Agronomics

Soybean Fertility

Most producers do an excellent job of providing adequate nutrients for top corn yields. The same is not always true for soybeans. It is a common practice in some areas to over-fertilize the corn crop, and then let the following soybean crop use up the residuals. If the corn crop is larger than expected, it may leave a deficit for some nutrients in the following soybean crop.

Soybeans require 14 mineral nutrients for growth: nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), boron (B), chloride (Cl), molybdenum (Mo) and nickel (Ni) to grow successfully. Most soils can provide adequate amounts of the minor nutrients, but growers need to pay close attention to the plants needs with regards to N, P, K, S and Fe.

The best soil fertility management of a crop always begins with regular soil testing. Soil testing is a valuable and inexpensive tool for ascertaining the nutrient and pH status of a particular field and provides the necessary information to make good decisions on nutrient needs. Many universities recommend that soil tests be taken every two, three or four years. Testing in shorter intervals is more costly and unlikely to provide additional useful information. Instructions for collecting, handling, and shipping samples are available from your state Extension soil testing labs or from private soil test labs.

Standard soil tests generally provide a measure of pH, buffer pH (or lime test index, which is buffer pH x 10), available P, exchangeable K, Ca, Mg, cation exchange capacity (CEC) and organic matter (OM).

Micronutrient testing is available for additional fees through most labs. However, reported soil levels do not always correlate well with nutrient application recommendations in agronomic crops. Tissue testing in-season is recommended for identifying and confirming micronutrient deficiencies indicated by soil tests.

When diagnosing a suspected in-season nutrient deficiency, take soil and tissue samples from a normal area of the field in addition to the affected area. Comparing these reports will give additional insight into possible nutrient issues and increased confidence in diagnosis and confirmation of a nutrient deficiency.

Soil pH
Soil pH is important because it has a direct effect on the availability to the plant of nutrients in the soil. The pH of a soil is a measure of the acidity, or concentration of hydrogen ions in the soil solution. Many chemical and biological processes in the soil are affected by pH, and maintaining pH in the proper range will maximize the efficiency of other crop inputs and decrease the risk of yield losses. The optimal pH range for maximizing soybean yields is 6.0 to 6.8 (in mineral soils). Figure 1 shows the availability of essential nutrients as a function of soil pH.

Figure 1. Relative availability of plant nutrients by soil pH. Adapted from Fundamentals of Soil Science, 8th Ed. 1991. Henry D. Foth, Wiley Publishing

Most nutrients are maximally available in the optimal pH range for soybeans. In highly acidic soils, N, P, K, S, Ca and Mg may become limiting, even if soil tests report adequate levels being present. Alternatively, Fe, Mn, B, Cu, Zn and Mo may be limiting in alkaline soils with pHs above 7.5.

Fertilizer Requirements
Compared to corn, wheat, and sorghum, soybeans remove significant amounts of nutrients per bushel of grain harvested. Nutrient uptake in soybeans early in the season is relatively small; however, as they grow and develop, nutrient uptake increases. Soybeans need an adequate nutrient supply at each growth stage for optimum growth.owth.

High-yielding soybeans remove substantial nutrients from the soil. This should be considered in an overall nutrient management plan. For example a 50-bushel-per-acre soybean crop removes approximately 38 pounds of P2O5 and 62 pounds of K2O with the grain; in addition, approximately 12 pounds of P P2O5 and 50 pounds of K P2O5 can be removed with the stover.

Nitrogen is supplied to soybeans mainly by N fixation. Nitrogen fixation is the result of a symbiotic relationship between the soilborne bacteria Bradyrhizobium japonicum and soybean roots. In this symbiotic relationship, carbohydrates and minerals are supplied to the bacteria by the plant, and the bacteria transform N2 gas from the atmosphere into ammonium-N for use by the plant.

Fields that have never had soybeans in them or have not had soybeans in them for several years (e.g. CRP land) will have no naturally present B. japonicum, or levels will be too low for adequate infection and nodulation. In these instances, an inoculant seed treatment, applied directly to the seed or in-furrow is necessary. Both liquid and dry formulations are available, but since bacteria are living organisms, care must be taken to protect them from high heat that may kill them.

If the plants are well nodulated, additional N application is generally not recommended. In fact, providing additional N can make the bacteria “lazy” and fixation less efficient. Soybeans are heavy users of N, removing a total of 160 pounds per acre and about 55 pounds with the stover for a 50-bushel-per-acre soybean crop. Soybeans use all the N they can fix plus N from the pool of available N in the soil. Mid-season N applications to high yield potential fields have been researched, but results have often been inconsistent. If there are high numbers of nodules on the roots, additional N should not be necessary.

