10. Lime, Phosphorus, Potassium, Sulfur, Manganese, Copper, Magnesium, and Calcium Management for Small Grains

By Carl Crozier, Ron Heiniger, and Randy Weisz

ROUTINE SOIL TESTING FOR LIME, PHOSPHORUS, POTASSIUM, AND MICRONUTRIENTS

Soil testing prior to planting is an essential component of a small grain fertility management program. Different fields can vary so widely in pH and nutrient levels that it is impossible to predict optimum application rates without soil test results. It is much more economical to prevent yield losses associated with nutrient deficiencies than to try to correct them once visible symptoms appear. Soil sample boxes, information sheets, test results, and recommendations are provided free of charge by the Agronomic Division of the North Carolina Department of Agriculture & Consumer Services (NCDA&CS). The NCDA&CS Web site is at http://www.ncagr.com/.

Producers should sample each field once every two to three years. For example, fields in a corn-wheat-soybean rotation could be sampled following each soybean harvest. Lime and fertilizer rate recommendations will be given for the next crop to be grown (corn), and the following wheat-soybean double-crop. In addition to providing rate recommendations for specific crops, soil test reports can be used to monitor changes in soils over time.

Commercial laboratories can also provide soil testing services, but producers need to be aware that soil test field calibrations in North Carolina are based on use of the Mehlich-III nutrient extractant, the Mehlich pH buffer, and an alkaline extract for humic matter, all of which are used by NCDA&CS. Although some commercial laboratories will use the Mehlich-III nutrient extractant upon request, the authors are unaware of any currently using the Mehlich pH buffer to determine acidity or the humic matter extraction to determine soil class.

To ensure that the laboratory results represent the actual fertility status of the field, follow specific sampling guidelines. These guidelines can be found in SoilFacts: Careful Soil Sampling, The Key To Reliable Soil Test Information, AG-439-30, which is at http://www.soil.ncsu.edu/publications/Soilfacts/AG-439-30/.

Each soil sample should consist of 15 to 20 cores collected from a relatively uniform area of less than 20 acres. Subdivide fields with distinct soil types or slopes. For conventionally cultivated field crops, collect cores to represent the depth of the plow layer (usually 6 to 8 inches). For established no-till fields, collect cores from the upper 4 inches.

DIAGNOSTIC SOIL SAMPLING

If samples are collected to diagnose an observed problem rather than for routine purposes, submit separate samples to represent the surface soil (0 to 8 inches) and the subsoil (8 to 16 inches). In this case, submit both soil samples and plant tissues from the affected “bad” area and a nearby unaffected “good” area for analysis.

DIAGNOSTIC PLANT TISSUE ANALYSIS

Tissue analysis can determine if an adequate amount of fertilizer has been applied or if a particular nutrient is limiting crop growth. Depending on the nutrient and the timeliness of the testing, application rates can be adjusted for the current crop or for the next crop. Plant tissue analysis is particularly useful in determining the crop need for mobile nutrients, such as nitrogen, sulfur, and boron, and for diagnosis of deficiency symptoms for manganese, copper, or zinc. Tissue testing at Feekes Growth Stage 5 is an important tool in determining fertilizer requirements for topdressing small grains.

Taking a Tissue Test at Feekes Growth Stage 5

First, be sure the wheat is really at this growth stage. Stage 5 occurs when the leaf sheaths of the wheat are strongly erected and splitting the stem shows the growing point to be about 1/2 inch above the root crown (see description of Growth Stage 5 in “Feekes Growth Stages for Small Grains” at http://www.smallgrains.ncsu.edu/Guide/Chapter1.html). Once the first node of the stem, or joint, is visible at the base of the plant, the crop has reached Growth Stage 6. Samples taken during Growth Stage 6 or later usually show lower nutrient concentrations and can result in higher-than-needed fertilizer recommendations. Sampling before Stage 5 can result in nutrient recommendations that are too low. Obtaining a representative tissue sample is similar to obtaining a representative soil sample. Unusual areas of the field should be avoided. If major differences in top growth or crop color are evident in large areas of the field, these areas should be sampled and fertilized separately.

