Soil Sampling and Fertility Management

 Pawel Wiatrak and Jim Camberato

It is very important to perform soil tests and apply lime and fertilizer if needed to maximize yield potential. More information on these topics can be found at http://www.clemson.edu/agsrvlb/new_page_16.htm.

Plant nutrient applications through fertilizer and/or lime should be based on soil tests. Soil tests help determine the soil pH and nutrient levels, and the need for lime and/or fertilizer applications. These tests can be performed by the Clemson University Agricultural Services Lab, which provides soil analyses and nutrient recommendations on a fee basis.

Local County Extension office can provide soil sample bags and submissions forms, and also give advice on taking soil samples. The local office will also mail the soil samples to the lab.

Following are steps for obtaining a representative soil sample:

  • Divide field into 2.5 acre areas (also used in the precision ag sampling) based on similar management history and soil characteristics - sample should not represent a field over 10 acres.
  • Take into consideration difference in surface color is the most evident feature separating soil types, indicating possible variability in texture, organic matter content, and drainage.
  • Nutrient removal rates by crops are important in determining residual soil fertility levels. Therefore, cropping history is another important factor to consider when defining a sample area.
  • Construct an accurate map of the field to take samples from the same areas each year, preferably in the fall after harvest to enhance the relevance of annual comparisons.

Taking soil samples:

  • Obtain 10 to 20 soil cores (brush away surface plant residue material) from a 6 - 8 inch depth for tilled and 3 - 4 inches for no-till areas in a zigzag pattern throughout the area to ensure good representation.
  • Place soil cores in a clean plastic bucket and mix thoroughly.
  • Fill a one-pint sample box with the soil subsample.

Note: In most sandy Coastal Plain soils where deep tillage is practiced to break root restrictive hardpans, a sample of the top 4 inches of subsoil can be used to determine the availability of potassium, sulfur, and magnesium. These nutrients readily leach through the sandy surface layer but are retained in the upper part of the subsoil. The results of subsoil samples can be used to adjust fertilizer recommendations made from the analysis of the A horizon sample.

Soil samples are analyzed for soil pH and the plant-available contents of potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), zinc (Zn), copper (Cu), boron (B), and manganese (Mn). Based on these soil test results, recommendations are made regarding the correction of soil pH and fertilization requirements to achieve top yields.

These recommendations are based on several years of soil test calibration research conducted in South Carolina and neighboring states. Soil analysis procedures determine the amount of each nutrient available for crop uptake. The new soil fertility ratings and recommendations have been posted at http://www.clemson.edu/agsrvlb/new_page_16.htm. Nutrient levels on the soil test report are indexed into the following categories:

  • Low: soil plant nutrient element level is deficient and an application of this element will result in a significant yield increase. If the soil temperature is low (<60oF), part of the fertilizer, especially P, should be banded beside the row for row crops when planted in the spring. A high application rate is needed to meet the crop requirement, compensate for soil interaction, and build soil reserves. 

 

  • Medium: soil plant nutrient element level is adequate for moderate agronomic crop yields, but a yield response can be expected about 50% of the time from an application of this element. For moderate yield goals, there is probably a sufficient amount of this plant nutrient element without the need to add more than that expected to be removed by the crop; however, for high yield goals, the recommendation should be greater than that needed to compensate for crop removal. 

 

  • Sufficient: soil plant nutrient element level is in that range adequate to meet the crop requirement as well as that needed for consistent high crop yield production. A maintenance application rate is recommended to compensate for expected crop removal. Maintaining the surface soil within the "Sufficient" range will ensure that the subsoil essential plant nutrient element level will not be depleted. 

 

  • High: this soil plant nutrient element level can adversely affect crop yield and product quality, and a further increase can lead to crop yield decreases as well as plant nutrient element imbalances. Therefore, no addition of this element is recommended, unless needed to compensate for expected high crop removal. 

 

  • Excessive: this soil plant nutrient element level will adversely affect plant yield, create nutrient element deficiencies due to imbalances, and can lead to potential ecological damage to the surrounding environment.

