AGRI-BRIEFS
  AGRONOMIC NEWS ITEMS 		From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1999, No. 5

PHOSPHORUS AND POTASSIUM FOR DOUBLECROPPED SOYBEANS AND WHEAT - HAVE YOU PROVIDED ENOUGH FOR BOTH CROPS?

As a result of better management and genetic improvements, national soybean yields continue to increase. Yields averaged 39 bushels per acre in 1997 and 1998. Winter wheat yields have also continued to increase, with an average of 45 bushels per acre in 1997 and 47 bushels per acre in 1998. In some states, soft red winter wheat yields are likely to average above 50 bushels per acre in 1999. Soft red winter wheat yields have averaged 70 to 90 bushels per acre in the last few years, in many fields. Yields approaching 100 bushels per acre are realistic and attainable for many soft red winter wheat farmers.

Soybeans and wheat both require an ample supply of phosphorus and potassium for good yields. The seed of these two crops contain significant amounts of phosphorus and potassium, which are removed from fields at harvest. Soybeans remove about 0.80 pound of P2O5 and 1.40 pounds of K2O per bushel, and wheat removes about 0.55 pound of P2O5 and 0.34 pound of K2O in every bushel. At the national average yield levels, soybeans and wheat together will remove an average of 56 pounds of P2O5 and 70 pounds of K2O per acre.

Sustained high yields require replacement of harvested nutrients, particularly on low- to medium -fertility soils. If soybean yields averaged 55 bushels per acre and wheat averaged 75 bushels per acre, the combined nutrient removals would be 85 pounds of P2O5 and 103 pounds of K2O per acre. High yields require good soil fertility and fertilization programs, particularly for the doublecrop demands.

Since soybean prices are still low, you might ask, “Just how responsive are soybeans alone to phosphorus and potassium?” A recent scientific journal reported the results from six years of research at the West Tennessee Experiment Station. Phosphorus and potassium fertilization rate responses were evaluated in a comparison between a soybean cyst nematode (SCN)-tolerant (races 3 and 4) and a SCN-resisitant variety. The study was conducted on a soil testing low in phosphorus and potassium. With 0-30-30 (N-P2O5-K2O) applied annually, soybean yields increased an average of 14.3 bushels per acre per year for the cyst nematode- resistant variety. Estimated returns above fertilizer and application costs were $36.15 per acre per year (assuming $4.50 per bushel for soybeans) for the cyst-resistant variety. The best economic response by the cyst nematode- susceptible, soybean variety was to 0-60-60 applied annually. Yields were increased an average of 16.4 bushels per acre per year and resulted in estimated returns above fertilizer and application costs of $47.10 per acre per year.

Wheat also responds to phosphorus and potassium on soils with lower than optimum fertility. Benefits from improved phosphorus and potassium nutrition include: higher yields, less disease susceptibility, timely maturity, and efficient nitrogen utilization.

Plan to provide the nutrient needs of your wheat crop this fall and the demands of doublecropped soybeans. Determine your nutrient removal in harvested crops, test your soil, and apply recommended phosphorus and potassium to boost yields and raise the potential for profits.


—CSS—

For more information, contact Dr. Cliff S. Snyder, Midsouth Director, PPI, P.O. Drawer 2440, Conway, AR 72033-2440. Phone (501) 336-8110. E-mail: csnyder@ppi-far.org.
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