The tissue sample is taken by cutting a handful of wheat tissue at 20 to 30 representative areas in the field. The top growth should be cut approximately 1/2 inch above the ground. Soil particles clinging to the tissue must be brushed from the tissue, and dead leaf tissue must be removed. The individual sample should be placed in a clean paper bag that is large enough to allow good mixing of the total sample. After thorough mixing of the sample, take approximately three handfuls of tissue from the mixed sample, place them in a sample envelope or a clean paper bag, and send or take them directly to the laboratory. Samples not delivered within 24 hours need to be dried in a 160° oven to prevent spoilage. Never package tissue samples in plastic bags.

Taking a Tissue Test after Jointing (Feekes Growth Stage 6 and Later)

Tissue samples may be taken after Feekes Stage 6 to diagnose a crop nutrient problem. Be sure to collect samples from both a “good” area and a “bad” area. In this situation, the upper four leaves from at least 60 plants should be clipped and placed in a sample bag for immediate testing.

SOIL pH AND LIME RECOMMENDATIONS

Lime rate recommendations on the NCDA&CS soil test report are designed to raise the soil pH to a target level of 6.0 for mineral soils, 5.5 for mineral-organic soils, and 5.0 for organic soils. Lime should be applied as early as possible to allow time for neutralizing soil acidity. Liming rates cannot be determined based on soil pH alone; they also depend on residual soil acidity and residual credit for recently applied lime. For more information see SoilFacts: Soil Acidity and Proper Lime, AG-439-17, which is at http://www.soil.ncsu.edu/publications/Soilfacts/AG-439-17.

Proper pH is critical in obtaining good crop growth and yield. All classes of small grains grow best when the pH is near the target level for each soil class. If pH is too low, soluble aluminum and acidity can limit root growth and nutrient uptake. If pH is too high, micronutrients like manganese, iron, copper, and zinc can become unavailable. Small grains are particularly sensitive to manganese deficiencies on sandy coastal plain soils due to low cation exchange capacity, and on organic soils due to low mineral content. In addition to raising soil pH, lime also supplies calcium needed by crops. Dolomitic lime should be used to supply magnesium as well, if recommended by the soil test (which is indicated by a “$” is in the Mg column on NCDA&CS soil test report).

Stunted growth, nutrient deficiency symptoms, and low yield commonly occurs when soil pH is not maintained in the proper range. Often nutrient deficiencies are the result of low or high pH rather than a lack of adequate amounts of the nutrient in the soil. Growers experiencing problems with crop growth should always consider pH as a possible cause.

PHOSPHORUS RECOMMENDATIONS

Phosphorus is an important component of DNA and the proteins that control energy transfer in the plant. It is also part of the proteins that make up the cell wall. Phosphorus controls the intake of materials into the plant cells. It plays a key role in germination and early plant growth, promotes winter hardiness, stimulates the growth of the wheat kernel, and has a role in determining when the plant reaches maturity.

Phosphorus Deficiency Symptoms

Purpling of the leaf margins and bottom leaf surfaces of the lower plant leaves and purpling of the leaf sheaths at the base of the stem are symptoms of phosphorus deficiency. Slow growth or stunting is another sign of phosphorus deficiency. Phosphorus-deficient plants are slow to mature, and green heads are often found in spots in the field at harvest. Deficiency symptoms are often found on waterlogged, cool soils in late winter or early spring.

Phosphorus Fertilizer Rates

As noted previously, a good soil test is the best way to determine fertilizer requirements. The following phosphorus recommendations are made only as guidelines and should not replace the use of soil testing as the primary means of determining crop nutrient needs.

A wheat crop yielding 40 bushels per acre typically requires 40 pounds of P₂O₅ (25 pounds in the seed and 15 pounds in the straw). Mineral soils, such as those found in the coastal plain, bind phosphorus and usually test high in available phosphorus. Heavy organic soils do not bind phosphorus, resulting in a movement of phosphorus to the lower soil horizons or to drainage waters. Soils high in clay content, such as those found in the piedmont, bind phosphorus very tightly making it unavailable to the crop. Consequently, both heavy organic soils and soils high in clay content often test low in available phosphorus even though high amounts of phosphorus fertilizer are applied every year. Care must be taken on these soils to apply phosphorus in a way that limits the interaction between the phosphorus fertilizer and the soil. Since animal wastes are high in phosphorus, soils where heavy applications of animal waste have been applied will have high levels of available phosphorus. Table 10-1 shows the recommended rates for phosphorus fertilizer in the different regions and major soil types of the state.

