Ohio Agronomy Guide, 14th Edition

Bulletin 472-05

Chapter 3: Soil Fertility

By Dr. Robert Mullen, Dr. Ed Lentz, and Dr. Maurice Watson

Elements Essential for Plant Growth

Higher plants require 16 nutrient elements to complete their life cycle (Table 3-1). Of these, 13 are extracted from the soil and can be found in organic or inorganic fertilizers. The 13 essential nutrients are thought of as three distinct groups—primary macronutrients, secondary macronutrients, and micronutrients. Primary macronutrients are the elements needed in the largest quantity; these are nitrogen (N), phosphorus (P), and potassium (K). Secondary macronutrients are needed in lower concentrations and are calcium (Ca), magnesium (Mg), and sulfur (S). Micronutrients are needed in even lower concentrations and include iron (Fe), manganese (Mn), boron (B), chlorine (Cl), zinc (Zn), copper (Cu), and molybdenum (Mo).

The carbon (C), hydrogen (H), and oxygen (O) utilized by a plant come from carbon dioxide and water. Little can be done to control the availability of these three except through drainage, irrigation, and modification of the physical condition of the soil. On a dry-matter basis, C, H, and O make up more than 94% of the plant biomass. This means the remaining 6% of the biomass is made up of the other 13 nutrients. Even though their amounts seem small, deficiency of only one essential element can limit the growth potential of a plant.

Removal of the primary and secondary macronutrients (N, P, K, Ca, Mg, and S) by various crops is reported in Table 3-2. The values listed in the table indicate average nutrient removal and represent only those nutrients found in the harvested portion of the crop. The values reported are not the quantities of nutrients needed to generate the crop yields shown. Keep in mind crop nutrient content can vary widely under different growing conditions, and soil nutrient availability is determined by various fixation and release mechanisms.

Fixation and release mechanisms that control soil nutrient availability are strongly influenced by soil pH. Figure 3-1 shows the relative availability of 12 essential nutrients at different pH levels for mineral soils. Figure 3-2 shows the relative availability of the same elements at different pH levels for organic soils. Chlorine is not reported because it is usually present in sufficient amounts.

Figure 3-1. Relative availability of elements essential to plant growth at different pH levels for mineral soils.		Figure 3-2. Relative availability of elements essential to plant growth at different pH levels for organic soils.

Primary Macronutrients

Nitrogen (N)

Most nitrogen (N) contained in soil is in the organic form. One percent organic matter represents approximately 1,000 lbs nitrogen per acre in the top six inches of soil. Despite the abundance of nitrogen in organic matter, this form is unavailable to the crop. Organic nitrogen must be mineralized (converted into ammonium) by soil microbes to become plant available. The rate of mineralization is controlled by soil pH, moisture, temperature, and aeration and is highly variable. Crops grown on organic soils (greater than 20% organic matter) typically require less fertilizer nitrogen than crops grown on mineral soils due to mineralization of organic nitrogen. Ammonium present in a warm, well-aerated soil is quickly converted (one to two weeks) to nitrate. Soil nitrate is highly mobile and is susceptible to both denitrification (finer soils) and leaching (coarser soils) losses.

Fertilizer nitrogen sources can be classified into three categories—inorganic, synthetic organic, and natural organic. There is little difference between sources of nitrogen when properly applied at equivalent rates. Typical fertilizer nitrogen sources and their analysis are presented in Table 3-3. To assure maximum agronomic and economic production, nitrogen should be managed such that losses are minimized. Best management practices that decrease the potential for nitrogen loss should be selected—such as subsurface injection or dribble banding of liquid nitrogen rather than broadcast application, incorporation of surface-applied urea (conventional till) or application coinciding with a rainfall event (no-till), and spring application of nitrogen for summer crops rather than late fall application.

If nitrogen is to be applied in the fall for a spring crop, anhydrous ammonia should be selected and should only be applied if the soil temperature is below 50°F (other sources of nitrogen for fall application are not promoted). There is a risk, however, that warm, wet conditions may prevail in the spring that can lead to significant losses of nitrogen, requiring later additions of nitrogen to maximize production. Spring-applied nitrogen should be applied as close to planting as possible but be aware that application of anhydrous ammonia followed closely by planting can result in decreased germination. Allow some time to pass between anhydrous application and planting. Application of nitrogen at sidedress is typically the most efficient method.

