Phosphorous and potassium exist naturally in the soil as a part of rock, clay and other minerals that make up soils. Levels of phosphorous in the soil can be between 100 to 3,000 pounds of total P per acre. Potassium exists in higher quantities of 10,000 to 50,000 pounds of total K per acre. These levels are substantial but plant available P and K are the important measure in crop production. Due to the buffering of the soil solution quantities of nutrient from these sources along with the associated fixation and release with fertilizer addition or crop removal does not affect soil test level on a 1:1 basis.
The buildup formulas for P and K fertilization found in the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa give us some indication of the amount of fertilizer needed to change soil test levels 1 ppm for both P and K. This concept plays a role in what we need to do when we have a year like 2012 where drought substantially reduces yield, or where we have high soil test levels due to past practices, and want to know how long until we would draw down to maintenance limits.
The buildup equations in the Tri-states indicate that it takes 20 lbs./A of P2O5 to change soil test P levels one ppm. For potassium the equations indicates 6 to 10 lbs./A of K2O are required to change soil test 1 ppm depending upon the soil CEC.
Just to use phosphorous as an example a 150 bushel/acre corn crop will remove 55 pounds of P2O5/acre in the harvested grain. Thus the 150 bushel corn crop would move soil test level approximately 3 ppm.
View a new Factsheet, Interpreting a Soil Test Report AGF-514-12. This will help you understand the various factors found on a soil test report and how they relate to soil fertility needs for a crop.