Technical Abstract: 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 swine manures with a range of total P (6.8'4.9 g P per kg), water-soluble P (4.3'8.0 g P per kg), and C:P ratios (31'67). The manures were generated by feeding swine one of four low phytate barley (Hordeum vulgare L.) mutants or a normal variety. A total of thirteen soils were incubated with the five manures for 14 days with phosphorus solubility determined by extraction with water and sodium bicarbonate at regular intervals, and microbial P determined by fumigation'extraction. When manures were applied at the same rate, extractable P concentrations were greater in soils that received more total P in manure. When manures were applied at the same total P rate to the soils, extractable P concentrations were greater in soils that received the least C in manure (i.e. lower C:P ratios). The increase in extractable soil P following manure application was inversely correlated with the C:P ratio of the manures, but only weakly correlated with soil properties assumed to regulate P sorption, including calcium carbonate, pH, clay, and the degree of P saturation. These results suggest stimulation of the microbial biomass by added organic C is of considerable importance in determining soil P solubility following manure application.