PHOSPHATE ENRICHMENT IN THE SANDY LOAM SOILS OF WEST-FLANDERS, BELGIUM

The last three decades, pig breeding has evolved towards a specialised , large scaled, land independent bio-industry in the province of West- Flanders. Subsequently, in certain regions, very high amounts of liqui d pig manure are produced each year. This pig slurry is used as a fert ilizer at a rate which very often exceeds normal agricultural practice s. Because of the nonequilibrium between the phosphorus crop requireme nts and the P-inputs, phosphates accumulate in the soil. However, the phosphate sorption capacity of a soil is limited. Once the sorption ca pacity is exceeded, phosphates will start leaching through the soil pr ofile. Since, during winter, in these areas, the groundwater table is situated at a depth of less than 1.0 m, phosphate breakthrough might t ake place. In the sandy loam soil region (+/- 1000 km(2)) of the provi nce, an inventory of the P status of the soil was made. The region was sampled according to a regular grid with 2 km intervals. At random, s ome sample points were only 500 m apart. This resulted in a total of 2 96 samplings. In view of fertilizer recommendations, lactate extractab le P of the plough layer (0-30 cm) was determined. A maximum value of 101 mg P 100 g(-1) of air dry soil, a minimum value of 6 mg P 100 g(-1 ) and a median value of 31 mg P 100 g(-1) were found, indicating that for half of the spots monitored, the P status of the soil is high to v ery high. An oxalate extraction was done to investigate the phosphate saturation of the soil profile (0-90 cm). Based on a critical phosphat e saturation degree of 30%, more than half of the soil profiles are ph osphate saturated. Phosphate leaching at a rate higher than 0.1 mg ort ho-P l(-1) at a depth of 90 cm can be expected. Therefore, a restricti on of the P fertilization should be highly recommended. The geostatist ical processing of the data using block kriging resulted in a spatial continuous estimate of the phosphate saturation degree. A good agreeme nt was found between the pig density and the phosphate saturation degr ee of the soil profile.