Soybeans remove large quantities of phosphorus and potassium from the soil. Grain removal alone can remove 0.90 lbs. of phosphorus and 1.5 lbs. of potassium per bushel of soybeans. A 60-bushel soybean yield removes 90 pounds of K2O per acre, or the equivalent of 150 pounds of 0-0-60. If grown in a rotation with 180-bushel corn, an additional 45 pounds of K2O, or 75 pounds of 0-0-60 are needed, for a total of 225 pounds of 0-0-60 per acre every two years just to replace what is removed. Additional amounts are needed to account for efficiency factors and to meet any build-up needs.
Standard application rates of 100 pounds of 0-0-60 per acre commonly reported by Midwest farmers are not sufficient to sustain productivity at those yield levels. At that rate, fields are being mined of their potassium reserves. That steals potential from future crop years. Usually it appears as chlorosis, or yellowing, of the lower leaves' outside margin. This deficiency occurs as the plants are shifting toward high nutrient demand during the filling of the pods. If the roots are restricted, potassium soil tests are inadequate, or the soil is dry, symptoms will be more prominent.
Take soil tests, measure crop yields and be sure potassium rates are adequate to meet crop needs.
To maximize soybean yields, make sure a recent soil test indicates that your soil has an optimum level of phosphorus and potassium.