AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Fall 2001, No. 6
Soybeans remove about 0.80 pounds of P2O5 per bushel and 1.40 pounds of K2O per bushel at harvest. Wheat removes about 0.55 pounds P2O5 per bushel and 0.34 pounds of K2O per bushel. At the national level, soybean and wheat yields have been steadily increasing. On many farms, soybean yields are exceeding 50 bushels per acre, and wheat yields are exceeding 70 bushels per acre more often than in previous years. The combined phosphorus and potassium removal at harvest for both crops, at these yields, would be about 80 pounds of P2O5 per acre and 114 pounds of K2O per acre. Too often the typical phosphorus and potassium fertilization program fails to approach these levels, which may be cutting yields in many doublecropped fields. Failure to at least replace harvested nutrients will eventually cut yields, reduce profitability, and endanger sustainability as soil fertility is depleted.
To achieve sustained high yields and profits in wheat and soybean doublecropping systems, it is important to apply the nutrient needs for both crops in the rotation. Some farmers have been tempted to skip phosphorus and potassium fertilization of soybeans in recent years because of low crop prices. Where this has been the case for more than a year or two, and doublecropping is planned, it is time to reevaluate the nutrient management strategy. Past research has shown that both crops respond well to phosphorus and potassium fertilization. Wheat may require higher soil test phosphorus and soybeans may require higher soil test potassium levels as a result of the increased yield potential and higher nutrient demand. This observation may help explain the increased reports of potassium deficiency symptoms in soybeans in many states.
Good farmers recognize that nutrient uptake demand is much greater than nutrient removal. For example, a 70 bushel per acre wheat crop will take up about 47 pounds of P2O5 per acre and 142 pounds of K2O per acre, while a 50 bushel per acre soybean crop will take up about 52 pounds P2O5 per acre and 170 pounds of K2O per acre, for a doublecrop total uptake of 99 pounds of P2O5 per acre and 312 pounds of K2O per acre. The doublecrop harvest removal is equivalent to about 80 percent of the P2O5 and 37 percent of K2O uptake. Nutrient uptake will be limited by acidic soils, low soil phosphorus and potassium levels, inadequate phosphorus and potassium fertilization, soil compaction, poor soil drainage, drought, diseases, nematodes, temperature extremes, insects, and other stresses. Soil tests usually identify general soil phosphorus and potassium fertility requirements, but seldom consider other factors that may limit nutrient uptake efficiency. Adequate phosphorus and potassium can help plants better tolerate many of these stresses.
Adjustments to phosphorus and potassium fertilizer recommendations should be based on recent soil test results, knowledge of the yield potential, land tenure, and other factors that may affect nutrient management decisions. Whether the phosphorus and potassium fertilization decision is made by the farmer or in concert with a crop adviser, dealer, or Extension agent, it should start with an understanding of the doublecrop nutrient demands. Know what it takes to achieve high yields and profits ….. and take action.