VARIABILITY OF THE PHOSPHATE SATURATION DEGREE OF THE SANDY LOAM SOILS IN WEST-FLANDERS, BELGIUM

The variability of the phosphate sorption capacity (PSC) and the phosp hate saturation degree (PSD) of the soil profile was studied in the sa ndy loam soil region (+/-1000 km(2)) of West-Flanders at different sca les. In the frame of a regional inventory, the soil profile was sample d at 2000 m intervals on a square grid. In total, 296 observations wer e made. Additionally, in this area, 21 fields were sampled in detail: 15 augerings per field were made. Finally, on one field, seven observa tions were made at the micro scale: five augerings within a circular a rea of 30 m(2). All soil samples were analyzed for ammoniumoxalate ext ractable Fe, Al and P and the PSC and the PSD were calculated. The coe fficient of variation (CV) of the PSC of the whole soil profile 0-90 c m was found to be 7% on the micro scale, 20% on the field scale and 28 % on the regional scale. The CV of the PSD was found to be 9%, 21%, an d 48% respectively. This means already quite a proportion? about 20%, in variation in PSC and PSD is situated at short distances (< 5m) Most of the variability in PSC is already found at the field level. On the contrary, the CV of the PSD is twice as high on the regional scale co mpared to the field scale. This may be explained by the regional struc ture of the P inputs, due to the local intensification of pig breeding activities. The criterion phosphate saturation is stochastically appr oached at the field scale. In the study area, the mean field averaged CV of the PSD was 21%. Taking into account the uncertainty when sampli ng, supposing a lognormal distribution of the PSD, the critical PSD of 24% is translated into a practical threshold PSD of 26% if 15 measure ments per field are taken and a probability of 95% is pursued.