By Dorivar Ruiz Diaz
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, the daily rate of nutrient uptake increases. Soybeans need an adequate nutrient supply at each developmental stage for optimum growth. High-yielding soybeans remove substantial amounts of nutrients from the soil. For example, a 40-bushel/acre soybean crop removes approximately 30 pounds of P2O5 and 50 pounds of K2O with the grain. An additional 10 pounds of P2O5 and 40 pounds of K2O can be removed with the stover. This should be taken into account in an overall nutrient management plan.
Nitrogen is supplied to soybeans mainly by nitrogen fixation, and fertilizer nitrogen application is not recommended if the plants are well-nodulated (Figure 1). Soybeans are heavy users of N, removing a total of 130 pounds per acre, and about 44 pounds with the stover, for a 40-bushel-per-acre soybean crop. Soybeans use all the N they can fix, plus N from the pool of available N in the soil. Nitrogen fertilizer application to soybean seldom results in any yield benefit. Instead, efforts should focus on ensuring proper inoculation.
Figure 1. Nodulated soybeans (left) compared to soybeans without adequate nodulation (right).
Phosphorus (P)Source : ksu.edu
Phosphorus applications should 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 for soybeans. Keep in mind that soybean seeds are easily injured by fertilizer, therefore, no direct seed contact with fertilizer is advised.
Soybean 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 the grain is removed. As with P, a soil test is the best index of K needs. Soils testing very low or low should be fertilized with K, either as a banded starter at planting or broadcast and incorporated. Potassium should not be placed in contact with the soybean seed because of possible salt injury. Yield increases from K can be significant, and in some cases, more than yield responses to P for soils testing low or very low.
Sulfur is a mobile nutrient 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. Recent studies in Kansas 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.
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 (yellowing of leaves) 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 western Kansas. 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 deficiency. If iron chlorosis has been a common problem in the past, producers should select a soybean variety tolerant to iron deficiency. It may be beneficial to use a chelated iron in-furrow application. Foliar iron treatments seldom result in a yield increase.
Zinc, manganese, and boron are other nutrients that can be limiting in soybeans. The need for zinc should be determined by soil tests. Zinc fertilizer can be either banded at planting or broadcast pre-plant with little difference in response when applied at an adequate rate. Both organic and inorganic zinc 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 all influence tissue test results.
Some micronutrients also can cause phytotoxicity if prevalent in large quantities. Nutrient removal by soybean is very high in high-yielding environments, thus fertilizer application rates should be high or soil test levels will drop. Regular soil testing (every 2 to 3 years) is essential for optimum nutrient management. Soybeans take advantage of residual phosphorus and potassium, but keep in mind the total nutrient needs in the rotation.