NC State Extension Publications

 

Soil fertility programs are designed to optimize crop yields, improve farm efficiency, reduce the cost of production, conserve natural resources, and protect the environment by minimizing excessive use of fertilizers (particularly nitrogen and phosphorus). An effective soil fertility program considers soil testing coupled with plant tissue analysis. Plant tissue analysis helps determine sufficiency of nutrients not included in soil testing and determines if nutrients present in the soil are being taken up into the plant. In addition, appropriate management of tillage, crop residues, water, and pest management are needed to achieve high productivity levels.

Soil Testing Guidelines

Chemical aspects of soil fertility are monitored through soil testing and plant tissue analysis. The soil testing component of the program documents and involves the soil sampling strategy, the laboratory analysis process, interpretation of laboratory results and recommendations, selection of specific amendments or fertilizers, and application of materials with calibrated equipment.

Sampling

Because the reliability of the soil test can be no better than the sample you submit, it is essential that you take samples in a way that accurately represents the soil on your farm.

Sample depth for cultivated fields is usually approximately 6 to 8 inches because this is where fertilizer and lime are incorporated (Figure 6-1). For continuous no-till fields, a 4-inch sampling depth is recommended. Occasionally sampling these no-till fields with a 0- to 4-inch sample and a separate 4- to 8-inch deep sample can demonstrate the degree of pH and nutrient stratification.

Samples should be collected with a chrome-plated or stainless-steel sampling probe and a plastic bucket that are clean and free of lime or fertilizer residues. Multiple cores should be taken to represent the sampling area, and they should be mixed thoroughly before transferring to the soil sample box. Avoid sampling when soil is too wet as it will be difficult to mix the cores. As a rule, if it is too wet to plow, it is too wet to sample.

Samples should be collected three to six months before planting time. This will allow you to get the soil test report back in time to plan a liming and fertilization program. Growers in the coastal plain should plan to soil test every one to two years as the sandy soils in that region do not hold nutrients as long as soils in the other parts of the state. The nutrient levels in the silt and clay loams in the piedmont and mountains change less rapidly. Sampling these areas once every three to four years is usually sufficient.

For a more thorough description of soil sampling recommendations, see SoilFacts: Careful Soil Sampling-The Key to Reliable Soil Test Information.

Sampling Strategy

Traditional field sampling guidelines are to collect one composite sample consisting of 15 to 20 cores for no more than 20 acres of the same soil type (Figure 6-2). Although this approach has proven very useful for decades, better approaches are now recommended. Technological advances in GPS and GIS allow more intensive soil sampling schemes—called precision soil sampling—so that nutrients can be applied more precisely via variable rate equipment.

Zone sampling is a precision sampling technique that utilizes soil type information, field topography from observation, yield maps, and other knowledge of production history to distinguish management zones for sample collection. A zone may be as small as 2 acres but may be 10 to 15 acres if site characteristics and production history are uniform. Considerable time, knowledge of soils, field management, and production history are needed to delineate zones. In a zone, 15 to 20 cores are recommended for a composite sample.

Grid sampling is another precision sampling technique utilized by much of the fertilizer industry. Typically, a 2-acre grid is used to delineate sampling units. Samples are either taken as point samples where grid lines intersect or randomly within each 2-acre grid. This approach to sampling is easier than zone sampling because less expertise and time in delineating zones are required.

Several potential benefits of precision soil sampling and variable rate applications exist. Variable application rate technology should minimize nutrient deficiencies and reduce costs by adjusting application rates to meet localized lime and fertilizer needs in each sub-section of a field. If these benefits can be met, then more efficient nutrient utilization should lead to higher yield potential while also reducing runoff and leaching risk.

For a description of more detailed precision agricultural soil sampling schemes, see SoilFacts: Soil Sampling for Precision Farming Systems.

Laboratory Analysis

The Soil Testing Laboratory of the North Carolina Department of Agriculture & Consumer Services (NCDA&CS) Agronomic Services Division provides detailed chemical soil analyses including levels of most major plant nutrients (phosphorus, potassium, calcium, magnesium, and sulfur), several micronutrients (copper, manganese, zinc), and sodium. The Soil Testing Lab also determines pH, acidity, soil class, humic matter, percent base saturation, cation exchange capacity, and weight-to-volume ratio.

