Building soil phosphorus slowly and cheaply
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.