Linking Manure Properties to Phosphorus Solubility in Calcareous Soils

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 NaHCO 3 (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.