Plant Availability of Phosphorus in Swine Slurry and Cattle Feedlot Manure

To utilize manure P for crop production, P release and plant availability needs to be quantified. An incubation study was conducted to determine P availability from swine ( Sus scrofa ) and cattle ( Bos taurus ) feedlot manure in three soils. Treatments for each manure included temperature (11, 18, 25, and 32°C), water regime [constant 60% water‐filled pore space (WFPS) vs. four dry‐down cycles of 60 to 30% WFPS], time, and soils (Catlin silt loam, Sharpsburg silty clay loam, and Valentine fine sand). In another study, synthetic P fertilizer was used to determine the fraction of P that becomes unavailable with time to compare with manure P. Time, soil, and manure application were factors that influenced soil test P and water‐soluble P during incubation. At the low synthetic P fertilizer rate of 6 μg g −1 , about 12 kg P ha −1 , none of the applied P remained available in the Catlin soil while about one‐third remained plant available in the Sharpsburg soil and two‐thirds in the Valentine soil. At the high P rate, 68 mg kg −1 , 38 to 83% of fertilizer P remained available in the three soils. Phosphorus availability was 60 to 100% of applied cattle manure P and 52 to 100% of swine slurry P in the three soils. Phosphorus availability in the Sharpsburg soil was 100% of P in both manure types. Phosphorus availability from manure is high, and manure can be used similar to inorganic P fertilizer in soils where P‐based application is made in areas susceptible to P loss in runoff. In P‐deficient soils, a P availability of 70% should be used.