Urea applied in no-till production systems should coincide with an expected rainfall event. Volatilization losses of urea fertilizers without adequate incorporation (by tillage or rainfall) can be significant. Liquid nitrogen formulations should preferably be injected below the soil surface or dribbled in a band to minimize volatilization losses. Rates of application should be based on realistic yield goals determined from historical production levels. Specific rates of application are available in the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.

Phosphorus (P – P₂O₅)

Ohio soils can contain between 500 to 1,500 pounds of total phosphorus (P) per acre, most of which is unavailable to the crop. The level of available phosphorus is quite variable across the state, and field levels of phosphorus can be determined by soil test. Historical management and soil pH (of mineral soils) determine how much phosphorus is plant available. Phosphorus is taken up by the plant in two primary forms—H₂PO₄- and HPO₄²-. As soils become more acidic, H₂PO₄- is the primary species taken up, but excessive soil acidity can result in phosphorus deficiency as aluminum and iron concentrations increase, “fixing” phosphorus. Conversely, as soils become more basic or alkaline, HPO₄²- is the primary species taken up, but high pH can result in phosphorus deficiency as calcium phosphate precipitates. Plants take up both phosphorus forms indiscriminately, so there is no specific pH level that results in maximum phosphorus availability. In general, soil pH should be maintained between 6.0 to 7.5 to maximize plant available phosphorus.

Phosphorus fractions of typical phosphorus fertilizers are water soluble, citrate soluble, citrate insoluble, and total. Citrate soluble phosphorus is referred to as “available” phosphorus, and the portion of citrate soluble phosphorus that dissolves in water is water-soluble phosphorus. Phosphorus content of phosphorus fertilizers is expressed as percent citrate soluble phosphorus as P₂O₅-. Most commercial fertilizers contain 50% or greater water-soluble phosphorus and are suitable for row application. Typical phosphorus fertilizers and their analysis are presented in Table 3-4.

Despite the fact that plants take up the ortho form of phosphorus, poly-based forms of phosphorus are just as effective at satisfying plant needs. Poly forms of phosphorus added to the soil are actually converted to ortho forms relatively quickly.

Potassium (K – K₂O)

Ohio soils contain between 10,000 to 20,000 ppm potassium. Despite this high amount, only a small portion is actually plant available. Exchangeable (adsorbed to soil CEC—Cation Exchange Capacity) and solution potassium make up the plant-available portion. Potassium availability is dependent upon soil mineralogy and rainfall. Soils that develop from minerals high in potassium (feldspars and micas) have naturally high potassium fertility levels. Soils that developed under high rainfall conditions can be quite deficient in potassium because it has been leached out of the soil.

Muriate of potash is the primary source of potassium fertilizer used commercially. It is readily soluble and contains over 62% potassium (Table 3-4).

Secondary Macronutrients

Calcium (Ca)

Calcium is one of the most abundant nutrient elements found in the soil and is rarely deficient in Ohio. Calcium availability is strongly tied to soil pH, and soils that are maintained at adequate pH (>5.5) levels should have adequate calcium. If calcium is deficient, addition of lime to increase soil pH will remedy the problem. The ultimate source of calcium is lime (calcitic or dolomitic—both contain relatively high calcium amounts). Sources of calcium and their analysis are shown in Table 3-5.

Magnesium (Mg)

Magnesium deficiencies, while rare, can occur, primarily in the eastern half of the state. Like Ca, magnesium availability is strongly tied to soil pH. Soils with neutral or basic pH should have adequate magnesium. If soil magnesium and pH are low, use of dolomitic lime to neutralize soil acidity will remedy the problem. If magnesium is low and the pH is near neutral, application of one-half to one ton of 12% magnesium (dolomitic) lime will provide enough magnesium for maximum plant production. This will not result in over-liming of medium- or fine-textured soils.

Some propose an optimum calcium to magnesium ratio to provide proper nutrition for growing plants. There is, however, no economic benefit to this methodology. Depending upon soil mineralogy and soil pH, applying enough calcium or magnesium to reach an ideal ratio can be extremely expensive. Several research projects across the Corn Belt, including work in Ohio, have revealed that maximum production levels can be attained at several different Ca:Mg ratios.

Sulfur (S)

Sulfur deficiencies are rare but can be found in forage production systems, especially on sandier soils low in organic matter. Much like nitrogen, the primary form of sulfur in the soil is found in the organic fraction, and the form taken up by higher plants is highly mobile. For every 1% of organic matter, there are approximately 140 lbs of sulfur, which like nitrogen, must be mineralized to be plant available. Sulfur is deposited in large quantities from rainfall primarily due to industrial activities. As emission standards decrease sulfur release from industrial processes, S fertilization may become more important. Fertilizer sulfur is available from many different sources which are reported in Table 3-5.

