Integrated Crop Management is published by the Department of Entomology , Iowa State University , Ames, Iowa. The basic idea is that ammonium is not converted to nitrate in cold soils, so withholding applications until soils are cold reduces the risk of N losses by leaching and denitrification when soils have excess water in the spring.
Recent studies indicate that soil pH also deserves attention.
The first indication that soil pH is important came in a survey of 21 sites in 11 fields in Boone, Greene, and Hamilton counties in 1998. Weather conditions had been warm and wet, and many farmers were asking about possible losses of fall N. Opportunity to address this problem was provided by more than 50 students from Iowa and Ukraine participating in an exchange program that focused on science, agriculture, and the environment. The survey was designed with the assumption that time of N application was important and that landscape position within fields also may be important. In late April and early May, soil samples were collected to a depth of 12 inches by using intensive sampling techniques that would normally be considered impractical due to labor costs. Separate samples were collected within and between bands left by the ammonia applicators to enable calculation of fertilizer N recovery as nitrate and exchangeable ammonium.