Direct and Residual Fertilization in Soybeans

A common practice in the U.S. Midwest is to combine the quantities of phosphorus (P) and potassium (K) recommended for both corn and soybean crops into a single application. This biennial application of fertilizer is usually applied after soybean harvest. In a corn/soybean rotation, this results in a “direct” application to corn and a “residual” application to soybeans. Less common, although also studied by scientists, has been a biennial application applied after corn, resulting in a direct application to soybeans and a residual application to corn.

Traditionally, biennial applications of P and K have been adopted primarily to cut down on fertilizer application costs, but is this the best way to feed soybean the P and K that they need? To answer this question, we searched for studies that compared a biennial application to an annual one. In these studies, total rates of P and K were kept the same. The only difference was in how that rate was distributed over time. In the biennial application, a rate was applied that was twice that of the annual application. Annual applications resulted in “direct” applications for both corn and soybeans. In the biennial application, either corn or soybeans received the “direct” application, depending on when the application was made.

In Illinois, a study was conducted in two locations from 2001 to 2005. Various combinations of annual and biennial applications of P and K were studied in two tillage systems: no-till and chisel till. In the no-till system, annual versus biennial applications made after soybean harvest did not produce any differences in soybean yield. In the chisel-till system, one of 10 cases had a lower soybean yield (by 8 bu/acre) when both P and K were applied annually versus when they were applied biennially after soybean harvest.

Because this study looked at a variety of combinations of applications, it was possible to compare a biennial application of P and K after soybean harvest to a biennial application of P after soybean harvest combined with a biennial application of K after corn harvest. This allowed us to compare the effects of a larger, biennial, “direct” application of K to a “residual” K application. When soybeans were fertilized directly with these larger doses of K, yield decreased in 1 of 10 cases in both chisel-till and no-till. Yield depressions were 7 to 10 bu/acre.

Two studies have been conducted in Iowa. A much earlier one, conducted from 1968 to 1971, showed that soybeans did not respond to the differences between annual vs. biennial applications; however, in a more recent study conducted from 1994 to 1996, soybeans responded more frequently to a direct application of P than to residual P when soil test levels were optimum or lower. Differences were typically 3 bu/acre or less.

In Indiana, a study was conducted that examined K at four locations from 1997 to 2002 and compared annual, direct applications to soybeans to biennial, residual applications. Annual applications were half the rate of K as the biennial ones. In the majority of cases, there was no impact on soybean yields. In one case, soybean yield increased by 5 bu/acre with an annual application, and in another case, soybean yield decreased by 6 bu/acre.

The last study we examined came from Minnesota and looked at P exclusively. It was conducted from 1994 to 1998. It differed from most of the other studies in the primary comparison it made. Instead of using a biennial application made after soybean harvest as the basis of comparison, it used a biennial application after corn harvest. This created a larger, direct application of P to soybeans and a residual application to corn. This biennial application after corn harvest was compared to an annual application made before each corn and soybean crop. Soybean yield was not affected.

So what picture are all of these studies painting? First, it’s important to recognize that there have been only a few studies, so the picture is far from complete. Based on the information we have, there may be a small yield advantage to applying P directly to the soybean crop as an annual application when soil tests are lower. When it comes to a larger, biennial application of K, it may be best to apply that after soybean harvest rather than after corn. Soybeans may be sensitive to these larger applications.

Further Reading 

Buah, et al. 2000. Agron. J. 92:657–662. 

Fernandez, F. and S.M. Brouder. 2005. Proc. North Central Extension-Industry Soil Fert. Conf. 21:82–90. 

Hansen, N.C. and G.W. Rehm. 1999. Proc. North Central Extension-Industry Soil Fert. Conf. 15:16–25. 

deMooy, et al. 1973. Agron. J. 65:851–855. 

About the Author 

Dr. Scott Murrell serves as the U.S. Northcentral Director for the International Plant Nutrition Institute. He received his B.A. degree in history at Purdue University in 1986, studied ancient history as a Ph.D. student for a year at Yale University, and changed disciplines to receive an M.S. in soil fertility from Purdue University in 1991. He also received his Ph.D. in soil science from Texas A&M University in 1996. 

Dr. Murrell has spoken and written about many aspects of 4R Nutrient Management. He enjoys finding ways of quantifying the impacts of various nutrient management decisions, which has led him to work in the area of nutrient recommendation algorithms. 

Dr. Murrell is involved in many professional societies, and has received both the Werner L. Nelson Award for Diagnosis of Yield-Limiting Factors and the Soil Science Industry and Professional Leadership Award. He currently resides in West Lafayette, Indiana.

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