NC State Extension Publications

Identifying Nutrient Needs

Soil testing for lime, P, K, Mg, and micronutrients

Soil testing prior to planting a crop is an essential component of a soil 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). Producers should sample each field once every 2 to 3 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 second crop to be grown (wheat). In addition to obtaining 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. Some commercial laboratories will use the Mehlich-III nutrient extractant and the Mehlich pH buffer upon request. Nevertheless, we are unaware of other laboratories currently calculating lime rates according to the NCDA&CS protocol which considers acidity, soil class, and recent lime application.

To insure that the laboratory results represent the actual fertility status of the field, soil specific sampling guidelines should be followed (see: SoilFacts: Careful Soil Sampling – The Key to Reliable Soil Test Information, AG-439-30). Each soil sample should consist of 15-20 cores collected from a relatively uniform area of < 20 acres. Fields with distinct soil types or slopes should be subdivided. For conventionally cultivated field crops, cores should be collected to represent the depth of the plow layer (usually 6-8 inches). For established no-till fields, cores should be collected from the upper 4 inches.

Diagnostic plant tissue analysis and problem soil sampling.
If samples are collected to diagnose an observed problem rather than for routine purposes, then separate samples should be submitted to represent the surface soil (0-8 inches), and the subsoil (8-16 inches). In this case, both soil samples and plant tissues from the affected "bad" area and a nearby unaffected "good" area should be submitted for analysis. For seedlings less than 12 inches, collect the entire aboveground portion of 15-20 plants; from seedling to tasselling, collect uppermost fully developed leaf of 10-15 plants; and from tasselling to silking, collect the earleaf of 10-15 plants. There is no reliable system for testing plants after silking. Plant samples should be placed in a paper bag to permit drying, and shipped as soon as possible for laboratory analysis.

Lime Recommendations

Soil pH and Liming
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. Dolomitic lime contains magnesium as well as calcium, and can be used when soil test magnesium levels are low ($ in the Mg column of the recommendations section). 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 Use, AG-439-17.

Corn grows 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. If pH is too high, micronutrients such as manganese, iron, copper, or zinc can become unavailable. Micronutrient deficiencies are particularly problematic on sandy coastal plain soils due to low cation exchange capacity (CEC) and on organic soils due to low mineral contents.

Fertilizer Recommendations

Recommended rates of fertilizers along with other nutrient specific information is presented in Table 3-1. In most cases, fertilizer materials should be applied prior to planting with a few notable exceptions. Since nitrogen fertilizers are expensive and mobile, with potential for offsite pollution, application should be timed with periods of plant uptake. In sandy soils with low CEC, leaching of both potassium and sulfur may require split applications of these nutrients.

