According to Michigan State, 200 bushels of corn removes 70 pounds P2O5and 54 pounds K2O. 60 bushels of soybean removes 53 pounds P2O5and 84 pounds K2O. How will this reduce soil test levels?
200 bushels of corn reduces the soil P2O5test level by about 8 pounds and the K2O test level by over 13 pounds. 60 bushels of soybeans reduces soil P2O5almost 6 pounds and soil K2O over 9 pounds. These numbers are based on the old rule that adding/removing 9 pounds of P2O5will increase/decrease the soil P2O5test level by 1 lb, and adding/removing 4 pounds of K2O will increase/decrease the soil K2O test level by 1 lb.
To replace P and K removed by corn, we need 134 pounds MAP (11-52-0) and 90 pounds potash (0-0-60). Over 100 pounds MAP and 140 pounds potash is needed to replace P and K removed by soybeans.
Actually, these ratios change with soil test levels. According to the University of Kentucky, the 9:1 ratio of fertilizer P2O5to soil P2O5test level is accurate around soil test levels of 30 lb/ac. It takes over 14 pounds P2O5to build the soil P2O5by 1 pound when the soil test level recedes below 15.
In other words, it takes 50 percent more P2O5to build a soil test level by 1 pound P2O5in infertile soil than in fertile soil. This greater ratio also applies to nutrient removal: soil test levels will decrease 50 percent faster with P2O5removal in infertile soils.
Similarly, the 4:1 ratio of fertilizer K2O to soil test K2O is most appropriate to soil tests levels around 275 pounds/ac K2O. Soils testing around 175 lb/ac K2O require 5 pounds/ac K2O fertilizer — 20 percent more — to raise the soil test level 1 pound. Test levels in infertile soils also decrease 20 percent faster with K2O removal.
Also note that potassium gets fixed, or trapped, in clay soils, so as potassium levels decrease, more sites become vacant to trap new potassium. Therefore, K2O requirements increase even faster in these soils as test levels decline.
These results have important implications for soil fertility management: as soil test levels of P and K decrease, it will take more fertilizer per pound soil test value to repair them.