Soybean Fertility

A typical soybean fertility plan for North Dakota includes nitrogen, phosphorus, potassium, sulfur, and iron. Most North Dakota soils have sufficient quantities of the other nutrients.1

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Soybean plants utilize high amounts of nitrogen (N), upwards of 300 lb/acre for a 60 bu/acre seed yield.2 The total N requirement for soybean normally comes from N fixation and residual soil N. Use of inoculants in the North are currently recommended for fields that have never been planted to soybeans, where soybeans have not been grown in the past three to five years, soil pH is below 6.0 or above 8.0, soil has a high sand content, or a field has been flooded for more than a week, creating anaerobic conditions.3 A 13-year study at the Carrington Research Extension Center, comparing inoculation with an untreated check, shows an average yield advantage of 2.1 bu/acre and a 0.9% protein advantage on fields requiring inoculation.4

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Supplemental N is not normally needed in soybean if nodulation is good.1 Supplemental N applied at the R1 to R3 stages of soybean growth have not been shown to provide a yield advantage in North Dakota. High soil nitrate levels may increase the severity of iron deficiency chlorosis (IDC) where IDC is a problem.

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North Dakota soils are typically low in phosphorus (P). Soybeans respond best to broadcast P applications on medium to low P soils. Research in North Dakota and surrounding states have shown banding and broadcast P applications to be equally effective.5 In no-till systems, banded and broadcast applications have been shown to produce similar yields because soybean roots actively take up P near the soil surface. Narrow bands of fertilizer that contact soybean seed can cause stand reduction. University of Minnesota recommendations suggest that P fertilizer should be applied in the spring on soils with a pH of 7.4 or higher to reduce tie-up of P, allow more efficient use of P by soybeans, and keep soil levels high for soybean and the next crop.5 Potassium (K) should be applied to replace K used by the crop and keep soil levels high to maintain high productivity in soybean and the next crop.

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NDSU has found that, on soils with and without susceptibility to soybean IDC, yield potential can be best managed by seeding a high-yielding IDC-intolerant product in non-IDC soils and an IDC-tolerant soybean product in the IDC-susceptible soils. The best application of iron for the crop to reduce IDC is ortho-ortho-EDDHA iron chelate applied with water in-furrow.1

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Sulfur (S) is important to soybean growth and development but in many cases crop needs will be met by residual S in the soil. If the soils are loam or coarser in texture, the fall was wet, snowfall was normal to above normal, and/or spring was wet prior to planting, the application of 10 pounds of S as ammonium sulfate or gypsum would be recommended.1 S is suggested for fields with a history of reduced yields for crops susceptible to deficiency such as alfalfa and corn, soil organic matter in the top six inches is 2.0% or less, and no S has been applied on the field for many years.5

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Deficiencies of zinc, manganese, boron, molybdenum, nickel, chloride, or copper have not been recorded in North Dakota.1 Research trials conducted in North Dakota and Minnesota have shown that the soybean crop typically does not respond to the application of magnesium, boron, zinc, manganese, and copper.