INORGANIC AND ORGANIC-P IN SOIL SOLUTIONS FROM 3 UPLAND SOILS .1. EFFECT OF SOIL SOLUTION EXTRACTION CONDITIONS, SOIL TYPE AND SEASON

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 x 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 x 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 x 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 mug 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 mug P dm 3 in 1989 and 1990, respectively. P. was a major P component in soil solution and exceeded the amount of P(i) by about 5-20 times.