Some growers are considering a shift from a corn and soybeans rotation to continuous corn. More nitrogen (N) will be needed since soybeans will no longer provide some residual N. Other nutrient needs will also change, especially phosphorus (P). Corn, unlike soybeans, is planted early in soils that are more likely to be cool, moist and with a heavier residue cover. These conditions can suppress the uptake of P by corn and increase the likelihood of crop response to fertilizer P. Research shows that P, with some N, applied in a band two inches to the side and below the seed, boosts seedling access to a readily available supply of P.
According to agronomists with the Potash & Phosphate Institute (PPI), the nutrient needs for high-yield corn are known, as are those for soybeans. A 200-bushel corn crop will take up about 265 pounds of N per acre, 115 pounds of phosphorus pentoxide (P2O5), 265 pounds of potash (K2O), 65 pounds of magnesium (Mg) and 33 pounds of sulfur (S). A 65-bushel soybean crop takes up nearly 315 pounds of N (produced by N-fixing bacteria in the nodules of soybean roots), 60 pounds of P2O5, 200 pounds of K2O, 25 pounds of Mg and 20 pounds of S. Much of the N produced by soybeans is removed in the seed or is lost over the winter months. Some remains as residual N available for the following corn crop. Continuous corn will not have access to this N source, which might amount to 30 to 50 pounds of N per acre.