Phosphorus Forms in Manure and Compost and Their Release during Simulated Rainfall

Abstract The large accumulation of P in manure from animal feeding operations in localized areas has increased the potential for P export following land application. Impairment of freshwater quality by accelerated eutrophication has focused attention on manure management and the potential for P loss in runoff. Thus, we investigated the amounts and relative solubilities of P in manures and their composts using a modified Hedley fractionation and release of P during simulated rainfall (70 mm h −1 for 30 min) in laboratory columns (15‐cm diameter). Twenty‐four samples each of dairy manure, dairy manure compost, poultry manure, poultry manure compost, poultry litter, and swine manure were collected over 2 yr. Total P concentration ranged from 2600 to 40 000 mg kg −1 , mostly as inorganic P (63 to 92%). The distribution of inorganic and organic P fractions depended on manure and compost type. Most of the inorganic P, 80%, was water extractable (2030 mg kg −1 ), while 55% was hydroxide extractable in swine slurry (16 620 mg kg −1 ) and 38% acid extractable in poultry manure (9320 mg kg −1 ). The dissolved inorganic P concentration in leachate from manure and compost (10 Mg ha −1 manure application rate) during 30‐rain rainfall ranged from 34 (poultry litter) to 75 mg L −1 (poultry manure). The amount of P leached by five simulated rainfall events was significantly correlated to respective water extractable inorganic ( r 2 = 0.98) or organic P ( r 2 = 0.99) of each material. This suggests that water extractable P may be used to estimate the potential for land‐applied manures or composts to enrich leachate and surface runoff P.