The Efficient Fertilizer The Efficient Fertilizers

GuideSoil Defined

FeThe Soil ProfileThe soil profile comprises two or more soil layers called horizons, one below the other, each parallel to the surface of the land. Important characteristics that differentiate the various horizons are:

Color, texture, structure, consistency, porosity and soil reaction Thicknesses ranging from several feet thick to as thin as a fraction of an inch Generally, the horizons merging with one another and which may or may not be showing sharp boundaries

A HorizonThe uppermost layer in the soil profile or surface soil. It includes the mulch layer and plow layer. Living organisms are most abundant in this horizon, consisting of plant roots, bacteria, fungi and small animals. Organic matter is most plentiful, particularly in the mulch layer. When a soil is tilled improperly, the A Horizon may be eroded away.

B HorizonLies immediately beneath the A Horizon and above the C Horizon. It is called the subsoil. The B Horizon has properties of both A and C. Living organisms are fewer in number than in the A Horizon, but more abundant than in the C Horizon. Color is transitional between A and C as well. It is frequently higher in clay than either of the other horizons.

C HorizonThe deepest of the three. This is the material from which the mineral part of the soil forms. It is the parent material of soils. It may have accumulated in place by the breakdown of hard rock, or it may have been placed there by the action of water, wind or ice.

Fertile SoilA fertile soil contains an adequate supply of all the nutrients required for plant growth. The full potential of crops is not realized if a shortage of nutrients occurs at any time during the growth cycle. This is true even though plants are capable of remarkable recovery from short periods of starvation.

A fertile soil is not necessarily a productive one. The second major requirement is that the soil must be adequate for plant growth. This soil is based on environmental factors including texture, structure, soil water supply, pH, temperature and aeration.

Soil Texture, Structure and ColorSoil Classes and TexturesAn important factor in soil productivity is texture, defined as the relative percentage of sand, silt and clay. Soils are classified on the basis of texture of each of the horizons. The relative proportions of clay, silt and sand determine the soil textural class.

Clays are the smallest particles in soil; silts are somewhat larger in size, followed by sands that are coarse enough that the individual particles are visible to the naked eye. The following table shows the proportion of sand, silt and clay normally found in the various textural classes of soils.

SOIL CLASSES % SAND % SILT % CLAY

Sands 85+ 0-10

Loamy Sands 70-90 0-15

SOIL CLASSES % SAND % SILT % CLAY

Sandy Loams 43-85 0-20

Silt 80+ 0-12

Silt Loams 50-88 0-27

Loams 0-52 0-50 7-27

Sandy Clay Loams 45+ 0-28 20-35

Clay Loams 0-45 0-53 27-40

Silty Clay Loams 0-20 40+ 27-40

Sandy Clays 45+ 0-20 35-55

Silty Clays 40+ 40+

Clays 0.45 0-40 40+

Soil StructureThe arrangement of soil particles into groups or aggregates determines the "structure." A single mass or cluster of soil particles held together in a particular way imparts physical characteristics to the soil, such as a clod, prism, crumb or granule. Soil structure is often more important than the texture to the farmer. Soil structure can be changed to produce improved soil conditions for maximum yield and profits. Structure is especially important to water movement and in preventing root growth restrictions, both of which affect nutrient availability to the crop. Examples of various types of soil structure are shown at left.

Soil ColorColor in various types of soils is due primarily to the amount of organic matter and the chemical state of the iron and other compounds in the mineral fraction of the soil. Other minerals such as quartz, granite and heavy black minerals may also influence soil color. Unweathered parent materials tend to be gray in color, or else will have the color of the natural minerals from which they are derived.

The color of subsoils can reveal a great deal about the age and drainage conditions in the soil. Iron compounds can exist as oxidized forms (red), hydrated oxides (yellow), and as reduced forms (gray).

The Relationship Between Subsoil Color and Drainage

SUBSOIL COLOR DRAINAGE CONDITION

Red Excellent

Reddish Brown or Brown Good

Bright Yellow Moderately Good

Pale Yellow Imperfect to Fair

Gray Poor

Dark (Black) Variable

Soil Organisms

The mineral soil harbors a varied population of living organisms that play an important role in the dynamic changes occurring within the soil. Many groups of organisms live in the soil, and range from microscopic to those visible to the naked eye.

Some of the microscopic-sized organisms are the bacteria, fungi, actinomycetes, algae and protozoa. Most soil organisms depend upon organic matter for food and energy. Consequently, they are generally found in the top 12 inches of soil. One of the most important functions of soil microorganisms is the decomposition of crop residue. Some of it is converted into more stable organic compounds that are stable in the soil over long periods of time. But a large percentage of the organic material is released to the atmosphere as carbon dioxide. Also, nitrogen and other essential plant nutrients are released and made available to growing crops.

Rhizobium bacteria form a symbiotic relationship that results in nitrogen fixation in legume plants. These organisms penetrate plant roots, causing the formation of small nodules on the roots. They then live in symbiotic relation with the host plant. The beneficial effect of this process is realized when cultivated legumes, such as alfalfa, clovers, soybeans, etc., are inoculated at seeding with the proper strain of the rhizobium bacteria.

The millions of microorganisms in the soil play critical roles in plant nutrition, although many are unidentified. The improved understanding of the microbiology of plant nutrition is one of the important unmet challenges of crop nutrient management.

Harmful MicroorganismsSome soil microorganisms are harmful to soils and growing plants, in the form of diseases, toxins produced and denitrification. When the supply of air in a soil is limited, certain aerobic soil organisms can get their supply of oxygen by reducing highly oxidized compounds, such as nitrates. Further reducing action may result in free nitrogen (N2) being produced and lost to the atmosphere. This is not an environmental problem, because 78 percent of the atmosphere is N2 gas, but the result is a net loss of N available for the crop. Other microorganisms contribute to loss of N as NOx gases, potent greenhouse gases that can be an environmental problem.