Most of the year (generally April through November), routine NCDA&CS soil tests are provided at "no direct cost" to North Carolina residents. From approximately Thanksgiving through March, however, a peak-season fee of $4 is charged for the processing of all soil samples. The fee’s purpose is to encourage early sampling so the lab can operate more efficiently for faster customer service. The laboratory provides routine predictive analyses as well as more detailed diagnostic services that allow for the submission of linked soil and plant tissue samples from problematic areas.

The NCDA&CS Agronomic Services Soil Testing Section utilizes the Mehlich-3 extractant for nutrients, the Mehlich buffer to measure acidity for determining lime requirement, and a humic matter determination to distinguish soil classes for more appropriate lime recommendations. In September of 2017, the lab began measuring pH using a weak calcium chloride (0.01 M CaCl2) solution; this is referred to as a salt pH method.

Private soil testing laboratories are also an option. Because different laboratories may utilize different methods and make different recommendations, producers should verify these methods and recommendations and select appropriate options.

Interpretation of Laboratory Soil Results

For a soil test to be of value, field studies that relate crop performance to laboratory measurements are essential. These established correlations are the basis for valid fertilizer recommendations and are best attained using local data. With new crop varieties and management systems, these interpretations should be periodically verified.

Soil test interpretations and recommendations consider that fertility is not the only factor that limits yield. Soil pH, soil moisture, planting dates, crop varieties, weeds, insects, diseases, nematodes, soil physical conditions, and other variables can also limit production. Therefore, the goal of Agronomic Services’ soil test recommendations is not to achieve a specific yield but to prevent fertility from being a yield-limiting factor. In addition, soil test recommendations help curb excessive nutrient application, which is both economically and environmentally unsound. Data from numerous research studies around the world suggest that once the soil test nutrient concentration reaches a certain level, no crop response is likely even at very high crop yield levels.

Figure 6-3 elemental soil test index

A — expected response very high

B — response low to medium

C — little or no response expected

D — test levels far in excess of plant requirements, no response.

The publication Crop Fertilization Based on North Carolina Soil Tests describes the index rating system and the interpretation of the NCDA&CS soil test report.

Figure 6-1. Routine soil samples should be collected from the to

Figure 6-1. Routine soil samples should be collected from the topsoil layer (6–8 inches deep). For problem investigations, separate subsoil layer samples can be collected to detect nutrients that leach below the topsoil.

Figure 6-2. Recognizably different soil region, if of significa

Figure 6-2. Recognizably different soil region, if of significant acreage, should be sampled separately since they can vary in pH and nutrient composition.

Figure 6-3. Graph illustrating the index rating system for P and

Figure 6-3. Graph illustrating the index rating system for P and K and the interpretation of the NCDA&CS soil test report.

Modified from Hatfield AL. 1972 and published in "Crop Fertilization Based on North Carolina Soil Tests"

Plant Tissue Analysis

Plant tissue analysis is useful because it determines sufficiency of nutrient concentrations actually present in the plant. Some nutrient elements are not included in routine soil testing, and there are conditions where nutrients present in the soil are limited from being taken up into the plant.

Plant tissue analysis measures the nutrient concentrations within plants, which is useful for nutrients such as nitrogen (N), iron (Fe), and boron (B), which are not included in routine soil testing at NCDA&CS. Additionally, if crop growth is limited by a factor such as soil pH, soil moisture, planting dates, crop varieties, weeds, insects, diseases, nematodes, soil physical conditions, or other conditions, nutrients present in the soil may not be taken up by plants. Tissue analysis coupled with soil analysis can help determine the causes of resulting nutrient deficiencies.

Pay close attention to collection of appropriate plant samples. For soybean seedlings (less than 12 inches tall), collect the entire tops from 20 to 30 plants cut at 1 inch above the soil line. For larger plants up to the time of full bloom, collect the most recent mature leaves, including the petiole, from 20 to 30 plants. Typically, this is about the fourth leaf down from the bud of the plant. Sampling after pod set is not recommended because there are no established sufficiency levels for later growth stages.


