Tilled under soybean plants contribute organic matter that decomposes and enriches the soil. This process adds essential nutrients like nitrogen, phosphorus, and potassium to the soil, improving its fertility. The exact nutrient content will depend on the specific soil conditions and the growth stage of the soybean plants at the time of tilling. Tilling your crop into the soil also improves soil conditions. The organic matter lightens the soil and improves drainage.