Nitrogen Fixation by Crop

CROP LB/ACRE N FIXED

Alfalfa 196

Ladino Clover 178

Sweet Clover 116

Red Clover 112

White Clover 103

Soybeans 98

Cowpeas 89

Lespedeza 85

Nitrogen Fixation by Crop

CROP LB/ACRE N FIXED

Vetch 80

Garden Peas 71

Winter Peas 54

Peanuts 42

Adapted from "Fertilizers and Soil Amendments" by Follett, Murphy and Donahue.

Cation Exchange CapacityEach soil colloid contains a net negative electrical charge due to its structural and chemical makeup. Soil colloids have the ability to attract and hold positively charged elements by electrical attraction. Most chemical compounds when in solution dissolve into electrically charged particles called ions. Ions with positive charges are called cations and ions containing negative charges are referred to as anions. Consequently, positively charged cations such as potassium (K+), calcium (Ca++), magnesium (Mg++) and ammonium nitrogen (NH4+) are attracted and held to the surface of soil colloids much like a magnet attracts and holds iron filings.

Montmorillonite clay and organic colloids have more surface area exposed than kaolinite-type colloids and, therefore, have a higher net negative electrical charge. Thus, montmorillonitic soils have more capacity to hold positively charged nutrient ions, or cations. This characteristic is called cation exchange capacity (CEC). Knowledge of a soil’s CEC is basic to understanding how to manage lime and fertilizer additions. Since kaolinite clays have less surface area exposed, they have lower (CEC) values, meaning less capacity to hold nutrients.

CEC helps to explain why certain fertilizer elements such as positively charged potassium, calcium and magnesium, as well as ammonium nitrogen are not as easily leached from the soil as the negatively charged ions, or anions, of nitrate nitrogen, sulfates or chlorides.

Cations adsorbed on the surface of soil colloids, and those contained in the soil solution, are available for plant use. Adsorbed cations, however, can be replaced by other cations present in the soil solution through the process of cation exchange. These replaced cations may then combine with an anion and be leached from the soil.

For example, when large amounts of a fertilizer material such as muriate of potash (KCI) are applied to the soil, the KCI dissolves in soil moisture and disassociates into K+ and Cl- ions. The K+ in solution tends to exchange with Mg++ adsorbed on the clay and organic matter. The K+ is held on the soil particles, and the Mg++ combines with Cl- to form MgCl, a soluble compound that is then leached from the soil with rainfall. As plants remove nutrients from the soil solution throughout the growing season, the concentrations change, and this dynamic exchange of nutrients continues.

The force by which cations are held by soil colloids will depend upon several factors. The smaller the cation and the less water it has adsorbed, generally the tighter the cation is held on the soil particles. Hydrogen ions, therefore, are more tightly held and more difficult to replace than larger and more hydrated cations such as ammonium, calcium, magnesium and potassium. Divalent cations (two charges) are generally held tighter by soil colloids than monovalent cations (one charge). Therefore, calcium and magnesium, divalent cations, are more difficult to replace than the monovalent cations such as potassium and ammonium. Soils with high sand and silt content have a lower percentage of clay and organic matter, and thus have lower CEC. This explains why coarse-textured soils require more frequent applications of lime and fertilizer.

Determining Soil Cation Exchange CapacityThe cation exchange capacity (CEC) of a soil is typically expressed in terms of milliequivalents. A milliequivalent is defined as "one milligram of hydrogen or the amount of any other element that will displace it." When applied to soils, milliequivalents are generally expressed on the basis of 100 grams of oven-dried soil. One milligram of hydrogen per 100 grams of soil equates to 10 parts of hydrogen per one million parts of soil. An acre (top 6 2/3 inches) of soil weighs about 2,000,000 pounds. Therefore, 10 parts per million of hydrogen (whose atomic number is one) equals about 20 lb/acre of hydrogen.

This calculation provides a standard of measurement for converting the milliequivalent of other elements to pounds per acre. The standard is one milliequivalent of hydrogen equals 20 lb/acre of hydrogen. Since the atomic weight of hydrogen is 1, to convert a milliequivalent of other elements to pounds per acre, multiply its atomic weight by 20. Remember, divalent elements have two positive electrical charges and replace two hydrogen ions; therefore, to arrive at the equivalent atomic weight of divalent cations, divide its atomic weight by 2.

Example:Atomic weight of calcium = 40Valence = 2Equivalent weight = 40/2 = 20

Therefore, one milliequivalent of calcium is equal to the equivalent weight of calcium multiplied by 20 lb/acre of hydrogen. Calcium equivalent weight of 20 x 20 lb/acre hydrogen = 400 lb/acre.

CEC by a LabOne laboratory method of determining the exchange capacity of a soil is to remove all of the adsorbed cations by leaching a weighed portion of soil with a salt solution such as one normal ammonium acetate. All of the adsorbed cations are replaced by the ammonium ions. All excess ammonium ions are then removed by leaching with alcohol. The adsorbed ammonium ions are then removed from the soil by extracting with a different salt, such as one normal potassium chloride. The potassium ions replace the adsorbed ammonium ions. The quantity of ammonium ions in the leachate can then be

measured, and is then expressed as milliequivalents per 100 grams of soil — the CEC value. This laboratory procedure is laborious and time consuming. Generally, an estimate of the soil's CEC value is sufficient.

