Phosphorus (P) and potassium (K) management get easier when you understand what a soil test reading is, and what it isn’t, says Farm Journal Field Agronomist Ken Ferrie. “It’s an indicator of nutrient availability. It’s not a measurement of the total supply in the soil.”
With P, the test reading is not an absolute value, but only an indication of whether the soil is likely to supply sufficient P for the crop. (Most soils hold 3,000 lb. to 4,000 lb. of P, but only a very small amount is in plant-available form.)
“Say your soil test shows 46 lb. of phosphorus per acre and you need 70 lb. per acre for a 200-bu. corn crop,” Ferrie says. “If you apply 24 lb. per acre, the soil test should read zero at the end of the year. But it doesn’t work that way. All the 46-lb. number means is the soil should supply enough ortho-phosphate for the crop. When farmers think the soil test reading is the amount of phosphorus actually available to the crop, they are misinterpreting the number.
“The number 46 in this example relates only to the extraction method used by that particular soil test laboratory, and there are many methods. It was never intended to mean there are 46 lb. of available phosphorus in the soil,” he explains.
It’s the same with K, a soil test attempts to predict how much K will become available for the next crop, not how much is actually present in the soil.
“Many soils contain enough potassium to meet crops’ needs for 500 to 1,000 years,” Ferrie says. “But we continue to apply potash fertilizer because not enough soil potassium will become available in time for the crop to use it during the growing season.”