Do Soybeans Really Need “P” Fertilizer After Corn?

Many corn and soybean growers in the Midwest tend to only apply “P” fertilizer for the corn crop in a corn soybean rotation. They rely on the residual affects of the fertilizer applied to the corn to carry the soybean crop. Research from the University of Minnesota has shown that if the soil P test level is high, soybean yields should be fine without additional P fertilizer application for the soybeans. If the soil P level is low or very low, soybean yield losses can be great if no “P” fertilizer is applied that year. Another factor is the rate of “P” applied in the corn year. If you band apply the “P” to the corn and cut the broadcast rate 50% as suggested by University guidelines, you will have substantial yield losses in the soybeans the following year on low “P” soils. These facts are shown in research conducted from 1998-2000 at Waseca, MN. The table shows that under low soil “P” levels, soybean yield losses will occur when the preceding corn crop was fertilized with a 40-50 lb/a band rate of “P”. The data also shows that 80-100 lb/a broadcast “P” applied to the corn was not enough “P” to achieve the same soybean yield on the low “P” site.

With the high price of fertilizer right now, growers really have two choices if there soil “P” test level is low or very low. One choice is to band apply a lower rate of “P” for their corn crop and then plan on applying “P” fertilizer for the soybeans the next year. They will be hoping the price of “P” fertilizer is lower when they fertilize for the soybean crop next year. The other choice is to apply the full rate of broadcast P recommended to the corn the first year and save the cost of fertilizer application when they plant soybeans the next year.

If a grower has built his soil test levels up to a high level through good management the past many years, he can cut his rate of “P” and band apply for the corn and not worry about the soybeans. The high soil “P” test will provide for his soybeans. This is one of the many benefits of having built soil test “P” levels to the high range while fertilizer prices were lower in the past.

Soybean response to “P” applied in Previous Corn Crop
University of Minnesota, 1998-2000
Corn Soybean “P” App.
Method
High P
Soil Test
Low P
Soil Test
High P
Soil Test
Low P
Soil Test
Tillage System Tillage System P2O5
Lb/a
P2O5
Lb/a
Soybean
Yield bu/a
Soybean
Yield bu/a
One-pass One-pass None 0 0 56 38
Starter 40 50 56 47
Spring Broadcast 80 100 55 52
Chisel Chisel None 0 0 56 34
Starter 40 50 56 50
Spring Broadcast 80 100 56 54
Low P site was 3-4 ppm
High P site was 19 ppm
All “P” fertilizer applied before corn planting in the fall or spring.
No “P” fertilizer applied before soybean planting.