"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.
Are there some soils that will not build or build very slowly?
"Yes – unfortunately we don't have these identified, although we believe there are not many in the state," he said. "The build rate of 18 – 20 pounds of fertilizer to raise soil test by 1 part per million is an average of many soils, although most were medium to fine textures."
He said that coarse soils, with less clay tend to take less fertilizer to raise soil test, whereas heavy clays, tend to take more phosphorus to raise soil tests. In addition, higher pH soils (greater than 7.5) especially with free calcium carbonate tend to take more fertilizer phosphorus to raise soil test phosphorus.
"In saying this, we have had studies on high clay soils with free calcium carbonate that have similar build rates as above. For some soils, we just don't know the phosphorus build rate that is needed," Gelderman said. "If you have applied extra phosphorus fertilizer above what the crop removed for several years and the field or areas within the field have not changed in soil test, these soils are likely fixing added phosphorus and will not release this phosphorus easily to the plant."
Crops growing on these soils typically respond to band applied phosphorus better than broadcast phosphorus, Gelderman said. He added that an annual soil phosphorus tests should be taken to monitor trends and adjust applications accordingly.
"Don't use just one year's soil test to make soil test phosphorus adjustments, but at least use two to three test years to determine trends," he said.
The bottom-line for increased soil phosphorus test levels is that it can be a slow and expensive process building soil phosphorus levels and should only be used if fields are owned and then only if current fertilizer phosphorus prices are significantly lower than typical prices.