Table 6-1. Soybean plant tissue sampling procedures and sample codes for the NCDA&CS Plant Tissue Laboratory.
Growth Stage Plant Part to Collect # Plants or Leaves to Collect
Description Code Description Code
Seedling S Entire top of plant cut 1 inch above soil W 20 to 30

Prior to bloom, during initial bloom, before pod set. (Sampling after pods begin to set is not recommended.)

E,B

(F, not recommended)

Most recent mature leaf

M

20 to 30


For laboratory guidelines on appropriate sample collection and report interpretation, see the NCDA&CS Agronomic Services, Plant/Waste/Solution/Media Analysis Section Plant Tissue Analysis Guide.

Nutrient Recommendations

Soybeans remove a relatively large amount of nutrients from the soil compared to other crops. Total nutrient uptake depends on yield, which varies based on variety, soil, cultural practices, and weather. Soybeans take up relatively small amounts of nutrients early in the season, but the daily rate of nutrient uptake increases for most nutrients around stages R2 to R4.

Soybean yields are best on soils that are medium-high in phosphorus and high in potassium. The estimated uptake and removal rate of the primary and secondary nutrients contained in a 50 bu/A yielding crop is shown in Table 6-2.


Table 6-2. Estimated nutrient requirements in 50 bu/A soybeans.
N P2O5 K2O Ca Mg S Cu Mn Zn

(lb per acre)

Uptake

277

56

148

49

19

35

0.05

0.06

0.05

Removal

188

40

74

19

10

23

0.05

0.06

0.05


While these quantities are high, this should not be interpreted as the amount of fertilizer to apply each season. Nutrient applications will vary according to soil type, residual nutrient status (as measured by soil tests), and soil pH. Fertilizer and lime applications should be based on current, accurate soil tests. Studies in North Carolina and several other soybean producing regions are evaluating whether or not soil test interpretations should be modified to generate different nutrient input recommendations for higher yielding crop systems. Currently it has not been clearly demonstrated that higher fertility inputs lead to higher yields. Nevertheless, improved management practices that lead to higher yields should reduce residual soil nutrient levels as measured by soil tests and lead to higher fertilizer input recommendations using current guidelines.

Macronutrients

These are the most important nutrients due to the quantity required by crops and the frequent application needed.

Nitrogen (N)

Effectively nodulated soybeans can usually fix sufficient N for optimum plant growth. When soybeans have been a regular rotational crop in a field with effective nodulation in the past, sufficient inoculum is likely to be present. For fields without a history of soybean production, or fields experiencing extended flooding or other severe disturbance, inoculation is strongly encouraged. Repeated field experiments have shown that inorganic N applications to nodulated soybeans are not likely to be profitable. Plant tissue analysis, however, should be a useful tool to detect N deficiencies should they occur. Visual symptoms of nutrient deficiency typically involve yellowing of lower leaves (Figure 6-5).

Phosphorus (P) and Potassium (K)

Phosphorus and potassium recommendations provided by soil testing are expected to resolve most yield limitations. Because soybeans are late feeders of K, very early application of K prior to planting is not recommended, especially on low CEC, sandy soils that are prone to leaching. P deficiency in soybean typically results in stunted plant growth, while K deficiency usually results in yellowing and necrosis of lower leaf margins (Figure 6-6).

Calcium (Ca) and Magnesium (Mg)

Calcium and magnesium can be supplied through the application of agricultural limes. If Mg is needed, growers should use dolomitic lime in pH management. Additional inorganic sources (calcium sulfate, potassium magnesium sulfate) are available in cases where soil pH is already at optimum levels but Ca and Mg are low. Magnesium deficiency in soybean usually causes lower leaf interveinal chlorosis (Figure 6-7). Calcium deficiency symptoms separate from low pH effects are not typically observed in North Carolina.