Table 3-1. Critical nutrients for corn production.
Element Common Deficiency Symptoms Common Fertilizer Forms1 Basis for Fertilizer Rate Suggested Rates if no Soil Test Data Available2 Notes
Nitrogen (N) Stunting, yellowing of lower leaves Urea (46-0-0); anhydrous ammonia (82-0-0); nitrate (+ammonium 34-0-0, calcium 15.5-0-0-19Ca, potassium 13-0-45, or sodium 16-0-0); ammonium (+chloride nitrate, phosphate, sulfate 21-0-0-24S); urea ammonium nitrate solution 28-32% R.Y.E.3, regional recommendation Grain: 1-1.25 lb N/bu or 120-160 lb N/ac4
Silage: 10-12 lb N/ton or 180-220 lb N/ac
Split N: 30-40 lbs./acre at planting, remainder at 15-24 inch stage. If irrigated: increase rate by 10-15%
Phosphorus (P) Stunting, purpling, dark green, delayed maturity, poor ear development Granular monoammonium phosphate (MAP, 11-52-0) and diammonium phosphate (DAP, 18-46-0), liquid ammonium phosphate (10-34-0) Soil test Grain: 30-50 lb P2O5/ac
Silage: 40-60 lb P2O5/ac
Starter band if no-till or cool, wet soils
Potassium (K) Lower leaf tip and margin burn, weak stalks, small ears, slow growth Potassium [+chloride (muriate 0-0-60), sulfate, nitrate, hydroxide, or magnesium sulfate] Soil test Grain: 80-100 lb K2O/ac
Silage: 100-120 lb K2O/ac
On deep sand, apply just before planting or split apply
Calcium (Ca) Terminal and root tip damage, dark green, weakened stems, ear disorders Lime, calcium sulfate (gypsum) Soil test Generally OK if limited to target pH
Magnesium (Mg) Interveinal chlorosis in older leaves, leaf curling, margin yellowing Dolomitic lime, magnesium sulfate (epsom salt), potassium magnesium, sulfate, magnesium oxide Soil test, tissue analysis If needed: 20-30 lb Mg/ac Generally OK if dolomitic lime used
Sulfur (S) Yellowing of young leaves, small spindly plants, slower growth and maturation Elemental sulfur; sulfate [+ammonium, calcium (gypsum), magnesium (epsom salt), potassium, potassium magnesium]; ammonium thiosulfate; sulfur-coated urea Tissue analysis or soil criteria If deficient: 20 lb S/ac Deficiency likely if sandy surface is 18+ inches deep
Zinc (Zn) Decreased stem length (rosetting), mottling-striping-interveinal chlorosis Zinc sulfate, zinc oxide, zinc chelates, zinc chloride Soil test, tissue analysis If deficient: apply 0.5 lb Zn/ac to foliage, or 6 lb Zn/ac to soil
Iron (Fe) Interveinal chlorosis of young leaves Ferrous sulfate, ferric sulfate, ferrous ammonium sulfate, iron chelates Tissue analysis
Manganese (Mn) Upper leaves pale green or streaked Manganese sulfate, manganese oxide, manganese chelate, manganese chloride Soil test, tissue analysis If deficient: apply 0.5 lb Mn/ac to foliage, or 10 lb Mn/ac to soil Overlining decreases availability
Copper (Cu) Stunting, leaf tip/shoot dieback, poor upper leaf pigmentation Copper sulfate, copper oxide, copper chelates Soil test, tissue analysis If deficient: apply 0.25 lb Cu/ac to foliage, or 2-6 lb Cu/ac to soil
Boron (B) Leaf thickening, curling, wilting, reduced flowering, pollination Boric acid, borax, solubor, borates Tissue analysis Avoid toxicity, apply only as needed
1 This table does not list all available chemical forms of fertilizers or recommend use of any specific form. Percent chemical analyses included are examples only, and may not reflect the composition of any specific commercial source.
2 Soil samples should be taken to avoid underestimating or overestimating actual needs.
3 R.Y.E. = realistic yield expected
4 NCDA&CS guidelines are 150-160 lb N/ac for sandy plain soils, 130-150 lb N/ac for piedmont and mountain soils, and 120-130 lb N/ac for organic soils.
5 NCDA&CS guidelines are 2 lb Cu/ac for mineral soils, 4 lb Cu/ac for mineral-organic soils, and 6 lb Cu/ac for organic soils.

Nitrogen (N) Management
Nitrogen is essential in the production of proteins and chlorophyll. Careful management is critical since N frequently limits crop yields, it is expensive, there are numerous sources and application strategies to chose from, and there is potential for runoff or leaching into surface or ground waters. The basic components of N management are rate, timing, form, and placement.

Rate: Table 3-2 shows the calculation of N rates for corn based on "realistic yield expected" (R.Y.E.). Decades of field experiments suggest that producing each bushel of grain requires 1 to 1.25 lb N, and each ton of silage requires 10-12 lb N. realistic yield expectation values are available for each soil series through the Natural Resources Conservation Service and are published in county soil surveys. Whenever possible, producers should use their own farm records to determine R.Y.E., since this more accurately reflects their level of management and actual field conditions. Realistic yield expectations can be calculated as the average of the best 3 out of 5 most recent years for a farm, field, or smaller area such as soil type within a field.

Table 3-2. Calculating N rates for corn based on realistic yield expected (R.Y.E.).
R.Y.E. x Factor Total N (lb N /acre) Starter N1 (lb N/acre) Sidedress N (lb N/acre)
Grain: 125 bu/acre (1-1.25 lb N/bu)
Low rate 1 125 31 94
High rate 1.25 156 39 117
Silage: 20 tons/acre (10-12 lb N/ton)
Low rate 10 200 50 150
High rate 12 240 60 180
1 Starter rate calculated as 25% of total N rate or denitrification during wet periods. Therefore, the majority of the N fertilizer should be applied as sidedress application.

Although prior legumes and other well-fertilized crops can leave considerable amounts of residual N in soils, it has not yet been possible to quantify the availability of this N to a subsequent corn crop in North Carolina. Weather events can lead to substantial leaching, runoff, or denitrification of residual N. As a general rule, the lower end of the recommended N rate range (1 lb N/bu grain or 10 lb N/ton silage) is more appropriate under conditions when residual N is expected.

Timing: Nitrogen fertilizer can be applied pre-plant and/or as a sidedress application prior to silking. Several studies have shown that split nitrogen applications with up to 40 lbs N per acre applied as a starter fertilizer and the rest at sidedress produce the highest yields. Since up to 35 percent of the total N used by the corn crop can be taken up after pollination, at least some of the N should be applied as a sidedressing. Excess N applied early can be lost on sandy soils or with irrigated corn due to leaching, and on finer textured soils due to runoff.

