Inorganic and organic P in soil solutions from three upland soils

Soil solutions from three P-deficient Cambisols were analyzed for inorganic orthophosphate (P_i), organically combined phosphorus (P_o), total phosphorus (P_t) and residual phosphorus (P_r=P_t–(P_o+P_i)). The solutions were obtained by centrifugation of soil samples wetted-up to 90% field capacity. Increasing the centrifugal force from 750 to 1400×g (for 60 minutes) increased the volume of soil solution obtained by 17–35%. Increasing the centrifugation period from 30 to 90 minutes (at 1000×g) increased the volume by 2–12%. The effect of the different centrifugation conditions on the P composition of soil solutions were not critical and had little effect on either P_t concentration or on the distribution of P between P_i, P_o and P_r fractions. Soil solutions were also obtained on a seasonal basis over a 2-year period. The soils, fresh from the field, were wetted-up to 90% field capacity and centrifuged at 1000×g for 60 minutes to isolate the soil solution. Although the soils were derived from contrasting parent rock, and had different Fe and Al sesquioxide contents, the P_t concentrations of the soil solutions and the distribution between the fractions were similar. Annual average P_t concentrations for the 3 soils ranged from 93 to 114 and 63 to 89 g dm^-3 during the first and second year, respectively. Seasonal changes were of a similar order as those resulting from differences in soil type. During May, June, August and October soil solutions had average P_t concentrations ranging from 82 to 111 and 51 to 119 g P dm^-3 in 1989 and 1990, respectively. P_o was a major P component in soil solution and exceeded the amount of P_i by about 5–20 times.