Sulfur (S)

Sulfur deficiency has not been commonly observed in North Carolina soybean fields, but deficiencies are most likely to be seen in very deep, coarse-textured soils. Since sulfate anions tend to leach and accumulate in subsoil clay layers, sufficient quantities may be present in some soils even though the topsoil samples analyzed at the laboratories indicate low levels. Sulfur deficiency symptoms include yellowing of the uppermost leaves (Figure 6-8). Sulfur deficiency is best diagnosed by plant tissue testing rather than soil testing. For additional information, see: Soil Facts: Sulfur Fertilization of North Carolina Crops.

Micronutrients

These nutrients are required in small amounts, but deficiencies can reduce yield because they are essential for specific functions.

Boron (B)

Boron deficiency in soybean affects rapidly expanding young tissues and can lead to misshapen and chlorotic upper leaves and abnormal plant buds (Figure 6-9).

Manganese (Mn)

Manganese deficiency is relatively common in eastern North Carolina soybean field areas with pH greater than 6.2 due to over-application of lime. At higher pH, Mn availability is reduced even if it is present in the soil. Manganese deficiency usually results in leaf interveinal chlorosis and stunted growth (Figure 6-10).

Molybdenum (Mo)

Molybdenum is required by the Bradyrhizobium bacteria that form nodules on soybean roots to fix nitrogen. As soil pH decreases, Mo availability decreases, but liming to the appropriate pH will usually correct this concern. If an inoculant is used, Mo should be applied with the seed treatment. Deficiencies in the growing season can be corrected by foliar treatment.

Lime

Soybean growth is known to be adversely affected by low soil pH. Yields are best when pH is near 6.0 on mineral soils, 5.5 on mineral-organic soils, or 5.0 on organic soils. Liming is recommended if soil pH is below recommended levels. Lime helps reduce toxic levels of aluminum and manganese, increases availability of molybdenum (which increases growth of Bradyrhizobium), and increases phosphorus, calcium, and magnesium (if dolomitic availabilities).

Liming should be based on current soil test results. Although soybeans respond well to lime, high pH above recommended levels can cause micronutrient deficiencies. Since lime is relatively immobile in the soil, it should be incorporated if possible for adequate pH adjustment in the root zone. In addition, lime is most effective when applied at least three months prior to planting soybeans, especially for no-till production. For no-till fields, earlier and more frequent liming at lower application rates may be desirable because lime is left on the surface instead of incorporated as with conventional tillage.

Figure 6-11 shows the availability of nutrients at various pH levels. The more narrow the bar, the less available the nutrient is at that pH.

Sources of Lime and Fertilizer

Product recommendations provided through a soil test report generally state lime or nutrient recommendations on an amount per acre basis. The recommendations do not include additional instructions on sources or combinations of specific nutrients due to the large number of alternatives (including numerous sources each of blended or co-precipitated granules, soil- or foliar-applied solutions, fertigation, animal wastes). All products should be evaluated based on composition analysis provided on the product label. North Carolina is fortunate to have an agricultural lime law that protects consumers. Liming materials should be evaluated based on their effective neutralizing value (ENV) which is determined by both chemical composition (calcium carbonate equivalence or CCE) and the particle sizes (finer particles react faster). See Soil Facts: Soil Acidity and Liming for Agricultural Soils.

Producers should develop their own specific nutrient management program. Lists of commonly available sources of nutrients can be found on the NCDA&CS website. Additional descriptions of fertilizer materials including selected specialty, alternative, or fertilizer efficiency enhancer materials can be found in the Fertilizer Use chapter of the North Carolina Agricultural Chemicals Manual. Consult Cooperative Extension agents, NCDA&CS regional agronomists, or other professional agronomists as needed.

Figure 6-4. Expectation of yield or growth (%) in response to in

Figure 6-4. Expectation of yield or growth (%) in response to increasing nutrient concentration and interpretation index of the plant.

Source: NCDA&CS

Figure 6-5. Nitrogen deficiency induced in the greenhouse (top)

Figure 6-5. Nitrogen deficiency induced in the greenhouse (top) and normal plants (bottom).

