Agriculture and Environment Extension Agriculture and Natural Resources Publications 1-2013 Soybean Nutrient Needs Clarke McGrath Iowa State University, [email protected] David Wright Iowa State University Antonio P. Mallarino Iowa State University, [email protected] Andrew W. Lenssen Iowa State University, [email protected] Follow this and additional works at: http://lib.dr.iastate.edu/extension_ag_pubs Part of the Agricultural Science Commons, Agriculture Commons, and the Agronomy and Crop Sciences Commons Recommended Citation McGrath, Clarke; Wright, David; Mallarino, Antonio P.; and Lenssen, Andrew W., "Soybean Nutrient Needs" (2013). Agriculture and Environment Extension Publications. 189. http://lib.dr.iastate.edu/extension_ag_pubs/189 Iowa State University Extension and Outreach publications in the Iowa State University Digital Repository are made available for historical purposes only. Users are hereby notified that the content may be inaccurate, out of date, incomplete and/or may not meet the needs and requirements of the user. Users should make their own assessment of the information and whether it is suitable for their intended purpose. For current publications and information from Iowa State University Extension and Outreach, please visit http://www.extension.iastate.edu. Soybean Nutrient Needs Building a sustainable soybean and corn phosphorus (P), and potassium (K) as well as a cropping system that consistently produces high smaller amount of sulfur (S) and some yields isn’t easy but it is possible. It begins with micronutrients. Although soybean requires improving soybean and corn yield using proven considerably less P and S than N or K, all are management strategies. Obtaining maximum important for plant growth and development. yield is only possible when the plant’s nutritional requirements are met and environmental stress is Soybean grown in Iowa does not require an limited. Maintaining a fertile growing application of N fertilizer. Leguminous crops, environment is a risk management strategy that like soybean, meet their demand for N through a produces higher yield and healthier plants that process called biological N fixation. Soybean are often better suited to withstand the yield- forms a symbiotic relationship with soilborne robbing effects of biotic and abiotic stress. A rhizobia bacteria (Bradyrhizobium japonicum) to well planned soil fertility program is a convert, or ‘fix’ atmospheric N gas to ammonia management strategy that leads to profitable (NH ) N, a form usable by the plant. The soybean production. A knowledge of what the 3 rhizobia bacterium attaches to new soybean roots nutrients do, which ones are needed, how much just behind the root tip then colonizes the plant. to apply, and when to apply them is an important In response to bacterial chemical signals, plant part a successful management strategy. root cells form nodules, which help protect the NUTRIENT REQUIREMENT OF SOYBEAN bacteria from oxygen. Nodules grow very rapidly and will continue to supply N to the plant Soybeans respond to fertile soils. It is a common for approximately six weeks. Nodules begin production practice in Iowa to fertilize the corn supplying N during the early vegetative growth crop and let the rotational soybean crop utilize residual soil nutrients the following year. This Table 1. Average nutrient removal rates for soybean and corn grown in the is a good management Midwest. practice, when coupled N P2O5 K2O S with a regular soil testing program, to --------------- pounds/bushel of grain -------------- † † provide enough Soybean grain 3.8 0.80 1.50 0.10 nutrients to produce two Soybean stover 1.1 0.24 1.0 0.17 high-yielding crops. Total 4.9 1.04 1.50 0.27 Corn grain 0.90†† 0.375† 0.30† 0.08†† High yielding soybeans Corn stover 0.45†† 0.16†† 1.10†† 0.07†† require large amounts of Total 1.35 0.54 1.40 0.15 nitrogen (N), Source: Heatherly, L.G and R.W. Elmore (5) except where noted; †Iowa State University Extension †† publication PM 1688 (17); International Plant Nutrition Institute; Amounts base on market moisture PM 3045 1 phase (V2-V3) and will continue through seed residues, a process highly influenced by rainfall fill (R5-R6). (9,10). The rate of P loss is much less because most plant P is organic and less soluble in water What is the demand for soil nutrients from one than K, which is found as a free cation in plant rotation cycle of average soybean and corn tissue. They also reported that P and K recycling crops? A bushel of soybeans removes about 3.8 of soybean is rapid because most P and K are in pounds of N, 0.8 pounds of P (P2O5) and 1.5 the leaves, which drop from the plant and pounds of K (K2O) (Table1). quickly decompose on the moist soil. Yield is the primary factor determining soil Opportunities for increased yield with P and K nutrient removal (10). Nutrient requirements for fertilization are likely