Evaluation of Sulfur-coated Urea (SCU) Applied to Irrigated Potatoes and Corn

Excessive rates of soluble N fertilizers are sometimes used on irrigated sands in Central Wisconsin to compensate for loss of some of the applied N by leaching. Use of a slow-release form of N or several small applications of a soluble N fertilizer may eliminate some leaching and subsequent loss of NO₃-N. This study was designed to evaluate these methods of reducing N losses by measuring yield and recovery of applied N by potato (Solanum tuberosum L.) tubers and corn (Zea mays L.) grain. Supplemental N treatments applied each year for 3 years were as follows: 1) urea in a single or split application, 2) SCU impregnated with a wax coating, and 3) SCU with only a sulfur coating. Recovery of applied N by tubers from a single urea application was extremely low in 1972 due to severe leaching induced by abnormally high precipitation. Slow-release SCU carriers produced higher yields and N recovery that year because their slow dissolution reduced leaching losses of N. However tuber yields and recovery of N from urea treatments were greater than from SCU treatments in 1973 and 1974 because of reduced leaching those years. In addition, the supplemental N (urea or SCU) for all potato treatments was protected from leaching in 1973 and 1974 by placement of the N into a relatively dry zone of the potato hill. Corn yields and grain N recovery were similar among carriers of N in 1972 and higher from urea than SCU in 1973 and 1974. The greater recovery of N from urea by corn grain as compared to potato tubers in 1972 apparently was due to the deeper rooting of corn. Furthermore the potatoes received twice as much water by irrigation as compared to the corn. Oat (Avena sativa L.) forage yields from both trials in 1975 indicated there was greater residual N in SCU plots than urea plots due to incomplete release of urea-N from SCU. Adequate dissolution of SCU-N within a single growing season occurred only under severe N-leaching conditions.