A soil pH too low or too high can adversely affect nutrient availability and hence, corn growth and yield. The best pH for corn grown in most South Carolina soils is between 5.8 and 6.5. If soil tests show a low soil pH, lime can be applied to enhance yield by:

  • Reducing toxicity of soil aluminum and/or manganese.
  • Improving uptake of phosphorus, potassium, and molybdenum.
  • Increasing availability of calcium and magnesium (with dolomitic lime).

It is important to perform soil tests and apply lime and fertilizer if needed to avoid yield losses due to nutrient deficiency. The following are some considerations regarding the nutrient management practices to help with maintaining high corn yields.

Low soil pH is one of the most frequent problems encountered in corn fields across South Carolina. Soil pH decreases over time due to use of high rates of nitrogen fertilizer, crop nutrient removal, and leaching of basic cations (like calcium). Symptoms of low pH include stunted plant development, uneven crop growth across the field, premature senescence of lower leaves, and intervenial chlorosis (yellowing between veins) on upper leaves.

Low soil pH can limit plant growth through aluminum and acid effects on plant roots, manganese toxicity, calcium, magnesium, or molybdenum deficiency, and reduced phosphorus availability. Additionally, under low pH conditions bacterial activity is reduced, slowing the breakdown of soil organic matter, crop residues, and organic fertilizer sources (such as animal manures). Thus nutrient release from these sources is lessened.

For most South Carolina soils, highest corn yields can be obtained when the soil pH is between 5.8 and 6.5. For soils with very high organic matter contents, such as Carolina bays, a soil pH between 5.0 to 5.5 is usually satisfactory. If needed, lime should be applied as early as possible to allow time for neutralizing soil acidity. The best time to apply the recommended rate of lime is following harvest of the previous crop to allow lime to react and correct soil pH prior to planting corn. Lime application will be slower in a dry soil, because water is required for lime to react with soil. Usually, liming materials with a high calcium carbonate equivalent (CCE) tend to neutralize soil acidity faster than those with a low CCE.

Faster activity of lime will be from finer material due to greater surface area. Use dolomitic lime when magnesium levels are low or anticipated to be low prior to the next time lime is needed. Dolomitic lime will supply 80 to 240 pounds of magnesium and 400 to 600 pounds of calcium per ton. Calcitic lime will supply 8 to 12 pounds of magnesium (usually not enough) and 700 to 800 pounds of calcium per ton.

Soil tests report both the soil and buffer pH. Soil pH is a measure of the hydrogen ion concentration in the soil (active acidity) whereas buffer pH is a measurement of total soil acidity (active + reserve acidity). The buffer pH is used to calculate the amount of lime needed to correct both the active and reserve acidity. In order to correct soil pH most of the active and reserve acidity must be neutralized. A soil with a low cation exchange capacity (CEC) has a low buffer capacity and will not require a lot of lime to correct soil pH, but may need lime applied frequently. A soil with a high CEC may require a large amount of lime to initially correct pH, but may not need another application for few years due to a high buffering capacity.

Excessively high pH reduces availability of several micronutrients (manganese, iron, copper, and zinc) and can result in micronutrient deficiencies. Zinc and manganese deficiency are the most likely to occur with corn.

Research in several states in the Southeast has shown that the placement of some fertilizers near the seed at planting may increase grain yields in years of good rainfall or when corn is grown with irrigation. Usually this fertilizer is placed 2 inches to the side and 2 inches below the seed. Higher yields will usually be achieved on sandy soils if the starter fertilizer is applied.

Applying phosphorus as a starter may be beneficial if the soil test shows phosphorus to be very low or if corn is planted in a cold soil. Research has shown that applying other nutrients in this manner may help to increase early plant growth, but may not always increase the final grain yield.

Even when soil fertility levels are optimum, nutrient deficiencies may occur if root growth and function are restricted by soil compaction and hardpans, nematodes, diseases, low soil temperatures, or excess rain over a prolonged period of time. If nutrient deficiencies are suspected, take leaf samples from both healthy and deficient plants for tissue test analyses. Soil samples should also be collected from the same areas. Contact your county Extension office for sampling procedures and mailing.

