Linking manure properties to phosphorus solubility in calcareous

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 kg(-1)), water-soluble P (4.3-8.0 g P kg(-1)), 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(-1) soil) or P (27.5 mg P kg(-1) 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(-1) soil). Water and NaHCO3 (Olsen) extractable P were determined at regular intervals, with microbial P determined by fumigation-extraction after each incubation. For N-based application (i.e., variable P amendment), extractable P increased with total P applied. For P-based applications, the increase in soil P was more closely correlated to microbial P concentration than manure P composition or soil properties. These results suggest that stimulation of the microbial biomass by added organic C is important in determining soil P solubility following manure application.