Availability of Phosphorus In Manure From Beef Cattle Feedlot Systems Evaluated by Soil Testing.

Characterizing nutrients in beef manure and assessing their crop availability is essential to improve manure nutrients management and the utilization of this resource in crop production systems. The objective of this study was to determine the crop- availability of beef manure P from different beef cattle feedlot production systems. Manure sampled from 109 Iowa feedlots was analyzed for the total concentration of several nutrients including P, water soluble P (WSP) concentration, pH, and other chemical properties. The WSP of the total P ranged from 10 to 86%. These samples were classified into five manure types according to animal feeding systems and manure handling and storage methods. These were bedded manure, open lots with soil or concrete floor, deep pit (slurry), and no feeding of corn distilled grain or gluten feed. Representative samples from each manure type with low, medium, and high WSP concentrations were selected for a soil incubation study. An aerobic incubation technique was used to assess potentially available manure P for three contrasting Iowa agricultural soils: Adair (fine, smectitic, mesic aquertic Argiudolls), Harps (fine-loamy, mixed, superactive, mesic typic Calciaquolls) and Nicollet (fine-loamy, mixed, superactive, mesic aquic Hapludolls). The P treatments were each manure type applied at a rate equivalent to 98 kg total P ha⁻¹, a similar rate of diammonium phosphate (DAP), and a control receiving no manure. The mixtures of soil and P sources were incubated for 7, 14, 35, 63, and 95 days at 80% water holding capacity and 25 °C. For each incubation time, soil was analyzed for pH and P by the Bray-P1, Mehlich-3, and Olsen methods. The results showed that the maximum STP increase for all P sources was reached by the 7-day or 14-day incubation times for all soils and soil test methods. Manure WSP concentration was not correlated to manure P availability as estimated by soil testing. The manure P availability compared with DAP ranged from 68 to 90 % for the five manure types, and differences were inconsistent across STP methods and soil types. This manure P availability range is narrower than the range currently suggested in Iowa (60 to 100%) for manure management plans.