Corn needs optimum levels of nitrogen for adequate growth and development. Nitrogen is critical in many plant processes, such as a key component of chlorophyll and the leaf enzymes associated with sugar production. It is also a major component of amino acids to build proteins, energy transfer compounds - such as ATP (adenosine triphosphate), which allow cells to use energy during metabolism, and nucleic acids such as DNA - genetic material that allows plant cells to grow and reproduce. Nitrogen is also vital in maintaining green leaf area and proper ear development.

Soil nitrogen exists in three general forms: organic nitrogen compounds, ammonium (NH4+) ions, and nitrate (NO3-) ions. Most of the potentially available nitrogen in the soil is in organic forms which are not directly available to plants until converted to NH4+ by soil microorganisms. Plants accumulate nitrogen in the inorganic or mineral forms, NH4+ and NO3-. Ammonium ions bind to the soil's negatively-charged cation exchange complex while nitrate ions exist dissolved in the soil water.

Nitrogen-deficient corn plants are pale green and have small, spindly stalks. Additional symptoms include a V-shaped yellowing beginning at the leaf tip and proceeding to the base of the leaf on lower leaves. Corn plants generally accumulate up to two-thirds of their total nitrogen 2 weeks before and 2 weeks after tasseling.

Recent research has shown, however, that many stay-green hybrids accumulate even a greater proportion of their total nitrogen during the grain-filling period. Therefore, nitrogen applications must be properly managed to ensure that adequate nitrogen is available to the plants during this period of development. The most profitable rate of nitrogen fertilizer depends on many factors such as soil type, amount of rainfall during the growing season, plant population, cultural practices, residual nitrogen from the previous year (especially following a year with low rainfall), and decomposition of organic matter under warm soil conditions.

The suggested rates of nitrogen need to be adjusted based on grower's experience. For most corn under dryland conditions a rate from 120 to 150 pounds per acre should be sufficient. For very sandy soils with yield potentials of 80 bushels per acre or less, 100 pounds of nitrogen per acre is adequate. Use from 150 to 180 pounds of nitrogen per acre on river bottoms or on soils with high water and nutrient holding capacities which produce yields of 130 bushels per acre or more.

When corn is irrigated, apply 180 to 225 pounds of nitrogen per acre. Reduce the rate by 20 to 30 pounds per acre when corn follows soybean in a rotation. When a legume cover crop is grown and turned under before planting corn, reduce the rate of nitrogen applied by 50 to 80 pounds per acre.

When animal manure is applied to the field, adjust inorganic fertilizer nitrogen rates for the nitrogen supplied by the manure. Since animal waste is highly variable in nutrient content (nitrogen, phosphorus, and potassium), laboratory analyses are needed to determine its nutritive value. Animal waste analyses for crop land application can be obtained from the Agricultural Service Laboratory of Clemson University. See your county Extension agent for submission forms and sampling instructions.

A broadcast application of nitrogen immediately before or after planting may be the most convenient method to apply nitrogen to the soil, but it is the least effective. Nitrate, the most prevalent form of nitrogen in the soil, is a very mobile compound in sandy soils and is subject to leaching during periods of heavy precipitation. Consequently, nitrogen fertilizer should be applied in a split application to increase the efficiency of nitrogen application. Apply 30 to 40 pounds of nitrogen at planting.

The start-up nitrogen application may be combined with a small amount of phosphorus. The early phosphorus application may help to improve its uptake under cool soil temperatures, which generally reduce phosphorus uptake under these conditions. The best method to apply this nitrogen is to band it 2 inches to the side and 2 inches below the seed or on the soil surface, 2 - 4 inches to the side of the row. Application too close to the germinating seed or emerging plant may cause severe salt injury. Also, the fertilizer salts may cause plants to wilt in low moisture soils by pulling water away from corn roots.

Another method is to apply the nitrogen in a band over the row. If banded over the row, use the lower rate of nitrogen and have the band at least 8 inches wide to avoid injury to the seed or seedlings. The remainder of the nitrogen rate for corn should be applied when the plants are 15 to 30 inches high (between 6 and 8 fully expanded leaves on the plants) - just before corn begins rapid growth and development. Corn will obtain this height usually 21 to 30 days after plant emergence.

