Although soybeans produce better yields than many crops on infertile and acid soils, applications of manure, lime, and commercial fertilizers are necessary for the production of high yields of seed or forage. Soils that are sufficiently fertile to grow corn satisfactorily will produce correspondingly good yields of soybeans. Soybeans, however, remove more phosphorus and potassium from the soil than corn. According to workers at the Illinois agricultural experiment station, soybeans yielding 20 bushels per acre remove slightly more phosphorus and more than three times as much potassium as a 40-bushel crop of corn. In each case, the straw was returned to the fields. Another comparison shows that a growth of soybeans that will produce 2 ½ tons of hay per acre will remove twice as much phosphorus and about five times more potassium than a 40-bushel crop of corn. Soybeans also respond favorable to application of lime; and unless soils are kept properly inoculated, applications of commercial nitrogen fertilizer aren’t necessary on fertile soils. Moderate applications of commercial nitrogen fertilizer are recommended on infertile soils, however, to stimulate the plant to grow rapidly during early stages.
Applications of commercial fertilizer to infertile soils also result in favorable changes in the composition of the seed. Applications of commercial fertilizer containing phosphorus increases the percentage of protein and phosphorus in the seed, whereas treatment with fertilizer containing potassium increases the oil content and improves the quality of the seed. Applications of chemical fertilizer containing both phosphorus and potassium to fertile soils generally have little or no effect on the composition of the seed or the yield. Because soil conditions vary considerably, it is not possible to make specific recommendations as to the kind and quantity of commercial fertilizers that should be applied. Fertilizers should not be applied in direct contact with the seed because soybeans are easily injured by such treatment. Rates of application up to about 125 pounds per acre may be made as starter fertilizer along the row. If application rates exceed this amount, at least a part of the fertilizer should be applied broadcast to avoid injury to the seed.