Cattle slurry manure applied to land increases the risk of phosphorus (P) movement to surface waters, which may lead to eutrophication. The water-extractable fraction of P in slurry manure is correlated with P concentration in runoff from soils amended with slurry smanure, and thus is an effective indicator of environmental P loss. We evaluated the water-extractable P (WEP) and readily soluble P (RSP; i.e., P extractable in a single water extract) contents in slurry manure from nine farms. On some farms, the additive Euromestmix® (MX) is used to complex N-compounds in the slurry manure, but the effect of MX on P in the slurry manure is not known. We also focused on methodological factors affecting the measurement of P in slurry manure. Drying the slurry manure before analysis decreased WEP and RSP contents. Dilution of slurry manure by varying the dry matter-to-distilled water ratio increased the water-extractable fractions. Analysis of calcium and magnesium contents in water showed that these minerals are involved in the release of P in water. Total RSP content of slurry manures from the nine farms ranged from 1.83 to 4.06 mg P per g dry matter. A substantial portion of total P in the slurry manure occurred in the water-extractable (46–71%) and the readily soluble P fractions (24–51%). The addition of MX tended to decrease the readily soluble inorganic P content of slurry manures.