Phosphorus Placement and Timing

Broadcast phosphorus on the soil just prior to planting. Do little or no tillage in heavy organic or clay soils to limit the amount of soil-fertilizer contact (and thus reduce nutrient binding). Banding phosphorus with the seed is possible using special drill attachments, but this practice has not increased yields or profits in North Carolina. If a phosphorus deficiency is found prior to Feekes Growth Stage 5, apply phosphorus. There is no benefit to applying phosphorus to a growing crop after Feekes Stage 6.

¹ This table does not list all available chemical forms of fertilizers or recommend use of any specific form.
² Soil samples should be taken to avoid underestimating or overestimating actual needs.

POTASSIUM RECOMMENDATIONS

Potassium has two key roles in plant growth. It regulates the opening of the leaf stomata, which control gas and water vapor exchange, and it regulates turgor and ionic balance in the cell, which controls water and nutrient flow into and out of the cell. Potassium influences grain quality and oil content, stem health and stiffness, and plays an important role in drought and disease tolerance.

Potassium Deficiency Symptoms

The most common deficiency symptom for potassium in small grains is stunted growth and early lodging. Plants with a potassium deficiency will have low vigor, poor drought or disease tolerance, and reduced kernel size. Under severe potassium deficiency, the leaf tip and margins on the lower leaves will bronze and eventually turn yellow and die. Deficiency symptoms are more likely on deep sandy soils or soils that are waterlogged and compacted.

Potassium Fertilizer Rate

The following potassium recommendations are only guidelines. Soil testing is the primary means of determining crop nutrient needs. A wheat crop yielding 40 bushels per acre typically requires 64 pounds of K₂O (16 pounds in the seed and 48 pounds in the straw). Because so much of the potassium in the plant is in the straw, most of it will be recycled in the soil. Most of the agricultural soils in North Carolina have adequate to high levels of available potassium. In particular, soils where animal waste has been applied will be high in available potassium. The exception to this rule is that available potassium is low on sandy soils in the coastal plain and tidewater regions of the state. Sandy soils do not bind potassium, so the potassium leaches below the root zone. Applications of potassium fertilizer in single or split applications will be necessary most years on sandy soils. Table 10-1 shows the recommended rates for potassium fertilizer on the major soil types in North Carolina.

Potassium Placement and Timing

Broadcast potassium just prior to planting. On sandy or very sandy soils with a high leaching potential, apply potassium in two applications, half at planting and the other half at Feekes Stage 5 when nitrogen is applied. There is no benefit to applying potassium to a growing crop after Feekes Stage 6.

SULFUR RECOMMENDATIONS

Sulfur increases kernel weight, kernel size, grain protein, and yield. Sulfur is required for the production of chlorophyll and many enzymes involved in the utilization of nitrogen. Consequently, a small grain crop must have adequate amounts of sulfur to use nitrogen fertilizer property.

Sulfur Deficiency Symptoms

Symptoms of sulfur deficiency include yellowing of young leaves, small spindly plants, slowed growth, and delayed maturation. Sulfur deficiency looks very much like nitrogen deficiency, except that with sulfur deficiency the young leaves at the top of the plant are the first to turn yellow. Sulfur deficiency symptoms usually occur in patchy spots across the field. Generally, sulfur deficiencies are only found on deep sandy soils. However, in recent years, sulfur deficiency symptoms have occurred in clay and organic soils during cool, wet weather when the plant is small. Periodic checks in the late winter and early spring can help identify fields with sulfur deficiency.

Sulfur Fertilizer Rate

A wheat crop yielding 40 bushels per acre typically requires 10 pounds of elemental sulfur (4 pounds in the seed and 6 pounds in the straw). While most of the agricultural soils in North Carolina will have adequate to high levels of available sulfur, sandy soils with low levels of organic matter usually are deficient in sulfur because sulfur is water soluble and easily leached. On sandy, sulfur-deficient soils, apply 15 to 25 pounds sulfur per acre at planting or with the nitrogen sidedress.