Moisture and nutrient deficiencies after this stage of growth will affect ear development. The nitrogen use efficiency can be improved by splitting the sidedress nitrogen portion (balance) into 2 sidedress applications for dryland corn and 3 applications for irrigated corn. For example, apply the first half of the sidedress balance when corn plants are 12 inches tall and the second half of the balance when corn plants are 24 - 30 inches tall for dryland corn. For irrigated corn, the timing of the first 2 applications would be similar to the dryland corn and a third portion would be applied 2 weeks later through the irrigation system.

All sources of nitrogen are equally effective for corn production when applied correctly. To increase the period of availability, growers may consider using slow-release forms of nitrogen or add a nitrification or urease inhibitor to the fertilizer. When nitrogen is applied with irrigation water (fertigation), the additional nitrogen may be applied over two or three applications. A 100 bushel per acre corn yield will remove 65 pounds of nitrogen in the grain. If harvested for silage, the grain and stover will remove 130 pounds of nitrogen per acre from the soil.

Phosphorus is an essential component in the structure of many plant compounds and plays an important role in energy storage and transfer within the plant. It is relatively immobile and moves very little in the soil profile under most conditions. Most of phosphorus is tightly bound to soil particles and unavailable to plants. Over-fertilization of soils with poultry litter, dairy wastes, and other animal manure contributes to build up very high levels of phosphorus and may result in leaching in sandy soils.

A fibrous and extensive corn root system is essential for optimal phosphorus uptake. Early in the growing season, deficiency symptoms (purple leaves) may appear during periods of cold weather even when soil phosphorus is adequate because root growth is slowed to a relatively greater extent than vegetative growth. Sugars accumulated in the leaves and stem of the plant are not moved fast enough to slowly growing roots and therefore trigger anthocyanin production resulting in purple plants.

Small ears, undeveloped kernels at the ear tips, and twisted or bowed ears indicate that phosphorus deficiency occurred during reproductive development. The rate of phosphorus fertilizer to apply can only be determined by soil testing.

A broadcast application is usually satisfactory, provided the fertilizer is incorporated into the soil. When the soil test for phosphorus is low or corn is planted in cold soil, initial growth may be improved if part of the phosphorus is banded 2 inches below and 2 inches to the side of the seed. However, this practice may not always result in higher grain yields.

A 100 bushel per acre corn yield will remove 34 pounds of P205 per acre in the grain. If harvested as silage, the grain and stover will remove 45 pounds of P205 per acre from the soil. The recommended rates of phosphorus for corn are shown in Tables 1-5.

Potassium is important to maintain plant's salt balance and regulate water and sugar movement within the plant. Because of its positive charge, potassium is involved in charge balance within the plant cells as well as making enzymes functional. Deficiency symptoms are usually first detected on older leaves and include the yellowing and dying of plant leaf margins beginning at the tips of lower leaves. The ears will not be filled at the tip and the stalks will be weak and may lodge.

Potassium movement depends on soil cation exchange capacity and will leach into the subsoil eventually in most sandy soils. A single application of potassium is sufficient on most soils but split applications are recommended on very sandy soils. The rate of potassium fertilizer to apply must be determined by soil testing. Plant will take up more potassium than needed (luxury consumption) if excess potassium is added to the soil. Also, a high rate of potassium application may induce magnesium deficiency on soils with low magnesium levels. A 100 bushel per acre corn yield will remove 24 pounds of K20 in the grain. If harvested for silage, the grain and stover will remove 100 pounds of K20 per acre from the soil. The recommended rates of potassium for corn are shown in Tables 1-5.

Table 1. Fertilizer recommendations for corn grain yield goal of 100 Bu/acre based on soil test results in South Carolina.*

Phosphorus

Potassium

 

Low

Medium

Sufficient

High

Excessive

 

pounds of N-P2O5-K2O per acre

Low

120-80-110

120-80-80

120-80-40

120-80-0

120-80-0

Medium

120-55-110

120-55-80

120-55-40

120-55-0

120-55-0

Sufficient

120-30-110

120-30-80

120-30-40

120-30-0

120-30-0

High

120-  0-110

120-  0-80

120-  0-40

120-  0-0

120-  0-0

Excessive

120-  0-110

120-  0-80

120-  0-40

120-  0-0

120-  0-0

* Source: http://www.clemson.edu/agsrvlb/new_page_16.htm.

