Field-scale evaluation of poultry manure as a combined nutrient resource for corn production

An on-farm study was conducted in Iowa from 2004 to 2006 at 18 sites to evaluate corn (Zea mays L.) grain yield (GY) and soil- and plant-test responses to poultry manure (PM) nutrient application at the field scale. A control and two target PM rates based on total N (PM-N) were applied in randomized field-length strips with three replications. Corn GY responded positively to PM applications. While N, P, and K plant and soil tests were related to PM nutrient rates, there was considerable variation, and relationships were probably influenced by the multiple applied nutrients. Soil-test P and soil-test K across sites increased linearly with increasing PM total P and K rates and with large increases from the high rates. This confirms high P and K crop availability. Grain yield responses to PM decreased linearly with increasing leaf chlorophyll meter (CM) and late spring soil NO₃–N test (LSNT) values but were not related to end-of-season lower corn stalk NO₃–N test values. No N test had a plateau relationship with GY, suggesting no excess N supply despite large PM-N rates. This confirms low first-year PM-N availability. The relationship between CM and LSNT indicated a critical LSNT value at 24 mg kg^–1, similar to that from previous small-plot research. This field-scale study showed that PM is a valuable nutrient resource. However, due to PM multinutrient content and differences in availability, the nutrient causing GY and plant- or soil-test results often cannot be clearly identified and results need careful interpretation for reliable use.