The common practice of applying phosphorus (P) only before corn in a corn-soybean rotation may not be the best option.

George Rehm, a University of Minnesota soil scientist, reached that conclusion after a P fertilization study.

"The study was done to see if we could reverse the situation and apply our phosphorus ahead of soybeans and let the corn live off what's left over rather than the other way around," Rehm reports. "Applying it before the soybean crop is probably just as good; in fact, a little better."

He points out that soils testing low in P should be fertilized every year. Those with medium P levels can be fertilized just once, before the corn or the soybeans, and high-testing soils don't need additional P for either crop.

The study, conducted through two cycles of a corn-soybean rotation, compared four phosphate application rates and two application methods: broadcast incorporated and deep banding. In the deep banded treatments, P was placed 4-6" deep in fall, then the crop was planted over the bands the following spring.

The results show that soybeans respond well to P fertilization when soil-test levels are low or very low, says Rehm. No significant yield difference was seen between broadcast and banded treatments.

Banding, Rehm points out, can work effectively for no-tillers. But it's more expensive than broadcasting, so growers using conventional tillage probably should stick with broadcast P applications.

(John Lamb, George Rehm, Andrew Scobbie and Thor Sellie, University of Minnesota)