Estimate of CECAn estimate of the cation exchange capacity of a soil can be made from soil test results. This can be accomplished by dividing the pounds per acre of the element as determined by the soil test by the milliequivalent weights of the cations. First, the equivalent weights of cations must be converted into pounds per acre. The cations used in the calculation of CEC are hydrogen, potassium, magnesium and calcium.

Conversion Table from Cation Weights to Pounds Per Acre

CATION ELECTRICAL CHARGE ATOMIC WEIGHT EQUIVALENT

WEIGHT

LB/ACRE OF ONE MILLIEQUIVALENT

(M.E.)

Hydrogen 1 1 1 20

Calcium 2 40 20 400

Magnesium 2 24 12 240

Potassium 1 39 39 780

Converting Soil Test to Milliequivalents for Cations

Conversion Table from Cation Weights to Pounds Per Acre

CATION ELECTRICAL CHARGE ATOMIC WEIGHT EQUIVALENT

WEIGHT

LB/ACRE OF ONE MILLIEQUIVALENT

(M.E.)

CATION SOIL TEST LB/ACRE 1 M.E. LB/ACRE M.E./100 GRAMS

Hydrogen 50 20 2.50

Calcium 800 400 2.00

Magnesium 120 240 0.50

Potassium 250 780 0.32

To arrive at the estimated cation exchange capacity of this soil, divide the lb/acre of each element as determined by soil test by one milliequivalent (m.e.) in lb/acre of each element. As shown in the table at left, for calcium, divide the 800 lb/acre soil test value by the 400 m.e. value, which yields a value of 2.0 m.e. per 100 grams of calcium. The sum of the m.e. per 100 grams for each of the four nutrients is the calculated CEC for that soil.

The proportion of adsorbed base cations (calcium, magnesium and potassium) relative to hydrogen is expressed in terms of percent base saturation. Generally, the higher the percent base saturation of a soil, the higher the soil pH and fertility level. In the above example, the percent base saturation would be:

((Ca 2.0 + Mg 0.5 + K 0.32) /5.32) X 100 = 53% Base Saturation

This base saturation number is then used with the appropriate calibration database for the area to guide fertilizer recommendations.

The table at right shows the CEC values for representative soils across the United States and illustrates the wide range of values that can occur.

CEC Values for Representative Soils

SOIL OR SOIL COMPONENT LOCATION CEC M.E./100 GRAMS

Grundy Silt Loam Illinois 23.6

Clarion Loam Iowa 19.1

Sac Silty Clay Loam Iowa 35.1

Delta Light Silt Loam Massachusetts 9.4

Cecil Sandy Loam South Carolina 5.5

Norfolk Sandy Loam South Carolina 3.0

Lakeland Sand Florida 1.5

Kaolinite Clay – 5-15

lllite Clay – 10-45

Montmorillonite Clay – 60-150

Humus – 140

Anion AdsorptionAnions are the opposite of cations, in that they contain a net negative charge. The most common anions in soils are chloride, sulfate, phosphate and nitrate.

In addition to cation-adsorbing capacity, soils also have the ability to adsorb anions, but to a lesser extent than cations. Anion adsorption is pH dependent and increases with a decrease in soil pH. Phosphates and sulfates are adsorbed more strongly than nitrates

and chlorides. Anion adsorption is not as important agriculturally as cation adsorption. Most agricultural soils have a pH higher than that at which anion adsorption is at its maximum strength; and with the exception of phosphate, and to a lesser degree sulfate, anions are largely lost from the soil by leaching.

Soil pHSoil pH is a measure of the acidity and alkalinity in soils. pH levels range from 0 to 14, with 7 being neutral, below 7 acidic and above 7 alkaline. The optimal pH range for most plants is between 5.5 and 7.0; however, many plants have adapted to thrive at pH values outside this range. Because pH levels control many chemical processes that take place in the soil – specifically, plant nutrient availability – it is vital to maintain proper levels for your plants to reach their full yield potential.

An acid is defined as a substance that tends to release hydrogen ions (H+). Conversely, a base is defined as a substance that releases hydroxyl ions (OH-). All acids contain hydrogen ions, and the strength of the acid depends upon the degrees of ionization (release of hydrogen ions) of the acid. The more hydrogen ions held by the exchange complex of a soil in relation to the basic ions (Ca, Mg, K) held, the greater the acidity of the soil.

NOTE: Aluminum (Al) also contributes to soil acidity, but for simplicity, further discussion of soil acidity will be limited to H as the cause of soil acidity.

pH Range

5.0 – 5.5 5.5 – 6.5 6.5 – 7.0

Blueberries Barley Alfalfa

Irish Potatoes Bluegrass Some Clovers

Sweet Potatoes Corn Sugar Beets

Cotton

Fescue

Grain Sorghum

pH Range

5.0 – 5.5 5.5 – 6.5 6.5 – 7.0

Peanuts

Rice

Soybeans

Watermelon

Wheat

Desirable Soil pH for Optimum Crop Production pH RangeThe desirable pH range for optimum plant growth varies among crops. While some crops grow best in the 6.0 to 7.0 range, others grow well under slightly acidic conditions. Soil properties that influence the need for and response to lime vary by region. A knowledge of the soil and the crop is important in managing soil pH for the best crop performance.

Soils become acidic when basic elements such as calcium, magnesium, sodium and potassium held by soil colloids are replaced by hydrogen ions. Soils formed under conditions of high annual rainfall are more acidic than are soils formed under more arid conditions. Thus, most southeastern soils are inherently more acidic than soils of the Midwest and far West.

Soils formed under low rainfall conditions tend to be basic with soil pH readings around 7.0. Intensive farming over a number of years with nitrogen fertilizers or manures can result in soil acidification. In the wheat-growing regions of Kansas and Oklahoma, for example, which have

soil pH of 5.0 and below, aluminum toxicity in wheat and good response to liming have been documented in recent years.

