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Fertilizing Soybeans

March 27, 2009
Wayne Schoper Brown/Nicollet Extension Educator

As we head into the 2009 planting season we know that there are a lot of things such as the weather and the markets that are out of our control. For example, last summers dry August cost farmers a lot of bushels of soybeans as it was very hot and dry at a very critical point in the development of the soybean plant. However, one thing that is in our control is the fertilizer that is applied to crops on an annual basis. When we think about fertilizer usage, we tend to concentrate on the corn crop. But we know, through research and experience, that soybean yield will also benefit from fertilizer use in much of the northern and western Corn Belt.

George Rehm is a retired soil scientist at the University of Minnesota. George is still heavily involved with research and outreach at the University. He had the following comments in a recent article regarding fertilizing soybeans.

Fertilizer Application on soybeans can be profitable - If soil test levels for Phosphorous (P) and Potassium (K) are in the low or very low range, application of one or both of these nutrients on soybeans can be profitable. In Minnesota, several studies have been conducted to measure the response of soybeans to phosphate fertilization. In general, the application of a reasonable rate of phosphate when P levels are in the low or very low range should produce a 15% to 20% increase in yield. Potash fertilization is profitable if soil test K is low or very low. In recent studies potash application for both 15 inch and 30 inch production systems increased yield by 6% to 7%. Other studies have measured increases in yield in the range of 12% to 15%.

Placement of Immobile Nutrients - In contrast to the corn crop, phosphate and/or potash fertilizer can be either broadcast and incorporated before planting or applied in a band at the time of planting. With conventional tillage systems, slightly higher yields were measured when broadcast applications were used. For no-till production systems, a banded application is preferred because there is only limited incorporation. When air seeders are used, the fertilizer can be applied at the time of planting. For this planting method, soybean seed and fertilizer should not be applied in the same narrow band.

Avoid fertilizer close to the seed - Soybeans, unlike corn and small grains, are sensitive to fertilizer placed close to the seed. The damage to emergence can be substantial. In University studies, fertilizer placed in direct contact with soybean seed or within ? to ? of an inch from the seed caused significant caused emergence problems. To avoid this situation, place the band of fertilizer so that there is at least 1 inch between the seed and the fertilizer.

Phosphate Source - Both fluid and dry fertilizers can be used to supply phosphate when needed. When used at rates to supply equivalent amounts of phosphate, all sources have had an equal effect on soybean production.

Important Numbers - The results of a soil test are necessary to make good decisions when making fertilizer applications for any crop including soybeans. If the soil test for P is 8 ppm (Parts per Million) or lower (Olsen test) or 10 ppm or less (Bray test), an economical response to phosphate fertilization can be expected. Likewise, a response to potash fertilization can be expected if the soil test K is 100 ppm or less.

Fertilizer use for soybean production is not an automatic practice. Use the soil test as a guide. If current soil test information is not current, take the time to take some new samples this spring.



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