Soil Nitrogen and Phosphorus Availability for Field‐Applied Slurry from Swine Fed Traditional and Low‐Phytate Corn

Swine ( Sus scrofa ) slurry contains nutrients essential for crop production but usually contains more P relative to N than is required by most crops, creating the potential for negative environmental impacts. Diet modifications such as low‐phytate corn ( Zea mays L.) have resulted in improved bioavailability of P and reduced manure P content. A field study was conducted to compare in situ availability of N and P at two sites. One site received three annual additions of manure from swine fed low‐phytate corn or traditional corn diets or inorganic fertilizer, surface applied to rainfed no‐till sorghum [ Sorghum bicolor (L.) Moench]. A second site received a one‐time application and incorporation of the same nutrient treatments to irrigated corn. Nutrient treatments were applied at rates intended to meet crop N needs. At both sites, an in situ soil core resin bag technique was used to determine available N and P during the growing season. Potentially mineralizable N was 70% of applied N and extractable P was 100% of applied P for manure from both diets. Incorporation of swine slurry reduced potentially mineralizable N to 40% the year of application and 30% the year after application and reduced extractable P to 60% the year of application and 40% the year after application for both diets. Modified diets reduced the P content of the manure but not the availability of N or P.