Factors Affecting Soil Acidity Rainfall Nitrogen Fertilizers Plants

RainfallRainfall contributes to a soil’s acidity. Water (H2O) combines with carbon dioxide (CO2) to form a weak acid — carbonic acid (H2CO3). The weak acid ionizes, releasing hydrogen (H+) and bicarbonate (HCO3). The released hydrogen ions replace the calcium ions held by soil colloids, causing the soil to become acidic. The displaced calcium (Ca++) ions combine with the bicarbonate ions to form calcium bicarbonate, which, be ing soluble, is leached from the soil. The net effect is increased soil acidity.

Subsoil AcidityEven if the top 6 inches of soil show a pH above 6.0, the subsoil may be extremely acidic. When subsoil pH's drop below 5.0, aluminum and manganese in the soil become much more soluble, and in some soils may be toxic to plant growth. Cotton and, to some extent, soybeans are examples of crops that are sensitive to highly soluble aluminum levels in the subsoil, and crop yields may be reduced under conditions of low subsoil pH. If you’ve observed areas of stunted plants in your field, take a subsoil sample in these areas. If the soil pH is extremely acidic (below 5.2), lime should be applied early in the fall and turned as deeply as possible.

Liming Soil PaysCorrecting soil acidity by the use of lime is the foundation of a good soil fertility program. Lime does more than just correct soil acidity. It also:

Supplies essential plant nutrients, Ca and Mg, if dolomitic lime is used Makes other essential nutrients more available Prevents elements such as Mn and Al from being toxic to plant growth.

Limestone Increases Fertilizer Efficiency and Decreases Soil Acids

SOIL ACIDITY NITROGEN PHOSPHATE POTASH FERTILIZER WASTED

Extremely Acid — 4.5 pH 30% 23% 33% 71.34%

Very Strong Acid — 5.0 pH 53% 34% 52% 53.67%

Strongly Acid — 5.5 pH 77% 48% 77% 32.69%

Medium Acid — 6.0 pH 89% 52% 100% 19.67%

Neutral — 7.0 pH 100% 100% 100% 00.0%

LIMING MATERIALCOMPOSITIONCALCIUM CARBONATE EQUIVALENT (CCE)Calcitic LimestoneCaCO385-100Dolomitic LimestoneCaCO3; Mg CO395-108Oyster ShellsCaCO390-110MarlsCaCO350-90Hydrated LimeCa(OH)2120-135Basic SlagCaSiO350-70GypsumCaSO4None

Liming MaterialsLiming materials contain calcium and/or magnesium in forms, which when dissolved, will neutralize soil acidity. Not all materials containing calcium and magnesium are capable of reducing soil acidity. For instance, gypsum (CaSO4) contains Ca in appreciable amounts, but does not reduce soil acidity. Because it hydrolyzes in the soil, gypsum converts to a strong base and a strong acid as shown in the following equation:

CaSO4 + 2H2O = Ca (OH)2 + H2SO4

The formed Ca (OH2) and H2SO4 neutralize each other, resulting in a neutral soil effect. On the other hand, when calcitic (CaCO3) or dolomitic lime (Ca Mg (CO3)2) is added to the soil, it hydrolyzes (dissolves in water) to a strong base and a weak acid.

CaCO3 + 2H2O = Ca (OH)2 + H2CO3

Calcium hydroxide is a strong base and rapidly ionizes to Ca++ and OH- ions. The calcium ions replace absorbed H ions on the soil colloid and thereby neutralize soil acidity. The carbonic acid formed (H2CO3) is a weak acid and partially ionizes to H+ and CO2

-2 ions. Therefore, the net effect is that more ca than H ions are released in the soil, and consequently, soil acidity is neutralized.

IMING MATERIAL COMPOSITIONCALCIUM

CARBONATE EQUIVALENT (CCE)

Calcitic Limestone CaCO3 85-100

Dolomitic Limestone CaCO3; Mg CO3 95-108

Oyster Shells CaCO3 90-110

Marls CaCO3 50-90

Hydrated Lime Ca(OH)2 120-135

Basic Slag CaSiO3 50-70

Gypsum CaSO4 None

Site-Specific Nutrient Management

Site-specific nutrient management is a component of precision agriculture and can be used for any field or crop. It combines plant nutrient requirements at each growth stage and the soil’s ability to supply those nutrients, and applies that information to areas within a field that require different management from the field average. Site-specific management allows for fine-tuning crop management systems along with 4R Nutrient Stewardship — the right source, rate, time and place of nutrient use.

Site-Specific Nutrient ManagementSite-specific management can be thought of as a series of layers of information about each field, as depicted in Figure 1. Each time a measurement is made (soil tests, scouting reports, yield data, etc.), another layer of information is added. Over time, multiple layers of information are added and become part of the database that can guide future crop management decisions. By geo-referencing each data point to its precise geographic location, these data layers can be "stacked" for analysis to determine the relationship between layers for any point in the field. For example, the relationship between nitrogen rate applied and yield obtained might be determined, and then its variability mapped as an additional "calculated" layer of information.

Precision AgricultureThe systematic implementation of best management practices into a site-specific system provides the best opportunity to develop a truly sustainable agriculture system. Managing the right source at the right rate, right time and in the right place is best accomplished with the right tools. Various technologies are available to help make decisions related

to nutrient management, from soil sampling to fertilizer application to yield measurement. These tools enhance the ability to fine-tune nutrient management decisions and develop the site-specific nutrient management plan for each field.

