Before fertilizing and planting the wheat, farmers should consider the phosphorus demands for their doublecrop systems. It may be time to revive or boost nutrient management programs to be sure that phosphorus supplies do not limit the doublecrop phosphorus needs.

It is important to ensure that phosphorus is plant-available early in the growing season. Phosphorus is critical in the metabolism of plants, playing a role in cellular energy transfer, respiration, and photosynthesis. Wheat produces two kinds of stems – the main stem and a variable number of tillers.

Early in its life cycle, wheat “decides” which tillers to develop. Factors such as phosphorus or nitrogen deficiency, compacted soil, or planting too deep can create stresses that reduce the initiation of tillers. Early season phosphorus deficiency can result in restrictions in crop growth from which the plant will not recover, even if the phosphorus supply is later increased to adequate levels.

Wheat removes about 0.55 pounds of P2O5 (equal to P(2.29)) per bushel and soybeans remove about 0.80 pounds of P2O5 per bushel. Soybean yields are exceeding 50 bushels per acre this fall in much of the South, and wheat yields have exceeded 70 bushels per acre on many farms in recent years.

The combined phosphorus removal at harvest for both crops, at these yields, would be about 80 pounds of P2O5 per acre. Too often the typical phosphorus and fertilization program fails to approach these levels, which may be cutting yields in many double-cropped fields. Failure to at least replace harvested phosphorus will eventually cut yields, reduce profitability, and threaten sustainability as soil phosphorus fertility is depleted.

Some farmers have been tempted to skip phosphorus 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 double cropping is planned, it is time to reevaluate the nutrient management strategy.

Research at several universities has shown that both crops respond well to phosphorus fertilization. To achieve sustained high yields and profits in wheat and soybean doublecrop systems, it is important to apply the phosphorus needs for both crops in the rotation.

Phosphorus uptake will be limited by acidic soils, low soil phosphorus, inadequate phosphorus fertilization, soil compaction, poor soil drainage, drought, diseases, nematodes, temperature extremes, insects, and other stresses.

Soil phosphorus tests, which differ among public and private laboratories usually identify general soil phosphorus fertility requirements, but seldom consider other factors that may limit nutrient uptake efficiency. Adequate phosphorus can help plants better tolerate many of these stresses, and it can help hasten maturity – a critical factor in a double-cropping system.

Adjustments to phosphorus fertilizer recommendations should be based on current soil test results, knowledge of the yield potential, land tenure, and other factors that may affect nutrient management decisions. To sustain production, farmers should at least consider replacing the harvest removal of P2O5 each year through progressive fertilization programs.

Most farmers recognize that wheat is quite responsive to phosphorus fertilization, but many wonder just how responsive soybeans are. In research on soils with low phosphorus levels, when other nutrients were not limiting, soybean yields have been increased by as much as 10 to 15 bushels per acre.

Whether the phosphorus fertilization decision is made by the farmer or in concert with a crop adviser, fertilizer dealer, or Extension agent, it should start with an understanding of the double-crop phosphorus demands. Know what it takes to achieve high yields and profits and take action this fall.

There are many equally effective phosphorus fertilizer sources: diammonium phosphate (DAP; 18-46-0), triple superphosphate (TSP; 0-46-0), or fluid ammonium polyphosphate (APP; 10-34-0 or 11-37-0). Although monoammonium phosphate (MAP: 11-52-0) is less common in the South, it is also an effective phosphorus source and is commonly used in the northern Great Plains region.

The water-soluble P2O5 concentration exceeds 40 percent in each of the dry fertilizers: DAP, TSP, and MAP. This means that the water-soluble portion of the available P2O5 in the dry fertilizer sources exceeds 85 percent. The water solubility is less important than the “available” P2O5, which is the sum of both the water soluble and citrate soluble fractions.

Determine your phosphorus removal in harvested crops, test your soil, and apply phosphorus this fall to benefit both wheat and soybeans and raise the potential for profits. If wheat has already been planted, but phosphorus fertilization was neglected, plan to surface broadcast phosphorus fertilizer as soon as the opportunity exists.

Dr. Cliff Snyder is Southeast director of the Potash and Phosphate Institute.e-mail: snyder@ppi-far.org