Micronutrients

Micronutrient levels across the state are adequate for maximum plant production, and deficiencies are rare. Specific field environments and soil conditions increase the potential of finding a micronutrient deficiency (Table 3-6). If a micronutrient is found to be deficient, remember that over-application of most micronutrients can result in toxicity. Plant tissue analysis is the best way to determine if a plant has a micronutrient deficiency.

Molybdenum (Mo) and boron (B) can reach toxic levels even when applied in small quantities. Recommendations should be followed closely when either of these elements is being applied. Boron should not be applied in the row for corn or soybean. Manganese (Mn) toxicity occurs on many soils in eastern Ohio when pH nears 6.0. Alfalfa and soybeans are especially sensitive to excess manganese. Foliar-applied manganese in excess of recommended amounts, or in small quantities of water, may burn leaves of wheat, oats, and sugarbeets.

Micronutrient deficiencies are strongly influenced by soil pH. Soils that are acidic (< 6.5) should have adequate levels of most of the micronutrient metals (Fe, Mn, Zn, B, Cu). Molybdenum behaves just the opposite of the other micronutrients; its availability increases as soil pH increases.

Animal Manure

Animal manure is a good source of plant nutrients and contains many of the elements essential for plant growth. It is especially important to farming operations that include livestock enterprises. It provides those operators the opportunity to utilize efficiently the waste produced. Soils should be tested prior to applying manure. Table 3-7 shows the average nutrient content (N, P, K) of three different manure sources. Because the nutrient value of manure is closely related to the dietary regimen, which can result in vastly different nutrient levels, manure should be tested to determine its nutrient value prior to land application.

Several independent labs located in and around the state are able to analyze manure samples. To view a list of testing labs, go to: http://agcrops.osu.edu/fertility. Manure should be analyzed for total solids, total nitrogen, ammonium nitrogen, phosphorus, potassium, calcium, and magnesium. Other nutrient analyses should be available upon request. To determine application rates and recommendations of rates based on soil-test nutrient levels and manure levels, see OSU Extension Bulletin 604, Ohio Livestock Manure And Wastewater Management Guide, available at county offices of OSU Extension and on the Internet at: http://ohioline.osu.edu/b604/index.html .

Calculating Fertilizer Rates

Once the analysis of a specific fertilizer material is known (typically displayed as a percent), rates of application can be easily computed using this equation:

Pounds of fertilizer material needed = nutrient rate (lbs per acre) ÷ percent analysis of material

For example, if the soil test calls for 150 lbs of Mg and a dolomitic liming material is used that is 12% Mg, the rate of lime required is 1,250 lbs per acre (150/0.12). This calculation works for all fertilizer materials.

Lime and Liming Materials

Proper use of both lime and fertilizer is necessary for high crop yields. To optimize production, soil acidity should be corrected prior to fertilizer application.

Liming benefits soil in the following ways:

• Supplies calcium and magnesium.
• Increases soil pH and the availability of phosphorus, molybdenum, and magnesium.
• Reduces harmful concentrations of aluminum, manganese, and iron.
• Increases favorable microbial activity, which results in an increased release of organically bound nitrogen, phosphorus, sulfur, and other nutrients.

Determination of Lime Requirement

Soil pH measures the active soil acidity or alkalinity. The lime requirement is determined using the buffer pH, or the lime test index, which measures potential soil acidity. Finer-textured soils that have relativelyhigh CEC (Cation Exchange Capacity) have the ability to buffer changes in soil pH by releasing adsorbed hydrogen; thus, more lime is required to affect soil pH. Coarser-textured soils have lower CEC values and have a diminished ability to provide hydrogen to replace that which is neutralized by liming; thus, less lime is required. The lower the buffer pH is below 6.8, the greater the lime requirement. Table 3-8 shows the relationship between the buffer pH and lime requirement to various pH levels. For organic soils, pH is used to determine lime requirement.

Liming Materials

Agricultural liming materials used for correcting soil acidity include all calcium and magnesium oxides, hydroxides, carbonates, silicates, or combinations sold for agricultural purposes. Commonly found liming materials are presented in Table 3-9.