Identify and Quantify Variability within FieldsVariability within fields is measured by soil sampling, field scouting, physical measurements, soil survey, and yield monitoring. Documenting this information in the GIS database for a field provides the basis for site-specific management decisions. Variability within fields comes from a variety of natural and man-made factors. Natural variability is largely due to physical properties of the soil including topography, texture and structure.

Man-made influences on soil variability include:

Crop rotation, livestock pasture, fences, tile drainage, fertilizer and manure application; Cropping systems and tillage operations affect soil tilth; and Compaction (a result of a combination of natural and man-made factors).

Site-Specific Equipment and Technology

EquipmentSpecial equipment is not required for site-specific management. Identifying areas requiring specific management can be done with conventional soil testing and scouting techniques. Different fertilizer rates can be applied to different areas by staking or flagging them, and then spreading the different areas separately. Estimates of within-field distances to identify these areas can be documented by measuring, counting rows, pacing or other relative means. But there are technology tools available that expand the capabilities for using site-specific management more effectively.

Technology ToolsGPS, GIS-based records and data analysis, sensors and variable-rate controllers are revolutionizing nutrient management to best meet crop needs and efficiently utilize available resources. Site-specific sampling, variable-rate fertilizer application and yield monitors are among the most common tools guiding today’s modern nutrient management systems.

Global Positioning System (GPS)Most of the tools for precision agriculture involve use of data collection or controller systems that utilize the global positioning system (GPS). Each set of data collected is associated with its specific geographic coordinates (latitude, longitude, and elevation). This allows the understanding of precise relationships among the different layers of data, the resulting yield data, and other measurements. These layers can then be analyzed to make recommendations for future decisions.

GPS systems are used on planting equipment for collecting geo-referenced planting data, starter fertilizer application, and other inputs. With proper controllers, variable-rate application of inputs can be added to the management plan. Each of these steps can be added over time, increasing the value of the initial investment.

As more advanced military-technology becomes available for public use and new technologies develop to support GPS, this tool will continue to become more valuable to farmers in implementing site-specific management.

Global Positioning System (GPS) Real-Time Kinematic System (RTK) Geographic Information System (GIS) Soil Surveys Intensive Soil Sampling Remote Sensing

Nutrient Management Plan DevelopmentThe value of GPS, GIS and remote-sensing technologies comes from incorporating the data into the management decision process. These tools can help to develop a comprehensive crop and soil nutrient management plan that can help improve production efficiency, increase yields and reduce potential environmental problems associated with crop production. The GIS system provides a means to monitor and evaluate nutrient needs, crop removal, and losses to the environment

Information IntegrationSite-specific management and the technology tools available require integration of many sources of information. Without the use of computers and GIS software, it is impractical to try to analyze all of the information available. Site-specific systems, including yield monitor data, generate large amounts of data that should be integrated into GIS and used to interpret the variability to move to a higher level of production, input efficiency and profitability.

Taking Advantage of Information Information Integration Step-wise Implementation Building a Digital Nutrient Management Plan for Each Field Documentation of Needs, Rates of Application and Yield Responses Moving Forward with Site-Specific Precision Agriculture Systems

Fertilizer Use and the 4RsFertilizers are a necessary component of sustainable crop production. When properly managed, fertilizers help address the challenge of increasing production in an economically viable way while retaining the ecological integrity of cropping systems. However, if nutrients are not adequately available within a crop production system, fertility is mined from the soil, and the crop will never attain optimal yields. Conversely, if nutrients are supplied in excess or without managing risks, the possibility of nutrient movement away from the cropping system increases, potentially negatively affecting the environment. In both situations, the profitability of the cropping system will be negatively impacted by lost yield or by lost inputs.

4R Nutrient Stewardship utilizes fertilizer best management practices (BMP) addressing the right fertilizer source, at the right rate, the right time, and in the right place. The 4Rs provide the foundation for a science-based framework to

achieve sustainable plant nutrition management. In short, 4R practices are good for the grower, good for the farming community, and good for the environment.

There is an existing need to improve the adoption of fertilizer best management practices to enhance the sustainability, efficiency and productivity of agricultural systems. Efficiency and productivity together are interwoven with sustainability. Striving to improve efficiency without

also increasing productivity simply increases the pressure to produce more on lands less suited to agricultural production. Conversely, squandering resources to maximize productivity can result in increased environmental impacts and decreased profitability.

The essential plant nutrients play a vital role in providing adequate food supplies and protecting our environment.

Nitrogen and the EnvironmentWhen soil nitrogen supply becomes low, plant stresses are immediate and yield losses are assured. The large demand crops have for nitrogen (legumes are an exception) means that supplemental sources must be provided for efficient and sustainable crop production. All these sources, when added to soils enter the nitrogen transformation cycle and are eventually converted to plant-available ammonium and nitrate-nitrogen. To meet crop management objectives, fertilizer best management practices must ensure that adequate amounts of nitrogen are used for profitable production levels, while minimizing any potential negative effects on the environment. This is best achieved by utilizing practices that address the 4Rs.

Much of the concern about nitrogen in the environment is due to the potential movement of unused or excess nitrate-N through the soil profile into groundwater (leaching). Because of its negative charge, nitrate-nitrogen is not attracted to the various soil fractions. Rather, it is free to leach as water moves through the soil profile. Soil type has an influence on the amount of and speed with which nitrate-nitrogen moves through a soil profile, with movement greater in sandy as compared to clay soils. Nitrogen loss as ammonia volatilization from surface-applied sources and as dinitrogen gas (N2) or nitrous oxide (N2O) from soil microbial activity is also a concern.