Source: Fertilizer N is available in numerous commercial forms. In addition to different chemical forms, N sources can be coated or treated with nitrification or urease inhibitors to increase the period of availability for crop uptake. Numerous animal and municipal waste sources are also available. Sources should be evaluated based on the costs of the material as well as handling and application costs, available nutrient content, and convenience.

Placement: N placement is determined by the source chosen, the need to obtain rapid N uptake when planting in cool, wet soils, and the available distribution equipment. Although most N forms can be broadcast on the surface (anhydrous ammonia must be injected to a depth of 5-10 inches since it is a volatile liquid), there are sometimes advantages to applying starter or sidedress fertilizer in bands. Studies have shown that fertilizer materials containing urea (urea or 30% UAN) will volatilize when applied to warm, dry soils, particularly in situations where large amounts of crop residue covers the soil surface. This results in a loss of N and lower yields. Growers using no-till practices should consider banding or injecting fertilizer materials containing urea.

Placement of starter fertilizer is particularly important. The small rooting zone makes it difficult for the young corn plant (less than 14 days from emergence) to take up adequate amounts of nitrogen. A recent study in North Carolina comparing broadcast, in-furrow, and 2 X 2 (2 inches to the side and 2 inches below the seed) banded applications found that banding N in a 2 X 2 band was the best method of placing starter fertilizer in both conventional and no-till fields.

Macronutrient Management

Phosphorus (P): Phosphorus is essential for rapid plant growth. Stunting and purplish or dark green coloration are common deficiency symptoms. Deficiencies are more likely with cool, wet weather when both root exploration of soil and P supply to roots are slowed down. Starter band application of P (along with some N) does not always lead to yield increases, but it can be an effective tool to enhance early season growth and reduce risks of losses associated with billbugs, competitive weeds, and summer droughts (see SoilFacts: Starter Fertilizers for Corn Production, AG-439-29).

Potassium (K): Potassium is critical in maintaining plant salt balance and regulating water and sugar movement through the plant. Since K is highly mobile, deficiency symptoms are usually first noted in older leaves. The soil cation (K+), can be held on soil particles, but leaching is likely in deep sandy soils where CEC is low. Split applications of K should be made before planting and at 21 days after emergence on very sandy soils.

Sulfur (S): Deficiency symptoms include the yellowing of young leaves, small spindly plants, slower growth, and delayed maturation. These symptoms usually occur in patchy areas across the field. Generally, S is not needed except on deep sandy soils. However in recent years, S deficiency symptoms have been found on clay and organic soils during periods of cool, wet weather when the corn plant is small. On known S deficient soils, 20 lb S/acre can be applied at planting or with the N sidedress.

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

Micronutrient Management

Due to their expense and the potential for inducing toxicity, applications are generally not made unless specific deficiencies are identified. Common problems include manganese deficiencies on overlimed soils and copper deficiencies on organic soils. Many sources of micronutrients are utilized, these differ in availability, unit cost, application method, and application rate. Producers should consider the total material and application cost of a treatment. Besides the elements listed in this publication, there are additional essential elements which are not generally recognized as limiting corn yields in North Carolina.

Copper (Cu): Common Cu deficiency symptoms include stunting, leaf tip/shoot dieback, and poor upper leaf pigmentation. Perhaps the best way to diagnose a Cu deficiency is by observing the leaf tip. Pigtailing or corkscrewing of the leaf tip is a sign of Cu deficiency. Organic soils are naturally low in Cu and often deficiency symptoms can be found on these soils. 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. Copper chelates and organic complexes can be applied foliarly at much lower rates than are recommended for soil applications. However, soil applied copper should have much longer residual effects.

Manganese (Mn): Manganese deficiency symptoms include pale to almost whiteish upper leaves or streaked yellowing of the upper leaves. Manganese deficiency can be distinguished from a Mg deficiency in that Mn effects the upper leaves while Mg effects the lower leaves. Manganese deficiencies commonly occur in overlimed soils. Avoid stockpiling of lime in fields and apply lime only as recommended by soil analysis. 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 applications due to soil reactions that tend to convert the manganese to unavailable forms.

Zinc (Zn): Zinc deficiency symptoms include decreased stem length (rosetting), mottling-striping, and interveinal chlorosis. Zinc deficiencies are most common if soil pH is high and soil P levels are high. As with other micronutrients, recommended rates are lower for foliar applications, but residual effects are greater with soil applications due to the higher rates.