Liming materials are labeled based on their effective neutralizing power (ENP). The ENP of a liming material considers the material equivalence, purity, fineness of grind, and percent moisture. Particle size of liming materials impacts their effectiveness at neutralizing soil acidity and their speed of reaction. Ag liming materials typically contain particles of differing sizes which results in longer-term acid neutralization. Smaller particles react quicker while larger particles dissolve slowly, affecting soil pH over a longer period. This is why liming is typically not necessary every year. When comparing liming materials and their associated cost, ENP provides a good way to identify the most economical source.

Lime Recommendations

Lime recommendations presented in Table 3-8 are in tons of material/acre and assume a liming material with an ENP of 2000.

Recommendations should be adjusted if:

• Another grade of liming material is to be used.
• Depth of plowing is different from that indicated (eight inches).
• A different crop is planted.

Adjustments for the Type of Liming Material

Although the activity of liming materials in the soil is the same, liming materials do differ. Liming materials differ in their purity, fineness, and moisture, which affect the rates of application necessary to alter soil pH. Fortunately, the ENP, required by the Ohio Department of Agriculture for all liming materials, considers all factors that allow direct comparison of liming materials.

The example given here assumes a three-ton-per-acre lime requirement (from Table 3-8). If a liming material that has an ENP of 2,000 lbs/ton is used, simply apply three tons of that material per acre. Assume that a liming material that has an ENP of 1,500 lbs per ton is chosen. To compute the new recommendation, divide the lime requirement by the quotient of the ENP divided by 2,000:

3.0 tons per acre ÷ (1,500 lbs/ton ÷ 2,000) = 4 tons per acre

If a liming material with an ENP of 1,500 lbs/ton is used to meet a lime requirement of three tons per acre, 4.0 tons of the liming material should be applied per acre.

Adjust for the Depth of Tillage

If plowing depth is different from eight inches, the lime requirement must be adjusted. To adjust the lime requirement, simply use a multiplier to change the lime requirement. Multipliers for differing depths of tillage are presented in Table 3-10. For example, assume that the lime requirement is four tons per acre, but the depth of tillage is only six inches. Multiply four tons per acre by 0.75 (multiplier from Table 3-10):

4 tons per acre * 0.75 = 3.0 tons per acre

It only requires three tons of lime per acre to achieve the desired change in soil pH to a depth of six inches.

No-Till Adjustments

Lime adjustments for no-till cultural practices are also based on tillage depth. Two soil samples are recommended for no-till row crop cultural practices—shallow sample (zero- to four-inch depth) and deep sample (zero- to eight-inch depth). The shallow sample should be submitted to determine the lime requirement. If lime is required to neutralize surface acidity, decrease the lime requirement from Table 3-8 by half. The deep lime requirement remains the same. When lime is applied to the surface, it should be lightly incorporated. If the slope of the field is steep enough to cause erosion, do not incorporate at all. When lime is not incorporated, do not use urea fertilizer for a year after lime application. Ammonium nitrate, anhydrous ammonia, or banded 28% solution are suitable N materials for this case.

Other Adjustments in Lime Recommendations

A minimum lime requirement of two tons per acre is recommended for forage legumes if the soil pH is 6.2 or less, even if the buffer pH is higher than 6.8 and lime would typically not be recommended.

Sandy soils are so weakly buffered that despite the soil pH being below optimal ranges, the buffer pH value is greater than 6.8. A one ton per acre lime requirement should be used if soil pH is 0.3 pH units below the desired soil pH. A two ton per acre lime requirement should be used if soil pH is 0.6 pH units below the desired soil pH.

Why Re-liming Is Necessary

Periodic liming is necessary because basic cations (Ca, Mg, and K) are removed by crops and can be lost by leaching and erosion. Ammoniacal forms of nitrogen fertilizers also contribute to soil acidity. Coarser-textured soils require more frequent applications of lime than finer-textured soils.

Acidic Subsoils

Most of the soils in eastern Ohio and the light-colored soils of western Ohio have acidic subsoils. If there is any question about the pH of the subsoil, the subsoil should be sampled and tested separately.

When lime applications are necessary to correct subsoil acidity, the lime requirement to increase pH to 6.8 should be used. The pH level in the topsoil should be 6.8 or above to promote downward movement of the lime. Only where the surface pH is maintained near 6.5 will the subsoil pH increase. Because this downward movement takes several years, the sooner the lime is applied, the better.

Lime recommendations of more than four tons per acre should be applied as a split application to achieve a more thorough mixing of the lime with the soil. Half the lime should be applied prior to tillage and half before subsequent tillage. For best results, lime should be applied at least six months before seeding. This allows the lime time to neutralize soil acidity. When a maintenance application is recommended, it can be applied any time in the cropping sequence.

