PHOSPHATE IMMOBILIZATION IN AN UNCROPPED FIELD EXPERIMENT ON A CALCAREOUS SOIL

From an uncropped field trial on a calcareous soil, residual phosphate was monitored as function of time. At the start of the experiment, su perphosphate at a rate of 150 kg P per hectare was applied. Extractabl e phosphate, using 0.5 M NaHCO3 as extractant, is declining in the con trol and fertilised plot at a different rate as function of ageing tim e, lasting more than 7 years. Intersection of the linear regressions e xpressing the influence of ageing time on the extractable phosphate oc curs at 11.37 +/- 6.48 years. Total P in the arable layer remains cons tant throughout the whole duration of the experiment.