Phosphorus and the EnvironmentPhosphorus has been associated with environmental pollution through the eutrophication of lakes, bays and non-flowing water bodies. The symptoms are algal blooms, heavy growth of aquatic plants and deoxygenation. Since phosphorus is insoluble relative to other essential nutrients, environmental degradation is associated largely with phosphorus movement when soil erosion occurs. Except on some organic soils, very low concentrations of phosphorus are found in drainage waters as the result of leaching. The major form of phosphorus entering surface waters in most agricultural watersheds is particulate-phosphorus associated with either clay soil fractions or organic matter. These fractions are the most easily eroded, and have a relatively high surface area that contains enriched phosphorus levels compared to soil particles that have greater resistance to erosion.

Sustainable Cropping Systems

Sediment-enriched phosphorus commonly contains two to six times that of soil phosphorus levels that are left behind. High-loading in surface runoff is usually associated with storm events. Storm flow concentrations of soluble phosphorus are often 10 times greater than base flow concentrations. Numerous research studies have shown that conservation tillage practices reduce soil erosion and the movement of phosphorus from agricultural lands. Conservation tillage is a BMP because it reduces erosion considerably by absorbing the impact of falling rain and slowing water runoff. If erosion is stopped, then phosphorus losses to the environment will beProduction demands, input requirements and environmental impacts taken together mean the risks for making the wrong nutrient use decisions is greater now than ever. When fertilizer BMPs result in increased production and input use efficiency, they also reduce losses to the environment. When making practice selection, the interconnectivity between practices addressing source, rate, time and place should be considered.

While the scientific practices governing the 4Rs are universal, practice implementation is site-specific; so there is not a common management plan or set of practices that will work for everyone in every location. Crop advisors are key in the efforts to increase adoption of 4R Nutrient Stewardship with growers.

Selecting BMPs for increasing nutrient efficiency and productivity while reducing environmental impact begins with addressing the scientific principles behind the 4Rs. Fertilizer BMPs should be selected based on these principles, and should then be used in combination with other conservation practices.

reduced to acceptable minimum levels.

Right Source Right Rate Right Time Right Place

Right Source:Ensure a balanced supply of essential nutrients, considering both naturally available sources and the characteristics of specific products in plant-available forms. Specifically, consider nutrient supply in plant-available forms, ensure the nutrient suits soil properties, and recognize the synergisms among elements.

Fertilizer Best Management Practices that Address the 4Rs YIELD LEVEL

  TIMING OF APPLICATION

  SPLIT- OR MULTIPLE NITROGEN APPLICATIONS

  ADEQUATE & BALANCED NUTRIENT SUPPLY

USE OF INHIBITORS

  CORRECT METHOD OF APPLICATION

  CREDITS

  SOIL & TISSUE TESTING

IRRIGATION WATER CREDITS

  EROSION CONTROL

  USE OF COVER CROPS

  LIMING TO CONTROL SOIL ACIDITY

Split- or Multiple Nitrogen ApplicationsConsider split-nitrogen applications according to plant growth stages and crop needs for both small grains and row crops. Preplant, starter, top-dress, side-dress and fertigation are some of the fertilizer application timing options. Plant-soil analyses can be helpful to determine additional nitrogen needs. Timeliness of application is essential to be sure crop yields do not suffer from nitrogen deficiency.

Agronomic Comparisons of Fluid and Dry FertilizersExperimental data from a wide range of studies overwhelmingly supports the conclusion that there are essentially no differences among the liquid, suspension and dry fertilizers when they are compared over the long term under conditions of similar nutrient rates, placements and chemical forms. The last is particularly important when comparing phosphate fertilizers. For instance, it would not be valid to compare a highly water-soluble phosphate in fluids with a solid phosphate of low water solubility. However, when solids such as diammonium phosphate (DAP), monoammonium phosphate (MAP) or ammonium polyphosphate were compared with fluids such as 10-34-0, 8-24-0 or 11-37-0 under similar conditions, long-term studies have shown these to be essentially equal in nutritive value. Similarly, long-term studies have shown solid urea or ammonium nitrate to be virtually equal to nitrogen solutions, such as urea-ammonium nitrate. Essentially, the same conclusions would be reached with dry and fluid NPK mixes.

Cautions in Comparing FertilizeFor valid comparisons, studies should be conducted for several years at the same location using the same experimental design to ensure that the variability inherent in field studies does not lead to faulty interpretations. If data are selected from one study, for one year, at one location, evidence can be cited to prove that solids are better than fluids, or vice versa, or that polyphosphates are better than orthophosphates, or vice versa.

Care must be exercised in comparing any solid or fluid fertilizers under field conditions. For example, concentrated superphosphate, 0-46-0 or CSP, can’t be compared directly with 10-34-0

solution or solid monoammonium phosphate, 11-48-0 or MAP, because the latter two contain nitrogen.

For valid comparisons, studies should be conducted for several years at the same location using the same experimental design to ensure that the variability inherent in field studies does not lead to faulty interpretations. If data are selected from one study, for one year, at one location, evidence can be cited to prove that solids are better than fluids, or vice versa, or that polyphosphates are better than orthophosphates, or vice versa.

Care must be exercised in comparing any solid or fluid fertilizers under field conditions. For example, concentrated superphosphate, 0-46-0 or CSP, can’t be compared directly with 10-34-0 solution or solid monoammonium phosphate, 11-48-0 or MAP, because the latter two contain nitrogen.

Fluids and Solids are Equal Agronomically

The relative equality of fluid and dry fertilizers should not be too surprising in light of the fact that the chemical constituents of the two physical forms are usually identical.