Table 2. Fertilizer recommendations for corn grain yield goal of 150 Bu/acre based on soil test results in South Carolina.*

Phosphorus

Potassium

 

Low

Medium

Sufficient

High

Excessive

 

pounds of N-P2O5-K2O per acre

Low

170-105-135

170-105-95

170-105-60

170-105-30

170-105-0

Medium

170-  80-135

170-  80-95

170-  80-60

170-  80-30

170-  80-0

Sufficient

170-  55-135

170-  55-95

170-  55-60

170-  55-30

170-  55-0

High

170-    0-135

170-    0-95

170-    0-60

170-    0-30

170-    0-0

Excessive

170-    0-135

170-    0-95

170-    0-60

170-    0-30

170-    0-0

* Source: http://www.clemson.edu/agsrvlb/new_page_16.htm.

Table 3. Fertilizer recommendations for corn grain yield goal of 200 Bu/acre based on soil test results in South Carolina.*

Phosphorus

Potassium

 

Low

Medium

Sufficient

High

Excessive

 

pounds of N-P2O5-K2O per acre

Low

220-130-170

220-130-120

220-130-75

220-130-40

220-130-0

Medium

220-105-170

220-105-120

220-105-75

220-105-40

220-105-0

Sufficient

220-  80-170

220-  80-120

220-  80-75

220-  80-40

220-  80-0

High

220-    0-170

220-    0-120

220-    0-75

220-    0-40

220-    0-0

Excessive

220-    0-170

220-    0-120

220-    0-75

220-    0-40

220-    0-0

* Source: http://www.clemson.edu/agsrvlb/new_page_16.htm.

Table 4. Fertilizer recommendations for corn grain yield goal of 250 Bu/acre based on soil test results in South Carolina.*

Phosphorus

Potassium

 

Low

Medium

Sufficient

High

Excessive

 

pounds of N-P2O5-K2O per acre

Low

270-155-220

270-155-145

270-155-85

270-155-50

270-155-0

Medium

270-130-220

270-130-145

270-130-85

270-130-50

270-130-0

Sufficient

270-105-220

270-105-145

270-105-85

270-105-50

270-105-0

High

270-    0-220

270-    0-145

270-    0-85

270-    0-50

270-    0-0

Excessive

270-    0-220

270-    0-145

270-    0-85

270-    0-50

270-    0-0

* Source: http://www.clemson.edu/agsrvlb/new_page_16.htm.

Table 5. Fertilizer recommendations for corn silage based on soil test results in South Carolina.*

Phosphorus

Potassium

 

Low

Medium

Sufficient

High

Excessive

 

pounds of N-P2O5-K2O per acre

Low

180-100-100

180-100-60

180-100-50

180-100-0

180-100-0

Medium

180-  50-100

180-  50-60

180-  50-50

180-  50-0

180-  50-0

Sufficient

180-  40-100

180-  40-60

180-  40-50

180-  40-0

180-  40-0

High

180-    0-100

180-    0-60

180-    0-50

180-    0-0

180-    0-0

Excessive

180-    0-100

180-    0-60

180-    0-50

180-    0-0

180-    0-0

* Source: http://www.clemson.edu/agsrvlb/new_page_16.htm.

Calcium (Ca)

Calcium is involved in the construction of plant cell walls. Consequently, when a young plant is deficient in calcium, the tips of new leaves will not unroll properly and will stick together. Other symptoms include dark green stems, weakened stems, and poor ear formation. Applying lime will supply enough calcium for optimum corn growth, and deficiencies will not occur if a favorable soil pH is maintained. A soil test level of 300 pounds per acre or greater will supply adequate calcium for corn production. Apply sulfate forms (gypsum is calcium sulfate) if calcium deficiencies occur at optimum soil pH.

Magnesium (Mg)

Magnesium is an important component of chlorophyll molecules. Deficiency symptoms include chlorosis (yellowing) between the leaf veins of upper leaves and often a reddish tint on the stem and underside of lower leaves. Magnesium may become limiting at low pH on sandy soils. Compact layers or hardpans and a high rate of potassium or calcium may also cause magnesium deficiencies in corn. Also, magnesium deficiency may be induced by high crop nitrogen uptake, but usually disappears after about two weeks. If the soil test level for magnesium is 32 pounds per acre or greater, the corn plant will have adequate magnesium.

