Potassium Management during the Rotation from Alfalfa to Corn

Abstract

High K fertilizer prices in recent years have made it imperative for growers to apply optimum K rates to alfalfa (Medicago sativa L.). Current university fertilizer guidelines in the Corn Belt do not change for the last production year, when alfalfa stand persistence is not a major concern. Furthermore, little is known about carryover of K applied to alfalfa on first‐year corn (Zea mays L.) grain and silage yields. In 2008 to 2010, on‐farm research was conducted on 10 fields with medium soil test potassium (STK) to determine response to K for alfalfa yield and quality in the last production year, and to estimate K carryover to first‐year corn. Alfalfa yield and relative feed value (RFV) and quality (RFQ) did not improve with K fertilization. Herbage K concentration and K uptake increased with K fertilization across sites, indicating that applied K was available during the season of application. When corn relied on carryover K alone, each 100 kg ha−1 increase in the index of available K increased corn grain yield by 0.5 Mg ha−1, decreased stover yield by 0.4 Mg ha−1, and did not affect silage yields. Regardless of K rate applied to alfalfa, additional K applied to corn increased corn stover and silage yields by 10 and 8%, respectively. This suggests that carryover K was less available than K applied to corn. On medium STK soils going into the last year of alfalfa, applying fertilizer K to first‐year corn rather than alfalfa may enhance economic return.