on some fields depending all crops increase concomitantly with yield. on soil test levels. For best results, P and K Average soybean and corn yields in Iowa during should be applied when the chance of a yield the 2001 to 2011 period were 48 and 168 bushels increase is significant and the expected yield per acre, respectively (19). A soybean crop increase is sufficient to pay for the applied yielding 48 bushels per acre would remove nutrients. approximately 182 pounds N, 38 pounds P2O5 and 72 pounds K2O per acre from the field. The only way to accurately estimate the P and K Nutrient removal in the grain from a single two- supplying power of a soil is with a series of year cycle of soybean-corn rotation producing properly collected soil tests over years. average yields would be 333 pounds N, 63 pounds P2O5 and 122 pounds K2O per acre. A Understanding the Iowa State University soil test portion of these amounts will be supplied from interpretation categories is an important step in soil nutrient reserves and N fixation by the the process of determining P and K fertilization. soybean roots. The remainder will need to be The likelihood that application of these nutrients added through decomposition of crop residue and will produce a positive yield response within the application of fertilizers and animal manure. each soil test interpretation category is: FERTILIZING FOR HIGH-YIELDING Table 2. Likelihood of increasing soybean SOYBEANS yield from fertilizer application to soil with varying soil test levels. Iowa State University recommends fertilizer P and K Soil Test Likelihood of Yield 1 applications be based on soil test levels and Level Increase estimates of nutrient removal by the grain (17). Very low 80% Studies show a strong linear relationship between Low 65% Optimum 25% grain yield level and P2O5 and K2O removal with grain harvest. Studies also show a good High 5% Very high <1% relationship between estimates of P2O5 removal 1 The rates recommended for these soil test categories are in grain to soil test levels over several years. The based on field response trials and attempt to optimize long- term profitability by avoiding yield losses where responses are relationship between K2O removal and soil test large and very likely, and provide a high probability of K levels is more variable but studies show a maximized profits at current prices. Source: Sawyer et al. (17) long-term soil K decrease of 3.0 ppm/year (10). Critical soil test and plant tissue test Soil K levels appear to be highly influenced by K concentrations of secondary nutrients such as loss from standing maturing plants and crop 2 calcium (Ca), magnesium (Mg), and S; and reported by Sesay and Shibles from research micronutrients such as iron (Fe), zinc (Zn), conducted in the late 1970s (18). However, Haq copper (Cu), and molybdenum (Mo) and their and Mallarino also reported that when averaged relationships with soybean yield response to over 48 farm research locations, foliar fertilization are not available for Iowa farmers. fertilization increased soybean yield by 0.80 This is mainly because no deficiencies have been bushels per acre. The few higher yield responses observed in the past. Suggested optimal tended to occur with low early P uptake, high concentrations for soil or plant tissue tests for soil cation exchange capacity, and/or deficient soybean suggested in a few Corn Belt states may rainfall in spring and midsummer. In subsequent not be applicable to Iowa conditions, and research at 26 fields that included other nutrient researchers in those states emphasize the poor mixtures, Mallarino and Haq (11) concluded that reliably of these guidelines. Prediction of soybean response to foliar fertilization across all potential yield response from micronutrient production conditions will seldom offset application cannot be predicted well based on fertilization costs. More recent research by concentrations in soils or soybean tissues. Mallarino and Kaiser at five fields (8) that However, Mallarino and colleagues at Iowa State included spraying N or N, P, K foliar fertilizers University began a project in 2012 to assess alone or in mixture with a fungicide showed that soybean yield response to several micronutrients only the fungicide increased yield, and the N and, if deficiencies are found, calibrate soil and foliar fertilizer decreased yield. plant tissue tests. INFLUENCE OF SOIL pH ON NUTRIENT FOLIAR FERTILIZATION AVAILABILITY Efforts to increase soybean yield through foliar Soil pH is an indication of the general chemical application of N, P, K, S and various micro- environment in a soil. It is a measure of the nutrients began in the 1970’s when Iowa relative acidity or alkalinity. A soil pH of 7.0 is research by Garcia and Hanway (2) reported neutral, with a value below 7.0 being acidic and increased yields from a specific N, P, K, S above 7.0 alkaline.