Building soil phosphorus slowly and cheaply
Lower phosphate fertilizer prices have generated interest among farmers to build soil phosphorus levels. At the same time, the practice is puzzling to some, explained Ron Gelderman, professor and SDSU Extension soils specialist.
"Why increase soil phosphorus levels? The answer has to do with the stability of phosphorus and farmers' desire to save dollars in the future by purchasing this fertilizer at today's lower prices," Gelderman said.
He explained that soil phosphorus is relatively stable and unlike nitrogen, phosphorus is not easily lost.
"In most soils, the phosphate ion will move into more stable forms and be released as the plant requires it. Therefore, due to current lower than expected prices, producers want to store the less expensive nutrient on their fields, especially those with lower soil phosphorus tests," he said.
Building soil test phosphorus levels from low to high, Gelderman said, will increase yield, even if no pre-plant phosphorus fertilizer is applied to those fields.
"In long-term studies at SDSU, it was found that low soil test phosphorus levels yielded as well as very high test phosphorus levels when the appropriate phosphorus rates were applied," he said.
Therefore, if a farmer builds soil test phosphorus levels from medium to very high, there is little need to apply additional phosphorus, except for adding a small amount with a starter.
"This practice of phosphorus management allows for reducing P rates in those years when phosphorus fertilizer prices are higher than average," Gelderman said.
How much phosphorus fertilizer does one need to increase soil test phosphorus?
Research at SDSU and many other Midwestern universities has shown an average of 18 to 20 pounds of applied phosphate fertilizer per acre is needed to raise soil test phosphorus one part per million. The 18-20 pound rate is over and above any phosphorus applied to replace phosphorus removed with the harvested crop.
Newly revised phosphorus removal rates are 0.27 and 0.61 pounds of phosphate per bushel for corn and soybean, respectively. For example, Gelderman said in a field that yields 180 bushels per acre corn crop, phosphate removal is about 49 pounds of phosphorus per acre; and in a field that yields 60 bushels per acre soybean crop, there is about 37 pounds of phosphorus per acre removed.
"Therefore applying, 120 pounds of actual phosphorus for each of two years in a corn-soybean rotation would raise soil tests about 6 parts per million. It can be a slow and fairly expensive process to build soil phosphorus tests," Gelderman said.
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