Soybean Nutrient Requirements

Fertilizing prior to soybean production is not a common practice in Iowa. Nutrient deficiencies in soybean are rare and current recommendations in Iowa are to apply additional nutrients only if soil tests indicate that a specific nutrient is at a low concentration or if deficiencies are identified in the field. Deficiencies are rare even though the soybean plant requires more nitrogen than corn, and as a result can fix up to 50% of its own nitrogen. In central and north central Iowa, the nutrient most often limiting is iron, and this deficiency is only found in calcareous soils with high pH where the iron is bound tightly to the soil. When nutrients are limited, processes such as water transport, photosynthesis, and protein, oil, and carbohydrate production do not occur at proper rates and growth and yield development decrease. Even though nutrients are usually not a limiting factor in production systems it is worthwhile to understand and identify deficiency symptoms so that they can be corrected during the season or in following years.  

Essential Nutrients

Specific nutrients such as nitrogen (N), phosphorus (P), and potassium (K) are the three most important soil supplied nutrients and are called essential nutrients. There are several other nutrients important for plant growth such as calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mo) that are needed at much lower concentrations and are limiting only in certain environments.

Nutrients either have a positive charge or a negative charge and this controls whether they bind to a soil particle (negatively charged soil will bind positively charged nutrients) or exist in the soil solution (negatively charged nutrients repelled by soil particles). Those that are found in soil solution (such as nitrogen) are more mobile and have the greatest potential to become limiting. Positively charged particles can also become limiting especially if they are bound tightly to the soil particle, as in the case of iron, making them unavailable to the plant.  

The demand for nutrients depends on the soybean growth stage. As the soybean plant accumulates biomass the amount of nutrients needed to support growth increases. The maximum nutrient demand occurs during seed fill. Since the soybean seed has high levels of protein, demand for nitrogen is extremely high during seed formation. Throughout the growing season nutrients are gathered from the soil and through nitrogen fixation; however, late in the season many of the nutrients are remobilized from the older tissue to support seed development (Table 1 and 2).

Table 1. Estimates of the amount of six important mineral nutrients removed from the soil and located in the grain of soybean.

Table 2. Estimates of the amount of six important mineral nutrients removed from the soil and located in the above-ground vegetative (non-seed) plant parts.

(Source of two tables: www.psu.missouri.edu/soyx)

Nitrogen Can Limit Growth

Nitrogen Deficiency and Fixation. Nitrogen deficiency is identified as a yellowing or chlorosis of the leaves lower in the canopy as nitrogen is remobilized to new growth. There are two sources of nitrogen available to the plant. First, the plant can take nitrogen from the soil solution accounting up to 50% of the total nitrogen needed for growth. The soil is the first choice as a nitrogen source because this process requires less energy compared with the second nitrogen source. The remaining nitrogen comes from the well-known process called nitrogen fixation, a process carried out by plants in the legume family (such as soybean, clover, and alfalfa). Fixation occurs in structures called nodules (small oval structures) that form on soybean roots. These nodules are formed early in the season (V1) through the association of the root with a soil bacterium (Bradyrhizobia japonicum). These nodules absorb N2 gas from the atmosphere and through a chemical reaction form NH3 (ammonia) that can be used by the plant.

These bacteria are present in almost all fields in Iowa and provide up to 50% of nitrogen needed by soybean. High nitrogen levels in the soil limit the number of nodules formed increasing soybean’s dependence on the soil for nitrogen. In these conditions nitrogen deficiency can occur later in the season and nitrogen application may be necessary. Soybean roots should be checked prior to growth stage R1 (flowering) for nodules and if they are active (check to see if they are pink inside). In fields where soybean has not been grown recently the addition of an inoculum that contains the bacteria would be a good management option to ensure nodulation.

Research conducted in Iowa and other soybean producing states have shown that nitrogen application at planting does not improve yield and only decreases nodulation while increasing the plant’s dependency on the soil for nitrogen. Application of foliar fertilizers in the season has not shown consistent results in Iowa and these applications appear to be beneficial only in specific years and at certain locations. Research in states like Nebraska, Kansas, and Missouri has shown that nitrogen application at planting and during the season can be beneficial on low organic matter soils where the soil can not supply the remaining N requirement after fixation.