Sulfur deficiency symptoms may also be seen in crops grown in organic or clay soils. Excess rainfall when the plant is small with a limited root system can leach sulfur below the shallow root zone. In these situations, apply 15 pounds sulfur per acre when deficiency symptoms are noted. Apply sulfur before jointing to avoid crop damage and increase the likelihood of an economic response.

How Late Can Sulfur Be Applied?

The best discussion of this issue is from an extension bulletin dealing with coastal plain soils in Alabama. Grain yield was reduced 23 percent if no sulfur was applied, compared to ammonium sulfate applied at Feekes’ Growth Stage 4. Applying ammonium sulfate at Feekes’ Growth Stage 8 (flag leaf beginning to emerge) increased yields, but only about half as much as the Growth Stage 4 application.

Based on the Alabama work and some general economic calculations:

• response to sulfate forms of sulfur is possible after jointing,
• it is only profitable to apply sulfur this late if the deficiency is severe and there is reasonable potential for a highly profitable yield (estimated 25 to 50 percent yield reduction and a plant tissue test indication that sulfur is deficient “D” and not just low “L”),
• sulfur should be applied as soon as possible to obtain any benefit,
• sulfate is the best form for late applications, followed by thiosulfate,
• do not apply elemental sulfur after jointing, as further delays due to microbial transformations are required prior to plant uptake,
• planning ahead to avoid deficiencies or correcting them at normal topdress time are the best long-term strategies.

CALCIUM AND MAGNESIUM RECOMMENDATIONS

Calcium deficiency symptoms include terminal and root tip damage, dark green stems, weakened stems, and poor ear formation. Magnesium deficiency symptoms include interveinal chlorosis in older leaves, leaf curling, and yellowing of the leaf margins. Generally, calcium and magnesium levels are maintained through dolomitic lime applications. If deficiencies occur and no pH change is desired, apply sulfate forms like gypsum (calcium sulfate), or epsom salts (magnesium sulfate).

MICRONUTRIENT MANAGEMENT (COPPER, MANGANESE, ZINC)

Due to expense and the potential for toxicity, applications of micronutrients (i.e. copper, manganese, and zinc) are generally made only when unless they are specifically recommended by a soil test or if specific deficiencies are identified. Common problems include manganese deficiencies on overlimed soils and copper deficiencies on organic soils. Sources of micronutrients differ in availability, unit cost, application method, and application rate. Consider the total material and application cost of a treatment. There are additional essential elements, but they do not generally limit small grain yields in North Carolina.

Copper Recommendations

Copper is required for the formation of chlorophyll and is an important part of metabolic processes in the plant. Proper levels of copper in the plant enhance protein content of the kernel and grain yield.

Copper Deficiency Symptoms

Common copper deficiency symptoms include stunting, leaf tip or shoot die-back, and poor upper leaf pigmentation. Perhaps the best way to diagnose a copper deficiency is by observing the leaf tip. “Pigtailing” or “corkscrewing” of the leaf tip are signs of copper deficiency. Organic soils are naturally low in copper, and deficiency symptoms often can be found in plants grown in these soils, particularly when the plant and root system are small. Wheat is very sensitive to copper deficiency and will be one of the first crops to show symptoms.

Copper Fertilizer Rate

A wheat crop yielding 40 bushels per acre typically requires 0.04 pounds of elemental copper per acre (0.03 pounds in the seed and 0.01 pounds in the straw). Table 10-1 shows the rate of copper to use when a soil test detects a low level or when deficiency symptoms are noted. Copper is commonly supplied as copper sulfate, although copper oxides, copper chelates or organic complexes, and copper ammonium phosphates are also applied either to the soil or as foliar sprays or dusts. Take care to avoid the overapplication of copper fertilizers since high concentrations of copper can be toxic to the plant.

Timing a Copper Application

Apply copper preplant to avoid the high cost of copper chelates, eliminate the chance of leaf burn, and allow a much longer residual effect. However, if deficiency symptoms occur, apply a foliar spray at a much lower rate than what is recommended for a soil application. Usually, copper chelates or organic dusts are recommended for foliar application. Do not apply copper after jointing.