Special Topics

No-till corn production
Since lime and fertilizers will be added to the surface or only placed in shallow bands, care should be taken to develop adequate fertility levels throughout the root zone depth prior to adopting no-till. Long-term no-till studies suggest that yields and soil fertility can be maintained even though all lime and fertilizer are applied to or near the soil surface. 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 soil warming is delayed and the cooler temperatures can reduce the rate of crop growth. Immobilization and volatilization losses of N fertilizers can be reduced by methods which move them below the surface residue layer. Subsurface placement is best, followed by surface dribbling. The least efficient form of N fertilization is surface broadcasting of urea.

Irrigated corn production
Due to higher yield potential and the possibility of leaching, N rates should be increased by 10-15% if irrigation is used. Additional, or split applications of, K and S may also be warranted. Water sources should be analyzed prior to investing in irrigation systems, and tidewater region producers should be aware that the salinity of coastal region rivers can vary dramatically with rainfall and tidal fluxes.

Reducing runoff and protecting water quality
There is increasing awareness that farming systems are open to the environment, and fertilizers applied to fields can move offsite into ground and surface waters. Producers should pay attention to any management practice with the potential for higher yields without increasing fertilizer rates, thereby leaving less residual fertilizer in the environment. Nutrient management planning is an efficient tool to optimize 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 (BMPs) include riparian buffer strips and field borders, which offer advantages for water quality, but are agronomically less efficient since they treat fertilizer after it has left the field. For more information see the following extension publications: SoilFacts: Agriculture and Coastal Water Quality, AG-439-10; SoilFacts: Best Management Practices for Agricultural Nutrients, AG-439-20; and SoilFacts: Nitrogen Management and Water Quality, AG-439-2.

Organic soils
The chemistry of organic soils differs in several respects from that of mineral soils. The most important soil fertility factors recognized in North Carolina are: target soil pH, Cu deficiency potential, P leaching potential, and the efficacy of organic compounds such as urease/nitrification inhibitors. Since organic soils contain less minerals and thus less aluminum, aluminum toxicity is less important at lower soil pH. In addition, micronutrients such as copper, manganese, and zinc become less soluble and thus more likely to become limiting as pH increases. Furthermore, the organic soils in North Carolina have a low natural pH and high CECs, requiring large amounts of lime to raise the pH. Therefore, the target pH of organic soils can be lowered to 5.0, and of mineral-organic soils to 5.5. Crops grown on overlimed organic soils should be monitored for copper, manganese, and zinc deficiencies.

Since organic matter contains P that can be mineralized for crop uptake, crop growth may be adequate on organic soils with lower soil test indexes than those for mineral soils. Nevertheless, inorganic P tends to leach from organic soils since fewer mineral adsorption interactions occur. Thus, producers should apply P fertilizers as near the planting date as possible and avoid applying P several months prior to planting.

As with soil-applied herbicides which are often less effective on organic soils, the efficacy of organic compounds such as urease/nitrification inhibitors needs to be verified in the organic soil environment.

Precision agriculture
Technological advances in computer information management, global positioning systems, yield monitors, and application equipment offer the potential to manage crops 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 SoilFacts: Soil Sampling for Precision Farming Systems, AG 439-36.

Currently, precision agriculture is being used to 1) identify areas in fields with different soil test indexes, and vary lime and fertilizer rates accordingly, 2) monitor and map crop yield and moisture content, and 3) recordkeeping and documentation of material applications. Variable lime or fertilizer applications should lead to more uniform yields across the field. Grid soil sampling to assess soil variability should consider natural soil boundaries and treat these as separate units, as is recommended for conventional soil sample collection. Yield maps have the potential to direct integrated problem analyses focusing on yield limiting factors such as drainage, soil texture, or perhaps weed or other pest infestation. Such efforts could lead to the design of site-specific schemes for N fertilization, tillage, plant population or variety selection, or pesticide applications. Streamlined farm recordkeeping is likely to become more important as farm sizes continue to increase and as nutrient and pesticide management come under increasing scrutiny.

Animal wastes and sewage sludge
All amendments should be tested and soils should be monitored for: desirable nutrients (usually N and P), excess nutrient accumulation (P, Zn, Cu), liming effects (i.e. lime-stabilized sludge or poultry layer litter), and toxic metals (cadmium (Cd), lead (Pb), mercury (Hg). Producers should rotate applications as much as possible to obtain nutrient benefits while minimizing excess nutrient and toxic metal accumulation. Since organic P forms can move deeper in soils than do inorganic fertilizer sources, they can be advantageous in no-till or conservation tillage systems. Nevertheless, this should also lead to caution about excess applications. There are several North Carolina State University Soil Science extension publications describing the use of animal and municipal wastes.



Cooperative Extension Soil Science Specialist
Crop and Soil Sciences

Publication date: Jan. 1, 2003

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