Phosphorus and Potassium Placement Methods for Corn and Soybean: an Iowa Perspective

Increasing fertilizer prices and awareness of potential impacts of excessive or badly applied nutrients on water quality has renewed interest in fertilizer management strategies that reduce nutrient inputs or improve efficacy. Fertilizer recommendations for phosphorus (P) and potassium (K) in Iowa and most states of the Corn Belt are based on soil testing and maintenance of desirable soil-test values by applying amounts removed with crop harvest. The typical Iowa farmer applies before planting corn the P and K fertilizer needed for corn-soybean rotations. A few farmers, mainly in the northern regions of the state and those using no-till management, also apply starter fertilizer for corn. Iowa research during the 1960s and 1970s showed that application of P and K fertilizer at rates of 20 lb P2O5 or K2O/acre or higher rates applied with planter attachment besides and below the seeds (commonly referred to as the 2x2 method) seldom was more efficient than similar amounts applied broadcast and seldom increased yield significantly in high-testing soils. Corn response to N-P-K starter was more likely for very early planting dates with wet and cold soil and/or high residue cover. Reports of corn responses to starter have been more frequent in northern regions of the Corn Belt, such as in Minnesota and Wisconsin.