Manganese Recommendations

Manganese is required for the formation of chlorophyll and is an important part of metabolic processes involving nitrogen. Proper levels of manganese in the plant enhance plant growth and the production of chlorophyll.

Manganese Deficiency Symptoms

Manganese deficiency symptoms include stunting, gray specks in the leaf, and pale to almost whitish upper leaves or streaked yellowing (interveinal chlorosis) of the upper leaves. To distinguish manganese deficiency from a magnesium deficiency, note that manganese effects the upper leaves while magnesium effects the lower leaves. Manganese deficiencies commonly occur in overlimed soils (i.e. mineral soils with pH greater than 6.5) with low cation exchange capacity. A common situation where manganese deficiencies are noted is the overlimed areas at the ends of the field where the spreader truck turned or where lime was stockpiled. Avoid stockpiling of lime in fields and apply lime only as recommended by soil analysis.

Manganese Fertilizer Rate

A wheat crop yielding 40 bushels per acre typically requires 0.25 pounds of elemental manganese (0.09 pounds in the seed and 0.16 pounds in the straw). Sandy soils in the coastal plain are typically low in available manganese; organic and clay soils in the tidewater and piedmont regions generally have high levels of available manganese. Table 1 shows the rate of manganese to use when soil test levels are low or when deficiency symptoms are noted.

Timing of Manganese Fertilizer Application

The best time to apply manganese on soils with low test levels is preplant. To correct a deficiency if the soil pH is high, apply foliarly. Manganese is commonly supplied as manganese sulfate, manganese oxide, and manganese chelates or organic complexes. Manganese oxide must be finely ground to be effective. Manganese sulfate can be effectively applied either to the soil or to the crop foliage. Manganese chelates and organic complexes are recommended only for foliar application due to soil reactions that tend to convert the manganese to unavailable forms. Application of foliar fertilizers may have to be repeated several times to correct severe deficiency symptoms on fields that have been overlimed. Once wheat is jointing, consider whether response to fertilizer is likely to outweigh crop damage due to traffic.

Zinc Recommendations

Zinc deficiency symptoms include decreased stem length (rosetting), mottling, and interveinal chlorosis. Zinc deficiencies are most common if the soil pH is greater than 6.5 and the soil phosphorus index is greater than 75. As with other micronutrients, recommended rates (Table 10-1) are lower for foliar applications, but residual effects are greater with soil applications.

SPECIAL CONSIDERATIONS FOR NO-TILL

Before a field is placed in 100 percent no-till production, test the soil and bring it to target pH and optimum nutrient levels. Once adequate fertility levels are achieved throughout the root zone, no-till production can begin. Long-term no-till studies suggest that yields and soil fertility can be maintained even though lime and fertilizer are applied to the soil surface without incorporation. As previously mentioned, routine soil samples in established no-till fields should be collected to a depth of 4 inches. Use of starter fertilizers is more important in no-till since plant development is delayed.

SPECIAL CONSIDERATIONS FOR REDUCING RUNOFF AND PROTECTING WATER QUALITY

Farming systems are open to the environment, and fertilizers applied to fields can move into ground and surface waters. Use management practices that maximize yield without increasing fertilizer rates, thereby leaving less residual fertilizer in the environment. Nutrient management planning optimizes the conversion of fertilizer to grain. Installation and management of water control structures can reduce fertilizer runoff and minimize drought stress in fields. Additional best management practices include riparian buffer strips and field borders, which offer advantages for water quality, but are agronomically less effcient since they treat fertilizer after it has left the field. For more information see the following extension SoilFacts publications: Agriculture and Coastal Water Quality, AG-439-10; Best Management Practices for Agricultural Nutrients, AG-439-20; and Nitrogen Management and Water Quality, AG-439-2. All can be found at http://www.soil.ncsu.edu/publications/Soilfacts/.

