Phosphorus and Potassium Response in No-till Corn and Soybean Production.

Phosphorus (P) and potassium (K) application guidelines for corn and soybean production in Wisconsin have not been evaluated for no-till production. The objectives of this study were to: 1) evaluate corn and soybean yield response to P and K application on very low testing soils; 2) assess grain removal of P and K; and 3) evaluate tissue nutrient concentrations at several growth stages. A field study was established in 2011 with soybean with alfalfa as the preceding crop for five years. In 2012 the plot area was rotated to corn and another study area was established with soybean. The study was a full factorial of P₂O₅ rates (0 to 101 kg ha^-1 in 34 kg ha^-1 increments) and K₂O rates (0 to 180 kg ha^-1 in 45 kg ha^-1 increments) applied broadcast in the spring of the year in each year of the study. Initial soil test levels were 8 and 1 mg kg^-1 P, and 59 and 48 mg kg^-1 K, at sites established in 2011 and 2012 respectively. Potassium responses for both crops were large in each year. Phosphorus yield responses were not evident for corn until 2014. Corn yield increased significantly with P application, but only where K was applied at rates greater than or equal to 90 kg K₂O ha^-1 annually, and the agronomic optimum P rate was 50 kg P₂O₅ ha^-1. Soil test K levels were 65 mg kg^-1 or greater in the spring prior to fertilizer application where P responses occurred. Where no K had been applied during the course of the study, soil test K was 53 mg kg^-1 in the spring prior to fertilizer application. Soybean did not respond to P application in any year. Data clearly demonstrate that K is more important than phosphorus for both corn and soybean.