Vergelijking van twee bemonsteringsstrategieën bij toepassing van de gecombineerde indicator voor de fosfaattoestand van de bodem

The phosphate application limits for grassland and arable land are outlined in the Fertilizers Act. These limits depend on the phosphate status of the soil. The phosphate status on arable land is currently determined by a water extraction (Pw value), on grassland by an extraction with ammonium-lactate-acetic acid (P-Al value). The sixth action program of the EU Nitrates Directive has indicated that the Ministry of Agriculture will change the methods for the characterisa tion of the phosphate status of the soil; a combined indicator based on the extraction with 0.01 M CaCl2 and P-Al value will be used to assess the phosphate status of the soil for both grassland and arable land. This change came into effect on January 1st, 2021.Currently, a stratified soil sampling design is required for the designation of low-phosphate and phosphate-fixing agricultural soils. For soils with higher phosphate status, accredited soil test laboratory may choose their own protocols which can be based on the W method, Z method (zigzag method) or cross sampling method. A change in the assessment of the phosphate soil status from one parameter (Pw value or P-Al value) to two parameters (P-CaCl2 value and P-Al value) may affect the accuracy of the soil phosphate status and hence the sampling protocol. Contrasting values for the intensity and capacity may be accompanied by an increase in the spatial variability of the phosphate status of the soil. Such an increase has consequences for the number of soil cores that have be taken depending on the area of the agricultural field. However, information about the size of the sampling error associated with combinations of the P-CaCl2 and P-Al number was missing. It was therefore not possible to propose the required soil sampling protocol for the combined indicator. This report addresses the question if the sampling strategy requires adjustment for different combinations of P-CaCl2- and P-Al values. The study also answers the question if the sampling strategy can be harmonized into one system that is adequately safeguarded and verifiable.