Soils from ten grassland sites and one forest site in western Ireland were sampled monthly for 12–17 months. Moist, composite samples were analysed for labile phosphorus (P) fractions extractable with the Olsen’s reagent and water (Pw). Molybdate analysis before and after extractant digestion was used to define inorganic and organic P fractions: Olsen P and Olsen molydate unreactive P (MUP); Pw dissolved reactive P (DRP) and dissolved unreactive P (DUP). All-soil P fractions exhibited significant covariance with a seasonal sine-curve parameter in ANCOVA, and this seasonal component of variation was statistically similar among soils for all parameters except Olsen P. Olsen P fractions followed a distinct seasonal trend of maximum winter and minimum summer concentrations. This was more pronounced for the Olsen MUP fraction, which displayed strong positive covariation with soil total N content, moisture content, and preceding effective precipitation (p = < 0.0001, 0.0027 and < 0.0001 respectively), and negative covariation with preceding temperature (p = 0.0011) — consistent with expected patterns of biological cycling. Seasonal variation in both Pw fractions followed an inverse trend to Olsen P variation, resulting in differing summer and winter Olsen P–Pw relationships. Pw DRP exhibited negative covariation with effective precipitation in the week, but not month, prior to sampling and with pH (p = 0.005 and 0.003 respectively), indicating a possible wash-out effect and physico-chemical control. Pw DUP exhibited strong positive covariation with temperature during the preceding month (p = 0.0004), indicating biochemical control, and was significantly and positively associated with grazing occurrence (p < 0.0001). Spikes in soil Olsen P and Pw fractions were attributed to an intensive grazing event during spring, and manure application during winter, but there was no peak in soil P following summer fertiliser application. It was concluded that less labile and organic P fractions buffer soluble and inorganic P fractions throughout the year.