SPECIAL CONSIDERATIONS FOR ORGANIC SOILS

Since organic matter binds with aluminum, aluminum toxicity is less important in organic soils with a lower soil pH than the target (6.0) for mineral soils. In addition, micronutrients like copper, manganese, and zinc become less soluble in organic soils, and thus are more likely to become limiting as pH increases. North Carolina’s organic soils have low natural pH and high cation exchange capacities. Consequently, large amounts of lime are needed to raise the pH. Therefore, the target pH of organic soils can be lowered to 5.0, and the target pH for mineral-organic soils set at 5.5. Crops grown on overlimed organic soils should be monitored for copper, manganese, and zinc deficiencies.

Since organic matter contains phosphorus that can be mineralized for crop uptake, crop growth may be adequate on organic soils despite low phosphorus indexes. Nevertheless, inorganic phosphorus tends to leach from organic soils since fewer mineral adsorption interactions occur. Thus, apply phosphorus fertilizers as near the planting date as possible rather than several months prior to planting.

Just as soil-applied herbicides are often less effective on organic soils, organic compounds like urease/ nitrification inhibitors may also be less effective in organic soils.

SPECIAL CONSIDERATION FOR PRECISION AGRICULTURE

Technological advances in computer information systems, global positioning systems, yield monitors, and application equipment offer the potential to manage fertility within each field by subdividing the field into many small units. More intensive management permits lime and fertilizer applications to be made only as needed within variable fields, thus avoiding localized nutrient deficiencies, cutting costs, and reducing the potential for negative offsite impacts. In practice, the underlying factors controlling yield variability need to be understood before efficient management decisions can be made (see Soil Sampling for Precision Farming, AG-439-36, at http://www.soil.ncsu.edu/publications/Soilfacts/AG-439-36/AG-439-36.pdf).

Currently, precision agriculture is being used 1) to identify areas in fields with different pH or soil test indexes, and vary lime and fertilizer rates accordingly, 2) to monitor and map crop yield and moisture content, and 3) for documentation of material applications. The cost of collecting grid soil samples or using a yield monitor must be returned by decreasing the amounts of lime or fertilizer applied, increasing crop yield, reducing negative environmental impacts, or by some combination of these benefits. Growers are more likely to increase profits by using precision farming practices in situations where pH or fertility levels are limiting wheat yields. An examination of the variability in soil pH or fertility within a field should indicate the potential for increasing crop yield through variable rate lime or fertilizer applications. If at least 1/4 of the field area has soil nutrient indexes below 25 or pH levels below the target value for that crop and soil class, then it is likely that precision farming practices will increase wheat yields and profits.

SPECIAL CONSIDERATIONS FOR ANIMAL WASTES AND SEWAGE SLUDGE

Animal waste and sewage sludge can be an excellent source of nutrients and organic matter for a wheat crop. Organic forms of phosphorus can move deeper in soils than do inorganic fertilizer sources. Consequently, they can be advantageous in no-till or conservation tillage systems. When applying animal waste as a fertilizer material for wheat, all amendments should be tested before application to determine optimum application rates. Soils that are being fertilized with waste materials should be tested to determine nutrient levels. The amount of waste material to be applied should be based on the need for desirable nutrients, such as phosphorus or potassium, and the requirement that levels of phosphorus, zinc, copper, cadmium, lead, and mercury should not exceed prescribed limits. Producers should rotate applications as much as possible to obtain nutrient benefits while minimizing excess nutrient and toxic metal accumulation. When lime-stabilized sludge or poultry litter is used, the soil pH should be carefully monitored to prevent overliming and possible manganese deficiency. Several SoilFacts publications may help, including: Land Application of Municipal Sludge, AG-439-3; Nutrient Content of Fertilizer and Organic Materials, AG-439-18; Permit Guidelines for Application of Municipal Sludge on Agricultural Lands, AG-439-6; Poultry Manure as a Fertilizer Source, AG-439-5; Swine Manure as a Fertilizer Source, AG-439-4; Using Municipal Solid Waste Compost, AG-439-19; and Waste Analysis, AG-439-33. More information can be found at
http://www.soil.ncsu.edu/about/publications/.

This file is a chapter from Small Grains Production Guide, 2004-05. Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the NC Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension Center.

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9/04—3M—JMG (Revised) AG-580

EO4-43980

Last Revised Sept. 2004