The matter of equality of various physical forms is even more predictable when one considers the limited variety of chemical forms presented to the plant root. Although a farmer may apply fertilizer nitrogen as anhydrous ammonia, urea, ammonium nitrate, urea-ammonium nitrate, calcium nitrate or several other forms, the same farmer may be assured that, within a fairly short time, the roots of his crops will be confronted mainly with nitrogen in the nitrate form (NO3). This is because various soil enzymes rapidly convert urea nitrogen to ammonium forms, and then soil microbiological processes fairly rapidly convert the ammonium forms to nitrate. So, for most of the growing season, plant roots "see" mainly nitrates unless a source of ammonium nitrogen is supplied during the season.

Despite the fact that farmers are offered a wide array of phosphorus-containing

fertilizers, these farmers are assured that their crops are really confronted with a very limited variety of chemical forms of phosphorus. First, the phosphorus in most fertilizers is present in the orthophosphate form. When an orthophosphate-containing fluid fertilizer is applied or an orthophosphate-containing dry fertilizer dissolves in the soil solution, the plant roots are confronted mainly with two phosphate species (H2PO4- and HPO4

=). If a fertilizer material

containing polyphosphates is applied, the polyphosphate is fairly rapidly converted in most agricultural soils to the orthophosphate form. So, regardless of the physical or chemical form of phosphate fertilizer, after a short while in the soil, plant roots "see" only two very similar forms of phosphate.

Potassium fertilizers are even more uniform than either nitrogen or phosphate fertilizers. The dominant source of potassium for both fluid and dry fertilizers is potassium chloride. Even when other sources are used, such as potassium phosphate or potassium nitrate, it is the potassium ion (K+) that the plant root deals with in the soil solution.

Implications of EqualityFluid and dry fertilizers of comparable chemical constituency are essentially equal agronomically when applied at equivalent nutrient rates under similar placements at the same time. This equality of the various forms of both fluid and dry fertilizers is a powerful management tool. It frees the farmer to choose from a wide variety of materials using a multiplicity of non-agronomic factors as criteria for the decision.

Soil TestingThe purpose of soil testing in high-yield farming is to determine the relative ability of a soil to supply crop nutrients during a particular growing season, to determine lime needs, and for diagnosing problems such as excessive salinity or alkalinity. Soil testing is also used to guide nutrient management decisions related to manure and sludge application with the objective of maximizing economic/agronomic benefits while minimizing the potential for negative impacts on water quality

SamplingThe soil testing program starts with the collection of a soil sample from a field. The first basic principle of soil testing is that a field can be sampled in such a way that chemical analysis of the soil sample will accurately reflect the field’s true nutrient status. This does not mean that all of the samples must, or will, show the same test results, but rather that the results must reflect true

variations within the field. Remember that the soil test recommendations for lime and fertilizer can never be more accurate than the accuracy of soil sampling.

Note: A separate chapter in the EFU Guide is devoted to soil sampling.

ACTORS AFFECTING NUTRIENT AVAILABILITY N P K S CA AND

MG MICROS

Soil pH X X X X X X

Moisture X X X X X X

Temperature X X X X X X

Aeration X X X X X X

Soil Organic Matter X X X X X

Amount of Clay X X X X X X

Type of Clay X X X X

Crop Residues X X X X X X

Soil Compaction X X

Nutrient Status of Soil

Soil Test ParametersIn addition to extracting solutions, several other parameters of each soil test are important in determining the final number that is printed on a soil report for any one soil sample. These parameters include:

Ratio of soil to extractant Shaking time, action and speed

Method of expressing the results (e.g., lb/acre, ppm, index systems) "Cut-off" levels for high test results Overall techniques used in the lab

The extractants containing the dissolved plant nutrients are analyzed to determine the concentration of the plant nutrient(s). Results are usually reported as parts per million (ppm), or pounds per acre (lb/acre). For most nutrients, ppm may be converted to lb/acre by multiplying by two (40 ppm of potassium = 80 lb/acre). For nitrate, sulfate and chloride, essentially all the nutrient forms present in the soil are extracted, and depth increments, other than the standard 6- to 7-inch surface layer, are sampled. For these measurements, ppm is converted to lb/acre by the following formula: lb/acre = ppm x 0.3 x depth increment in inches. For example, a 10 ppm nitrate N test on a soil sample taken

to a 24 inch depth would convert to 72 lb/acre (10 ppm x 0.3 x 24 inches). In this case, 72 lb/acre of nitrate nitrogen were present in the top 24 inches of the soil sampled.

Extracting available plant nutrients helps give an educated estimate as to the amounts of plant nutrients that will be available to a particular crop during the growing season. The amount of plant nutrients extracted will depend on the strength of the extracting solution and various other parameters. Soil test values are a relative number and should be interpreted as low, medium or high for a particular nutrient.

Calibration and InterpretationPerhaps the greatest challenge in soil testing is calibration of the tests. It is essential that the results of soil tests be calibrated against crop responses from applications of the plant nutrients in question. This information is obtained from field and greenhouse fertility experiments conducted over a wide range of soils. Yield responses from rates of applied nutrients can then be related to the quantity of available nutrients in the soil.

The results of long-term soil test calibration studies on different soil types are then utilized to establish recommended amounts of plant nutrients to apply to a particular crop at a given soil test level. For instance, if the soil test P level is in the range of 0–10 ppm (which is low), the P recommendation for a 150 bu/acre corn crop may be 100 lb/acre of P2O5; whereas, if the soil test P level is above 40 ppm (very high), the recommendation may be 0 to 20 lb/acre.

In this example to the right, more than 85 percent of the fields testing very low in a particular plant nutrient may give a profitable yield response to the added nutrient. At the very high level, there is only a 15 percent probability of a profitable yield increase to the added nutrient. These values are arbitrary, but they illustrate the idea of expectation of response.

