Key Points:

• Soil P and K test levels do not increase pound-for-pound with fertilizer applications.
• Fertile soils take 9 pounds fertilizer P₂O₅(phosphate) and 4 pounds fertilizer K₂O (potash) to raise soil test levels 1 pound.
• Lower fertility soil may require 50 percent more P₂O₅and 20 percent more K₂O to raise test levels 1 pound.

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.