Figure 6-6. Phosphorus deficient soybean (top, foreground) with

Figure 6-6. Phosphorus deficient soybean (top, foreground) with soil P-Index of 6 (very low) and leaf P of 0.26% (low); and K deficient soybean (bottom) with soil K-Index of 24 (low) and leaf K of 1.7% (near the critical level threshold of 1.5%).

Figure 6-7. Magnesium deficiency induced in greenhouse plants (t

Figure 6-7. Magnesium deficiency induced in greenhouse plants (top), and similar lower leaf symptoms observed in field soil with low pH (bottom).

Figure 6-8. Sulfur deficiency induced in the greenhouse (top) an

Figure 6-8. Sulfur deficiency induced in the greenhouse (top) and normal plants (bottom).

Figure 6-9. Boron deficiency induced in the greenhouse.

Figure 6-9. Boron deficiency induced in the greenhouse.

Figure 6-10. Manganese deficiency symptoms observed in the field

Figure 6-10. Manganese deficiency symptoms observed in the field.

Figure 6-11. The relative availability of different nutrient ele

Figure 6-11. The relative availability of different nutrient elements varies with soil pH as indicated by the width of each bar.

Application of Materials

Two critical factors in the application of lime and fertilizer materials are proper calibration to ensure the desired rate is applied and uniform product distribution to the field. In practice, achieving uniform product distribution requires careful maintenance and operation of equipment. See the Application Equipment chapter in the North Carolina Agricultural Chemicals Manual.

Broadcast Spreaders

Broadcast spreaders are widely used to apply fertilizer, lime, or amendments. Spreaders can be single-spinner, twin-spinner, or air-boom designs. A large spinner spreader typically consists of a hopper, a drag chain or belt, a discharge gate, a chute, and one or two spinners.

Spinner spreaders should produce a pattern that is heavy in the center and tapers to the edges. Desirable patterns for a spinner spreader are the “triangle,” the “oval,” and the “flat top” patterns (Figure 6-12).

Proper spacing of the swaths in the field is critical to apply product correctly. Swath spacing should be the width across the pattern where each side delivers 50 percent of the rate. If the swath spacing is too wide, some areas will not receive enough product between the passes of the spreader. If the swath spacing it too close, some areas will receive too much product. Be sure to check the spread pattern and maintain proper swath spacing in the field. Improper swath spacing or a bad spread pattern for products such as fertilizer can result in striped or uneven crop development (Figure 6-12). It essential to recognize the difference between the width of the spread pattern and the swath width. Figure 6-13 illustrates the spread pattern width as the maximum width the product is distributed. In contrast, swath width is the spacing of the applicator’s passes through the field.

Air boom spreaders use a high-volume air stream to suspend the product particles and convey them through tubes to diffusers spaced along the boom. The product is metered into an air chamber where the air stream catches the material and divides it into the tubes running to the diffusers. The product is uniformly distributed along the width of the boom with a very slight taper on the outside edges. As with the spinner spreader, proper swath spacing is critical. Because the air boom pattern has little taper, precise swath spacing must be maintained. Smooth delivery of material is important.

Fertilizer spreader settings, operation, and maintenance require considerable care and consideration of design settings. Check the discharge mechanism for blockage or wear. Check the drive mechanism to make sure it is functioning properly. Slipping wheels, worn belts, and worn chains can seriously affect performance and should be repaired. Check the spinners for holes in the bottom or in the vanes. Check for caked material on the vanes as well. Pay close attention to the speed of the spinner: excessively high or low speeds can cause improper application patterns. On air-boom spreaders, be sure to check the air chamber and tubes for blockages and leaks. Refer to your operator’s manual for correct settings and adjustments on all machines.

Figure 6-12. Streaks in soybean fields associated with patterns

Figure 6-12. Streaks in soybean fields associated with patterns of K fertilizer (top) and lime (bottom).

Figure 6-13. Spinner-spreader application patterns, with graphs

Figure 6-13. Spinner-spreader application patterns, with graphs indicating total spread widths and appropriate swath spacings.

Authors:

Cooperative Extension Soil Science Specialist
Crop and Soil Sciences
Section Chief, Soil Testing
NCDA&CS

Publication date: Nov. 21, 2017
AG-835

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