Organic Soils (Muck and/or Peat)

Organic soils usually do not benefit from liming unless the pH of the soil in the root zone is below 5.3. If the pH of the surface is near 5.3, but the subsurface pH is 4.0 or below, it may be necessary to lime and deep plow. A favorable pH to a minimum depth of two feet should be maintained to accommodate root penetration. In general, lime does not move downward further than plow depth in an organic soil. The pH of organic soils should be determined on samples taken to depths of 24 inches.

High Organic Matter Soils

Usually the most desirable pH range for organic soil is 5.3 to 5.8. When the organic matter content of soils is between 10% and 20%, the pH should be shifted as indicated in Table 3-11.

Diagnostic Methods

Soil Testing

Soil testing provides information about the nutrient level of the soil and the amounts of lime and fertilizer necessary to maximize production. Several testing labs are available in and around Ohio (to see a list of testing labs, visit: http://www.ag.ohio-state.edu:8000/%7Ecorn/library/testlabs.pdf ). Typical soil analysis includes:

• Soil pH and buffer pH
• Available phosphorus
• Exchangeable potassium
• Cation Exchange Capacity (CEC)
• Exchangeable calcium
• Exchangeable magnesium

Additional soil analysis can include:

• Organic matter
• Nitrate-nitrogen
• Soluble salts
• Total heavy metals
• Available manganese, zinc, and boron

Most soil testing labs report soil nutrient levels in parts per million (ppm). The Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa uses both ppm and pounds per acre (lb per acre) to identify critical nutrient concentrations. Make certain that similar units are being compared. To convert from ppm to lb per acre, simply multiply ppm by 2.

Soil Sampling

When sampling a field, randomly collect 20 to 25 soil cores to a depth of eight inches. Thoroughly mix the samples together to form a composite and submit a sample of the composite to a testing lab in an approved container. Soil samples should be collected either in the fall after harvest or in the spring. Soil samples should be collected about the same time each year to avoid extreme changes in soil-test information. If lime application is necessary, samples should be collected in the fall. Soil samples should not be taken during the growing season (with the exception of the pre-sidedress soil nitrate test). Soil samples should be collected every two to three years depending upon soil conditions. Coarser-textured soils should be tested more frequently than finer-textured soils.

For most crops and soils, optimum soil test values should be within the ranges indicated in Table 3-12.

Soil pH and Buffer pH

Soil pH measures the active acidity/alkalinity of the soil solution. Buffer pH (may be reported as LTI-lime test index, which is buffer pH multiplied by 10) provides a measure of the active and potential soil acidity, and determines the lime requirement. Coarser-textured soils typically have higher buffer pH values than finer-textured soils. As buffer pH decreases, the lime requirement increases.

Available Phosphorus

As mentioned earlier, Ohio soils typically contain several hundred pounds of phosphorus per acre. A great majority of this soil phosphorus is not plant available. To determine the amount of phosphorus that is and will become plant available, an acidic solution is mixed with the soil that dissolves aluminum phosphate precipitates. Available phosphorus of acid soils in the North Central Region is measured using the Bray-Kurtz-P-1 procedure.

Exchangeable Potassium

Much like soil phosphorus, soil potassium (K) is present in large quantities. Only a small fraction of this total amount is plant available. Unlike phosphorus, potassium is not precipitated with other ions in the soil solution, but is adsorbed on the CEC of the soil or trapped between soil mineral layers. To measure exchangeable potassium, a solution containing a high concentration of another cation is added to the soil (usually ammonium acetate). The ammonium from the solution knocks potassium off the CEC and other exchange sites. The amount knocked off is measured and used to identify how much will become plant available over the growing season.

Cation Exchange Capacity

Cation Exchange Capacity (CEC) measures the capacity of a soil to adsorb cations, including hydrogen (H+), calcium (Ca²+), magnesium (Mg²+), and potassium (K+). Other cations including aluminum (Al³+) and iron (Fe³+) are also adsorbed, but in slightly acidic to neutral soil, their amounts are small enough to be ignored. In slightly acidic to neutral soils, calcium and magnesium take up approximately 80% of the CEC, while potassium only occupies less than 5%. In acidic soils, aluminum and hydrogen can begin to occupy a larger percentage of the CEC.