  • Phosphorus applications should always be based on a soil test. Responses to direct P fertilization is generally consistent in soils testing very low or low in soil test P. Response to starter P fertilizer application in soybeans can occur, but it depends on several factors. The most important factor is the soil test level. Generally, warmer soils at soybean planting, compared to corn, also may contribute to typically lower response to starter fertilizers in soybeans. However, starter fertilizer in soybeans can be a good way to complement nutrients that may have been removed by high-yielding crops in the rotation like corn. Banding fertilizer at planting is an efficient application method. Soybean seeds are easily injured by fertilizer, therefore, no direct seed contact with P fertilizer is advised.
  • PotassiumSoybean seeds are relatively high in K and removal of K by soybeans is greater than for other crops on a per-bushel basis when only grain is removed. As with P, a soil test is the best index of K needs. Fertilize low testing soils with K, either as a banded starter at planting or broadcast and incorporated. Potassium should not be placed in contact with the seed because of possible salt injury. Yield increases from K can be comparable to those with P under very low and low soil test levels.
  • Sulfur is mobile in the soil (leaching is common) but fairly immobile in the plant. High soil test variability along with significant uptake by crops generates the need for proper S management — especially in sandier soils and fields with several different soil types. Deficiency symptoms in soybeans are pale-green to yellow leaf color without prominent veins or necrosis in the youngest trifoliate leaves. Recent Kansas studies suggest a low probability of soybean response to S application; however, S removal with soybeans can be significant, and more sensitive crops in the rotation such as wheat may require S fertilization after a soybean crop.
Photo Credit: Daren Mueller, Crop Protection Network
  • Iron deficiency symptoms appear in irregularly shaped spots randomly distributed across a field, primarily in fields with a previous history of iron deficiency. Different annual weather patterns can make iron chlorosis more or less prevalent. Iron chlorosis also differs under different soil conditions. In general, high soil pH and high carbonates (free lime) can increase the incidence of iron deficiency. Iron chlorosis can be a big limitation in some regions of the western Great Plains. Iron fertilizer using chelated sources and in direct contact with the seed (in-furrow) has shown significant yield responses in soils with a history of iron chlorosis. If iron chlorosis has been a common problem in the past, producers should select a soybean variety tolerant to iron chlorosis. It may be beneficial to use a chelated Fe in-furrow application. Foliar Fe treatments seldom result in a yield increase. Click here for more information on Iron Deficiency Chlorosis.
  • Others Zn, Mn and B are other nutrients that can be limiting in soybeans. The need for Zn should be determined by soil tests. Zinc fertilizer can be either banded at or broadcast preplant with little difference in response when applied at an adequate rate. Both organic and inorganic Zn sources (chelates and non-chelates) can be used, but chelates are considered more effective than the inorganic sources. Manure applications also are effective at eliminating micronutrient deficiency problems, including iron.

Monitoring nutrient levels with tissue analysis along with soil tests conducted during the crop season should be used to diagnose potential nutrient deficiencies. Stresses such as drought, heat and pest pressure can influence tissue test results.

Deficiency Symptoms

  • Nitrogen-deficient soybean plants appear pale-green to yellow with leaves maintaining dark-green veins (interveinal chlorosis)
  • Potassium is highly mobile in the plant. Consequently, potassium deficiency symptoms occur first on the lower leaves and progress toward the top as the severity of the deficiency increases. One of the most common signs of potassium deficiency is the yellowing along leaf margins followed by scorching and dieback.

Potassium deficiency in soybean. Photo credit: University of Missouri Extension

Iron chlorosis in soybeans. Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org
  • Iron Deficiency Chlorosis (IDC) is characterized by interveinal chlorosis of the leaves with the leaf veins remaining dark green.
  • Phosphorus deficiency may cause stunted growth, dark green coloration of the leaves, necrotic spots on the leaves, a purple color to the leaves, and leaf cupping. These symptoms occur first on older leaves.
  • Sulfur deficiency is characterized by stunted plants, pale green color, similar to nitrogen deficiency except chlorosis may be more apparent on upper leaves.
  • Magnesium deficiency first shows up in older leaves turning pale green, followed by interveinal chlorosis. As magnesium deficiency progresses, reddish and purple spots appear on soybean leaves.

Magnesium deficiency on soybean. Photo from AgWeb.com

As discussed above, nutrient removal by soybean is very high in high-yielding environments, so fertilizer application rates need to be high, or soil test levels will drop. Soybeans take advantage of residual P and K, but keep in mind the total nutrient needs in the rotation.

Due to differences in soil types across the soybean production regions, it is important to check the specific fertilizer recommendations from individual state Extension soils labs. For instance, in some production areas, soils may be naturally low in P, whereas in others, especially where manure is commonly used, additional need for P would be rare.

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Source: Information on soybean fertility is provided by Douglas J. Jardine, Professor Emeritus, Kansas State University 7/2020Images provided by Crop Protection Network and IPM images and AgWeb.com