NITROGEN AND PHOSPHORUS FORMS IN SOILS RECEIVING MANURE

Increasing production of beef, poultry, and swine from confined feeding operations in the Southern Plains necessitates that larger amounts of manure be applied to limited areas of agricultural land. If long-term manure applications of nitrogen (N) and phosphorus (P) in manure exceed crop removal, the accumulation of soil N and P can result in levels that pose a problem of concern to both surface and groundwater resources. We investigated the effect of beef feedlot manure, poultry litter, and swine slurry applications (100-1000 kgN and 37-270 kgP ha(-1) yr(-1)) for up to 35 years on the forms of N and P in 20 soils from the Southern Plains. Manure applications increased amounts of organic soil N but had little influence on the distribution or availability of N forms measured, In contrast with N, manure increased available inorganic P (IP) fractions 15 to 187 mg kg(-1) (bicarbonate extractable). Although manure also increased labile bicarbonate extractable organic P (OF) on average 162%, there was a general shift from OP dominace in untreated soils (64%) to IP in treated soils (60%). The major portion of this IP increase was Ca-bound (HCl extractable). The large amounts of Ca added in manure (2-60 g Ca kg(-1)) tended to dominate inorganic P availability reactions in manure-treated soils relative to untreated soils, even with soils of pH 6.0. Thus, current soil P tests, particularly acid-based extractants (Pray 1 and Mehlich 3), may overestimate soil P levels available for plant uptake or transport in runoff.