Magnesium saturation of the cation exchange capacity should be greater than 10% to ensure adequate magnesium availability. Magnesium is seldom a limiting nutrient if dolomitic limestone is applied as the lime source several months prior to planting corn. Use of calcitic lime sources (including some types of poultry manures) are often the cause of magnesium deficiency. If magnesium needs to be added to the soil without adding lime, apply 10 to 15 pounds per acre of magnesium as magnesium sulfate, sulfate of potash-magnesia, or magnesium oxide. Dolomitic lime is always the least expensive source of magnesium.

Sulfur is a key component of many compounds in the plant such as the energy producing chloroplast and several amino acids. Sulfur deficient plants are small and spindly, and have slower growth and delayed maturation. Sulfur deficiency is expressed as an overall yellowing of the plant or a yellowing of the new leaves. This response is in contrast to nitrogen deficiencies which are more apparent on old lower leaves. During periods of low temperature and/or with excessive rainfall in the spring, sulfur deficiencies on corn can occur shortly after seedling emergence, especially on sandy soils.

The plants often recover when roots reach the subsoil zone where sulfur has accumulated. Side-dressing with a nitrogen-sulfur solution or a sulfate-sulfur source will help the plants to recover. Elemental sulfur is not a good choice for alleviating sulfur deficiency because it is only slowly available and can decrease soil pH drastically. Sulfur, like nitrate, is relatively mobile in the soil and may be leached to the clay layer with rainfall. However, sulfur will accumulate in the subsoil unlike nitrate.

Soils with the clay layer within the root zone will generally have adequate sulfur for corn production. Apply 10 pounds of sulfur per acre when the depth to clay is greater than 15 inches. Apply sulfur with phosphorus and potassium fertilizers at planting or with the nitrogen side-dress. Split applications for deep sandy soils. Corn requires a relatively large amount of sulfur, generally 20 to 30 pounds per acre.

Zinc (Zn)

Because of its involvement in chlorophyll formation, zinc deficiencies will often appear as a bleached-colored chlorosis on the lower portion of new leaves. Also, deficiencies include decreased stem length (rosetting) and intervenial chlorosis. Deficiencies have been observed on young corn plants in cold soil during cool weather, but the plants usually grow out of the deficiency with warmer temperatures.

Severe zinc deficiencies may occur when deep sandy soils are limed to a soil pH of 6.5 or higher, which results in reduction of zinc available to plants. Soils having the suggested soil pH (5.8 to 6.5) will have adequate zinc for corn production if the soil test shows 1.8 pounds of zinc per acre or higher. To prevent zinc deficiency, apply 3 pounds per acre of actual zinc preplant or at planting, but only when soil tests are low.

Boron (B)

Boron deficiency may appear on new plant growth, especially on sandy soils low in organic matter. Symptoms of boron deficiency include shortened internodes, necrotic speckling on leaves, and poor tassel and ear formation. Boron occurs in the soil in both organic and inorganic forms, but only a small amount is available to plants. The best method for determining when boron is needed is plant analysis, not soil testing. High levels of calcium, potassium, or nitrogen levels can interfere with boron uptake. Soils high in calcium will require more boron. Generally, apply 1 - 2 pounds of boron per acre in split applications (with the nitrogen). Soils in the pH range 5.8 to 6.5 will have adequate boron for corn production.  

Manganese (Mn)

Manganese deficiencies may appear on new corn growth, especially on over-limed soils or with lime-amended biosolids. Manganese-deficient upper leaves become pale and chlorotic and are much narrower than normal leaves. Avoid stockpiling lime in the field and apply lime based on soil test recommendations. Soils in the suggested soil pH range of 5.8 to 6.5 will have adequate manganese for corn. If needed, manganese can be applied 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. In some soils the chelates may be better than Mn sulfate but not for foliar application.

Common Cu deficiency symptoms include stunting, leaf tip/shoot dieback, and poor upper leaf pigmentation. Copper deficiency is most likely on organic soils. Copper is commonly applied 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 should be applied as foliar application at much lower rates than soil applications. However, soil-applied copper will have much longer residual effects.