Linking manure properties to phosphorus solubility in calcareous soils: importance of the manure carbon to phosphorus ratio

Land application of manure can increase P transfer in runoff, although the risk depends in part on the characteristics of the manure. We assessed this for calcareous soils using manures from swine (Sus domesticus) fed one of five barley varieties (Hordeum vulgare L.), including four low phytate mutants and a normal variety, to produce manures with a range of total P (6.8-4.9 g P water-soluble P (4.3-8.0 g P kg-'), total N/P ratios (2.5:1-5.5:1), and total C/P ratios (31:1-67:1). Two experiments were conducted. First, manures were incorporated into three soils on a N (150 mg N kg- ' soil) or P (27.5 mg P kri soil) basis three times during a 7-wk incubation. Second, 10 additional soils were incubated for 2 wk following a single P-based manure application (82.5 mg P kg-i soil). Water and NaHCO3 (Olsen) extractable P were determined at regular intervals, with microbial P determined by fumigation-extraction after each incubation. Fo