To determine the CEC of a soil, use this equation:

CEC = ppm Ca/200 + ppm Mg/121 + ppm K/390 + 1.2 * (70 – [buffer pH * 10])

The CEC of a soil depends largely on the soil texture and the amount of organic matter present. The larger the CEC value, the more cations the soil is capable of adsorbing, which decreases leaching. Attempts to increase the CEC of a soil by adding clay or organic matter are impractical due to the amounts that would be necessary to affect a change. Liming acidic soils only affects the CEC slightly.

The normal range in CEC for different soil textures is as follows:

Percent base saturation of the soil CEC usually falls within the following ranges:

*Assuming the pH value is in the recommended range.

Calcium to Magnesium Ratio

The calcium to magnesium ratio is based on their percent saturation of the CEC. If the ratio of calcium to magnesium is 1:1 or less (less calcium than magnesium) and the soil calls for application of lime, calcitic (low magnesium, high calcium) lime should be applied. Most plants grow well across a wide range of calcium to magnesium ratios.

Magnesium to Potassium Ratio

To avoid potential grass tetany problems, the magnesium to potassium ratio should stay above 2 to 1. In other words, the percent magnesium saturation of the CEC should be twice that of the percent potassium saturation. As more potassium is taken up, less magnesium is absorbed, resulting in a nutrient imbalance in the forage. This imbalance can be transferred to the grazing animal in the form of grass tetany.

Soluble Salts

The soluble salts test indicates the concentration of all fertilizer and non-fertilizer salts in the soil. Excessive salt levels, known as saline soil conditions, can be toxic to plants, especially germinating or young plants. Saline soil impairs the ability of the plant to extract soil water, leading to a drought-like symptom. Obviously, the drier the soil gets, the more apparent the problem is. Saline spots in the field are typically characterized by good tilth and excessive moisture retention. Severe brine solution spills cause excessive soluble salt concentrations. To reduce the salt concentration, the soil should be well drained and leached with high-quality water. Natural rainfall gradually reduces the soluble salt level in most well-drained Ohio soils. Table 3-13 provides a guide for interpreting soluble salt levels.

Values in table should be reduced by one-half for prolonged droughty conditions. Toxicity of a single salt is greater than an equivalent amount of a mixture of salts.

Plant Analysis

Plant analysis is not a substitute for soil testing, but rather supplements soil testing by determining whether nutrient deficiencies are occurring. Plant analysis can be used to diagnose suspected nutrient deficiencies or detect nutrient deficiencies before plant growth is limited. Plant analysis also contributes to more economical and efficient use of fertilizer materials, avoiding excessive or inadequate application rates.

Used in conjunction with other data and observations, a plant analysis report aids in evaluating the nutrient elements in the soil-plant system. It also provides a way to evaluate the effectiveness of fertilizers added to the soil. Plant analysis can also help determine response to fertilizer treatment by answering the question: Was the nutrient element supplied by the fertilizer sufficiently absorbed by the plant? Plant analysis is especially useful for determining whether the soil is adequately supplying required micronutrients.

Plant Sampling

Each crop has its own sampling methodology and sampling techniques for the major agronomic crops as shown in Table 3-14. When sampling young plants (seedlings), collect the above-ground portion of 10 to 20 plants. Plant samples can be analyzed for all major nutrients (this does impact the cost of analysis). The desired concentration of a nutrient should occur within the sufficiency range for that nutrient. Table 3-15 lists the typical sufficiency ranges of nutrients for corn, soybean, alfalfa, and small grains. Nutrient concentrations slightly lower than shown indicate a marginal condition, which may adversely affect plant growth. The limits for these ranges vary depending on crop, plant part, and state of growth when sampled. These values relate specifically to a particular plant part sampled at a specific stage of growth. These values were selected after careful review of current literature and from the analytical results obtained from numerous samples collected from experiments conducted in Ohio.

When sampling plants in an obviously stressed area, it may be beneficial to submit a check tissue sample which is from an adjacent area that is stress free. This helps further determine if a nutrient deficiency exists. The relative concentration of elements also helps determine sufficiency or deficiency and should be considered when interpreting plant analysis. For example, the ratio of potassium to magnesium, zinc to phosphorus, and manganese to iron assists in diagnosing suspected magnesium, iron, manganese, and zinc deficiencies.

Interpretation of Recommendations

For specific fertilizer recommendations for corn, soybeans, wheat, and alfalfa, see Extension Bulletin E-2567, Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa, available at county OSU Extension offices or on the Internet at: http://ohioline.osu.edu/e2567/ .