EFFECT ON COLWELL SOIL-TEST-P OF STORING AIR-DRY SOIL SAMPLES AT ROOM-TEMPERATURE FOR UP TO 17 YEARS

We wished to determine whether soil-test P was affected by storing air -dry soil samples at room temperature. The soil samples had been colle cted from field experiments and air-dried (< 40-degrees-C) before meas uring soil-test P (bicarbonate-extractable P). The samples were from f ield plots that had been treated with different applications of fertil izer P (superphosphate, rock phosphate) one or more years previously. Soil-test P was measured on two different subsamples of the same sampl e: either A, in the year the sample was collected; or B, after the sam ple had been stored at fluctuating room temperatures either from 2 to 8 years (four field experiments) or 17 years (59 field experiments). T he room temperature ranged from 10 to 30-degrees-C, and averaged 17-de grees-C. The aim was to test whether soil-test P was systematically an d consistently different between subsamples A and B. Differences betwe en A and B were mostly small, and there were no consistent or systemat ic differences. For the Colwell soil test, applied to a range of south -western Australian soils, possible decreases in soil-test P due to co ntinued reaction with the soil could not be detected using bicarbonate -extractable soil P, and storage of air-dry samples at room temperatur e did not significantly affect soil-test P measured up to 17 years lat er. We conclude that, provided fertilizer P has had time to react with soil in the field, no further changes in Colwell soil-test P occur du ring air-dry storage for up to 17 years at room temperature.