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Soil sampling should be performed every two to three years and sampling should be done at the same time of year as previous efforts.1 If maintenance fertilizer is not applied annually, soil testing should be done every other year.2 Soil tests should include an analysis for macronutrients, micronutrients, buffer pH, organic matter, and cation exchange capacity.
When soil nutrient levels are below a critical level, appropriate build-up fertilizer should be applied. In addition, fertilize for the amount of nutrients removed by the previous crop, which is often referred to as maintenance or crop removal fertilizer. Macronutrients required by soybean include nitrogen (N), phosphorus (P), and potassium (K). Each harvested soybean bushel removes approximately 3.3 lbs of N, 0.73 lb of P2O5 (phosphate), and 1.2 lb of K2O (potash) (Table 1, left).3 Additional micronutrients that aid plant growth include calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), chlorine (Cl), and sulfur (S). Deficiency symptoms may appear if any of the nutrients are limited. For example, photosynthesis, N-fixation, and protein/enzyme synthesis are dependent on B, Ca, Fe, S, and Zn availability.
Figure 1. Nutrient availability based on soil pH. The wider “nutrient bar” indicates increased plant availability. Source: Illinois Agronomy Handbook. Permission to use granted by Dr. Emerson Nafziger, University of Illinois.
As yields increase, nutrient removal rates increase (Table 2, page 2). A portion of the required nutrients for these yield levels come from soil reserves and the remainder comes from fertilizer applied prior to critical stages of plant growth.
Soil pH levels influence nutrient availability and should be maintained at or above 6.0 for soybean production.2,7 N and P are most available to soybean at pH levels between 5.5 and 7.0. In low pH soils, nitrogen (N), phosphorus (P), sulfur (S), calcium (Ca), and magnesium (Mg), may become less available to growing plants (Figure 1, left).
Iron deficiency may result in reduced nitrogen fixation. Iron becomes less available at high soil pH levels, which may result in iron deficiency chlorosis (IDC). IDC symptoms include chlorosis (yellowing) between the leaf veins, which may be more pronounced on new growth. Iron deficiency is difficult to correct because soil binds the additional iron and very high application rates can be toxic to the plants. Seed product selection is the most important management option to reduce IDC risk.5 Applying a liquid iron chelate fertilizer with the seed at planting, may also reduce incidence.
Lime neutralizes soil acidity and adds Ca, a micronutrient essential to plant growth. Soil pH indicates the level of acidity or alkalinity, while buffer pH is used to determine the rate of lime application, if needed. Lime takes time to dissolve in the soil and neutralize acidity, so application should occur about 3 to 6 months before planting. Lime should be applied and incorporated a month or more before adding fertilizers, since it can interfere with the availability of other nutrients, especially P. Note the different sources of liming material in your area. Differences among products in their neutralizing efficiency (calcium carbonate equivalent and particle size) will influence optimum lime application rates.
As a legume, soybean plants produce or fix nitrogen in root nodules. Legumes have the ability to live in a symbiotic (mutually beneficial) relationship with a specific bacteria; in the case of soybean, Bradyrhizobium japonicum rhizobial species.6 The rhizobia “fix” nitrogen from the air into ammonia, which can be used by the soybean plant. Soybean plants will also utilize residual nitrate and nitrogen mineralized from soil organic matter. B. japonicum is specific to soybean and will not fix nitrogen in any other legumes. Likewise, the rhizobial species that fix nitrogen for alfalfa or other legumes will not nodulate and fix nitrogen on soybean.8
In order for nodules to form and produce N, the soil must contain a healthy supply of rhizobia. Most universities suggest if nodulated soybean have not been grown in a field during the previous three to five years, an inoculant containing B. japonicum should be applied to the seed.6,7 Inoculated seed and inoculants must be handled with care as the living organism can be killed by desiccation, direct sunlight, heat, caustic fertilizers, and pesticides.
Nutrient deficiencies that are identified during the growing season may be partially corrected by nutrient foliar applications. Application should be done at low rates and not during very warm parts of the day as plant tissue damage can occur. Some studies indicate that by spraying the soybean canopy between beginning seed (R5) and full seed (R6) growth stages, yield potential could increase. However, many on-farm trials showed that foliar fertilizer application produced inconsistent results, and even decreased yield in some areas.5 Iowa State University research, conducted across several locations and years, indicated there is a low probability of foliar fertilizers increasing yields.
Foliar soybean fertilization with macronutrients at early vegetative stages is likely to increase yield in 15 to 20% of the cases in Iowa.5 This research has shown no consistent difference between products, rates, or frequencies for applications tested. In nutrient-limited conditions such as sandy soils or high yielding irrigated fields, however, these products may be beneficial as the plant may not gain enough nutrients from the soil.
Sources: 1 Mengel, D.B and Hawkins, S.E. Soil sampling for P, K, and lime recommendations. Agronomy Guide AY-281-W. Purdue University. https://www.extension.purdue.edu/extmedia/AY/AY-281.html 2 Fernandez, F.G. and Hoeft, R.G. 2009. Managing soil pH and crop nutrients. Chapter 8. Illinois Agronomy Handbook. http://extension.cropsci.illinois.edu/handbook/ 3 IPNI estimates of nutrient uptake and removal. Table 4.5. Nutrient removal by selected crops. International Plant Nutrition Institute. 2014. http://www.ipni.net/article/IPNI-3296 4 Ferguson, R.B., Shapiro, C.A., Dobermann, A.R., and Wortmann, C.S. 2006. Fertilizer recommendations for soybean. G87-859. University of Nebraska-Lincoln. http://digitalcommons.unl.edu/ 5 Pedersen, P. 2007. Soybean nutrient requirements. Iowa State University Extension. https://crops.extension.iastate.edu/soybean/production_soilfert.html 6 Mueller, N., Elmore, R., and Shapiro, C. 2015. Soybean inoculation: When, where, and why? CropWatch. University of Nebraska-Lincoln. https://cropwatch.unl.edu/soybean-inoculation-when-where-and-why 7 Hoeft, R.G., Nafziger, E.D., Johnson, R.R., and Aldrich, S.R. 2000. Modern Corn and Soybean Production. MCSP Publications. 8 Conley, S.P. and Christmas, E.P. 2005. Utilizing inoculants in a corn-soybean rotation. Soybean production systems. SPS-100-W. Purdue University. http://www.extension.purdue.edu. Web sources verified 02/07/18. 131001060140