The tools of site-specific precision management now allow growers to manag

The tools of site-specific precision management now allow growers to manage more homogenous areas within fields. Some of those areas have much higher yield potentials than the database with which most of today’s soil tests were calibrated. This lack of calibration for high-yielding areas is one of the factors driving interest in using yield monitors and global positioning satellites to conduct strip trials to determine the adequacy of existing soil fertility programs. New precision ag tools have the ability to develop algorithms that allow for management of multiple site-specific zones within individual fields. This means a balanced crop nutrition prescription can be delivered to each square foot of every field.

When interpreting soil test results, several things should be kept in mind:

The chances of getting a profitable response to fertilization are much greater on a soil that tests low in a given nutrient than on one that tests high.

This does not rule out the possibility of a profitable response from nutrient application at a high level of fertility or lack of a profitable response on soils of low fertility. Soil tests are better at predicting the probability of a profitable response to nutrient application than predicting the actual quantity of nutrient that will be needed in any one year. Research in the United States and Europe shows that in any one season, a soil testing low in a nutrient often will not yield as well as a soil testing at an optimum level, no matter how much fertilizer is applied that year. Interpretation of soil test results and recommendations often becomes a matter of how to improve the fertility status of soils testing less than optimum. How much will be needed to change the soil from low to medium or high in that element? What will be the most economical level at which to maintain the nutrient status of the soil? With top-level management practices, yields increase and the probability of a response at any given soil test likewise increases. Wise use of soil testing incorporates a long-term approach to fertility management, in which site-specific soil test target levels are established for each field and nutrient management plans developed to reach and maintain the target levels.

RecommendationsThe goal of soil testing is to help farmers achieve economical optimum yields while protecting the environment. The basic philosophy of soil test fertilizer recommendations is:

Base them on soil test results; Recommend that lower-testing soils be built up to higher test levels by adding extra fertilizer; Apply maintenance amounts of plant nutrients to higher-testing soils to keep them there and to keep productivity high; and Do not apply specific nutrients to soils testing very high in these nutrients.

Individualized fertilizer recommendations use site- and grower-specific information, rather than laboratory-generated recommendations based on assumptions and generalizations. Computer programs are available that help personalize recommendations by considering the following:

Soil Test Calibration RelevancyHow appropriate is the calibration used in the standard recommendation for the field in question? Unusual soil types, a different climate, no-till or ridge-till culture, crop variety, cropping history and field variability are examples of factors that could cause differences.

Yield PotentialYield potential determines the economic value of each percentage change in relative yield and may influence the shape of the calibration curve.

Fertilizer PlacementBand placement often reduces lost yield as sub-optimal soil test levels are built to optimum levels because the short-term recovery of applied fertilizer by crop plants is improved. Some recommendation systems reduce the rate recommended when banding is used, compared to broadcast. However, rate studies have shown the optimum rate when banding is sometimes equal to or greater than the optimum broadcast rate. It is wise to build soil test levels to optimum regardless of placement method used.

Farmer Financial CircumstancesThe financial objective of farmers, like other investors with limited capital, is to maximize the return on the last dollar invested after considering all possible investment alternatives and their associated risks. Therefore, cash flow influences fertility management decisions.

Uniform and Balanced Nutrient DistributionBalance recommendations to ensure each nutrient is used efficiently.

Land Tenure (Period of Time the Grower Will Farm the Field)Soil test phosphate and potassium are capital investments, and buildup costs should be amortized over the expected time of ownership or operation. The longer the period of time benefits will be accrued from buildup, the lower the cost of buildup becomes and the higher the optimum soil test level becomes. Landowners and operators, as well as the environment, benefit from the development of agreements in which the costs and returns of soil test buildup

are equitably shared. Such agreements can help avoid the loss of productivity and accelerated erosion typical of run-down farms having impoverished soil fertility.

Soil Test Buffer PotentialSoil test buffer potential is the quantity of fertilizer required to change the soil test level, and is usually expressed as pounds of P2O5 or K2O required per ppm of soil test level change. Some low-pH and some high-pH soils fix applied phosphate readily, and increasing soil test phosphate is more costly, decreasing the optimum soil test level. Soil test phosphate and potassium levels are usually easier to change in sandier soils than on medium or fine-textured soils, except with very sandy soils, where potassium leaching becomes significant.

Recommendations When Levels Are High

Once soil tests are interpreted, possible approaches to a nutrient management plan may include the following:

Sufficiency: Add necessary rates of deficient nutrients so yields are not limited in present crop. Build-Maintenance: Add enough of needed nutrient(s) to supply present crop need, and gradually increase soil supply to non-limiting level. Replace crop harvest–removed nutrients to keep plant nutrient levels at non-limiting levels.

If soil tests high in a plant nutrient, applying more of that nutrient is not recommended, at least for the current crop. This is especially true if there is an abundance of the nutrient present to the extent that there is almost no chance of response even if the nutrient was not applied for several years. However, some laboratories assign the value high to a level that points to little or no response to applications of that nutrient that year.

Failure to apply any of these nutrients will result in soil test depletion. Also, under some conditions, crops will respond profitably to a nutrient even with a high test. For example, on early-planted corn, the addition of N, P and K as a row application may produce response on soils testing high.

Fertilizer application when soils test in the high range is influenced substantially by the factors discussed in the section on individualization of recommendations. Maintenance in the high soil test category will be appropriate for some growers and sites but not for others.

SOIL TEST CLASS PROBABILITY OF RESPONSE

Very Low Profitable response in all but rare cases

Low Profitable response in most seasons

Medium Average response over years is profitable

High Occasional profitable responses

Very High

.