A, however, 12-24-12 is availableat a better price. What is the difference in theapplication rate using 12-24-12?

1,000 lbs x .05 =50 lbs (recommendation based on lbs/A of nitrogen)

50 lbs/.12 =416.67 lbs/A (lbs. of 12-24-12/A that substitute for 5-10-5)

Most farm fertilizer dealers have the capability

to custom mix the fertilizer ratio recommend-ed in a soil test. However there are severalcommonly available commercial fertilizersthat can be used individually, or in combina-tion to meet the soil test recommendation.

Com m on Fertilizers

Urea 46-0-0

Ammonium nitrate 34-0-0

UAN (urea ammonium nitrate) 30-0-0

Ammonium sulfate 21-0-0

DAP (diammonium phosphate) 18-46-0

MAP (monoammonium phosphate) 11-52-0

Triple superphosphate 0-46-0

Muriate of potash 0-0-60

One way of comparing commercial fertilizersis to calculate the amount of actual nutrientsthey contain. This can be done by using thefollowing formula:

% Nutrients x 2,000 lbs. = Pounds Actual Nutrients/ton

The cost per pound of actual nutrients can be

used as a method of comparing the real cost ofcommercial fertilizers. The cost per pound ofnutrients can be determined by first calculat-ing the amount of actual nutrients per ton offertilizer (formula above), then dividing thefertilizer cost per ton by the pounds of actualnutrients.

Commercial fertilizers, animal manures, andcomposts are good sources of most of the essen-tial nutrients, however, some of the micro-nutri-ents may need to be applied as a supplementCalcium and magnesium can be obtained

through the application of a liming agent.Avoid over-applying any plant nutri ent.Excessive levels of any one of the essentialnutrients can throw the soil fertility out of bal-ance and can result in a reduction in plantgrowth. Excessive levels of some elements caneven be toxic to plants. This is especially thecase with many of the micro-nutrients.Excessive levels of N and P in the soil havebeen determined to be detrimental to waterquality.

Plant nutrients should always be applied based

on a reliable soil test and with a calibrated fer-tilizer spreader. Nutrient management planscan be developed through the CooperativeExtension Service that can balance the applica-tion of animal manures with crop productiongoals, so that the excessive application of Nand P can be avoided.

VIII. Soil pH

Soil pH is used as a measure of its relative alka-linity or acidity. Soil test results for pH are

Example :

Ammonium sulfate 21-0-0-24(S)

.21 x 2,000 lbs =420 lbs Nitrogen/ton

.24 x 2,000 lbs =480 lbs Sulphur/ton

Total Nutrients =900 lbs/ton


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based on a pH scale where 7.0 is pH neutral,above 7.0 is alkaline, and below 7.0 is acidic.Commercially available soil tests provide a pHmeasurement.

Soil pH is critical to healthy plant growth. SoilpH directly affects the availability of the essen-tial nutrients to plants. It is important to knowthe optimum pH for the plants to be grown.Soil pH also affects the adaptability of plantsin a given soil. For example, azaleas and blue-berries prefer an acidic soil and suffer whenthe pH nears neutral. Most agricultural plantsprefer a slightly acidic pH of 6.4, which is ade-quate for healthy plant growth. However,there are exceptions, so be familiar with thepH and nutritional needs of all the crops to begrown.

There are commercially available liming mate-rials to raise soil pH; these include limestoneand industrial byproducts. Any of these mate-

rials will effectively do the job. Be sure to note

the percent oxides in these materials; they willvary between 40 to 50 percent. Soil test recom-mendations for raising the soil pH are basedon pounds of oxides per acre.

Some recommendations will directly prescribethe required pounds of oxides per acre, whilesome other recommendations will prescribetons of limestone per acre. If the recommenda-tion is in tons of limestone per acre, theassumption is that the liming material is 50percent oxides. The amount of materialapplied per acre will have to be adjusted if it isnot 50 percent oxides.

For example, a soil test indicates a soil pH of6.2 and recommends that 1,500 pounds ofoxides be applied. After shopping around, thetwo best choices are aglime (50 percent oxides)and industrial stackdust (41 percent oxides) Two questions arise: (1) What are the applica-tion rates for each material?, and (2) What is

the cost per acre for each material?

Example : Soil test indicates 6.2 pHRecommendation: 1,500 lbs/A oxidesChoices: Aglime 50% oxides

Stackdust 41% oxides

Aglime1 lb/.50 =2 lbs (material needed for 1 lb. oxides)2 x 1,500 =3,000 lbs (material needed for acre recommendation)3,000 x (cost/lb) =cost/acre of aglime

Stackdust1 lb/.41 =2.44 lbs (material needed for 1 lb. oxides)2.44 x 1,500 =3,660 lbs (material needed for acre recommendation)3,660 x (cost/lb) =cost/acre of stackdust

Industrial byproduct liming materials are typi-cally in the oxide form (CaO). The form isimmediately available to react in the soil.Aglime materials (CaO3) will vary in mesh size(fineness). A typical, good aglime will be 80percent material that is 90-100 mesh.Limestone of this fineness is soluble and is

available to react in the soil. The other 20 per-cent of material will take 6 months or more toreact; this provides some slow release benefits.

It is important to remember, when applyingliming materials, that recommendations arebased on applying enough oxides to raise theplow layer (top 9 inches of the soil) to thedesired pH level. Liming applications that arenot incorporated in the soil (surface applied, ex.pasture) are limited to 1,500 lbs/acre per year.Applications above this can lead to crop injury.

Soils with a pH above 7.0 can restrict plantgrowth. There are some commercially avail-able sulphur-based materials that will effec-tively lower the soil pH, however, most soils inthis region are naturally below pH 7.0.Ammonium sulfate is a commercial fertilizer

that contains 25 percent sulfur. This product iscommonly used as a soil sulphur source thatwill slightly lower soil pH, however, severe pHreductions will require another choice. Checkwith the Extension Service.

The addition of any liming or acidifying mate-rials should always be based on the results ofa reliable soil test. Soil tests can be obtainedthrough the Extension Service and most farmfertil izer dealers.

basic soil and plant fertility

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