Other Nutrient Deficiencies

Iron Deficiency and Management (link to pictures) – This is a common deficiency in the Upper Midwest and occurs in calcareous soils with high soil pH. This deficiency is identified as iron deficiency chlorosis and symptoms will show chlorosis (yellowing) between the leaf veins. The yellowing will occur on new growth because this nutrient is not mobile within the plant. Iron is necessary for nodule formation and function, and when deficient can result in reduced nitrogen fixation. This occurs because of the high pH soil and the high levels of calcium iron molecules become tightly bound to the soil particle and are not released for plant uptake. This nutrient deficiency is difficult to correct because the soil binds any additional iron and too high of a rate can become toxic to the plant. There are some management options that have shown some promise to reduce this problem. The most important management option is variety selection. Second, the application of iron coated to the seed at planting. Third, is the use of crops like oat or wheat that have the ability to remove the bound iron and improve availability to the plant. The last option is to use increased plant populations, but often seeding rates are so high that it is not economical.

Phosphorus – Stunted growth, dark green coloration of the leaves, necrotic spots on the leaves, and a purple color to the leaves occurring first on older leaves.

Potassium (link to pictures) – Occurs on older leaves first, with the visual symptom of chlorosis at the leaf margins and in-between the veins. All, but the newest leaves may show potassium deficiency symptoms in severe cases.

Calcium – necrosis of new growth especially the tips of new leaves and is very rare in Iowa.

Sulfur – similar to nitrogen deficiency, but the chlorosis occurs on newer leaves because sulfur is not mobile in the plant.

Molybdenum – rarely occurs in Iowa, only in acidic soils and plants turn a light green color due to lack of nitrogen fixation.

Foliar Fertilization

Nutrient deficiencies that are identified during the growing season can partially be alleviated through the application of foliar nutrients. Application of these foliar products should be done at lower rates and not during very warm parts of the day as burning of plant tissue can occur having a more detrimental effect. Researchers had a theory that if nutrients were applied during the late seed development stages that leaf senescence could be delayed and yield may increase. Some experiments showed that by spraying the soybean canopy between R5 and R6 you could increase yield. However, many on-farm trials showed that foliar fertilizer produced inconsistent results, and even decreased yield in some areas. Recent research done in Iowa by Iowa State University conducted across several locations and years has shown that there is a low probability of foliar fertilizers increasing yields. Foliar fertilization of soybean with macronutrients at early vegetative stages is likely to increase yield in 15 to 20% of the cases in Iowa. The research has shown no consistent difference between products, rates, or frequencies of application tested. Across all research conducted the expected average response to foliar fertilizer in Iowa is about 1 bu/acre. This is not enough to make foliar fertilizer profitable. However, in nutrient limited conditions such as on sandy soils or high yielding fields that are irrigated, these products may be beneficial as the plant can not gain enough nutrients from the soil.

Banding Fertilizer in Soybean

It has been asked many times if banding of fertilizer can gain the same results in soybean as it does in corn. With this you find many concerns about the impact of germination, emergence, and subsequent yield. Results from trials conducted in Iowa and Minnesota are very consistent with the results from other fertilizer trials. The placement of fertilizer very close to the soybean seed does not improve yield. A reduction in emergence resulted in a reduction in yield, when dry condition occurred at planting. When growing conditions were good, the reduction in emergence did not correspond with a reduction in yield. So to summarize the data, placement of fertilizer in a band close to the soybean seed at planting is an unnecessary risk that will also decrease your profitability.

Soil Sampling is the Only Way to be Sure

The majority of soybean produced in Iowa will not show any nutrient deficiencies. This lack of evidence does not mean that a problem does not exist. It is recommended to take soil nutrient samples should be taken every two years to monitor nutrient conditions. The best manager will sample his fields every year. No-tillage systems should be monitored closely as nutrients can become stratified and not accessible to the plant. Based on these nutrient analyses, decisions can be made whether to apply any nutrients or not. Recommendations for the application of phosphorus, potassium, or other nutrients can be determined by consulting Iowa State University Extension publication 1688 (pdf). Also read another Iowa State University publication on how to Take a good soil sample to help make good decisions (pdf).

More Information

Diagnosing nutrient deficiencies in soybean (pdf)