Interpretive Summaries:

At three sites in eastern Colorado, various three to four-year crop rotations were implemented. Wheat has been grown every year at all sites, corn every year at Sterling, and sorghum every year at Walsh. In the original experimental design, half of each plot received P fertilizer every year, and the other half received no P fertilizer. The purpose was to determine if organic P cycling would satisfy the P needs of the crops in these no-till dryland cropping systems. By 1991, the available P levels on summit and sideslope positions had become so low in the no P treatments that wheat emergence and growth were negligible. In 1991, P fertilization of the no P plots was initiated at wheat planting (i.e., every three to four years). Annual P applications were continued on plots where originally implemented. Eight years after P fertilizer treatments were altered, wheat yields on the summit and side slope soils remain lower than where P was applied every year. This occurred despite the fact that the recommended rate of P was band applied below the seed at planting to each wheat crop. Soil analysis and yield data of other crops are in the process of being analyzed. This study is demonstrating the importance of P fertilization in the sustainability of intensive crop production in eastern Colorado. CO-11F