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 inor ganic orthophosphate (P(i)), organically combined phosphorus (P(o)), t otal 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 solutio n 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 s olutions were not critical and had little effect on either P(t) concen tration or on the distribution of P between P(i), P(o) and P(r) fracti ons. Soil solutions were also obtained on a seasonal basis over a 2-ye ar period. The soils, fresh from the field, were wetted-up to 90% fiel d capacity and centrifuged at 1000 x g for 60 minutes to isolate the s oil solution. Although the soils were derived from contrasting parent rock, and had different Fe and Al sesquioxide contents, the P(t) conce ntrations of the soil solutions and the distribution between the fract ions 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 secon d year, respectively. Seasonal changes were of a similar order as thos e resulting from differences in soil type. During May, June, August an d 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.