SOIL-PHOSPHORUS DETERMINATION USING 3 EXTRACTION PROCEDURES, THE EFFECT OF SAMPLING DEPTH AND COMPARISON OF PHOSPHORUS-FERTILIZER RECOMMENDATIONS FOR GRASSLAND

Three procedures for extraction of soil P: Morgan (Na acetate + acetic acid, pH 4.8), Olsen (NaHCO3, pH 8.5) and 0.01 M CaCl2 solutions were compared. The comparisons were based on 368 soil samples, sampled to a range of depths of between 1 cm and 10 cm, taken in County Cavan in the Republic of Ireland. Soil P concentrations ranged from very low to very high (0.7 mg/l to 405.2 mg/l (Morgan test)). There was a linear relationship between Morgan-P and 0.01 M CaCl2-P: CaCl2-P = -0.66 + (M organ-P x 0.235) [R-2 0.97**]. However, the relationships between Mor gan-P and Olsen-P and between 0.01 M CaCl2-P and Olsen-P were not line ar: Morgan-P = 5.5 + (Olsen-P x - 0.22) + (Olsen-P-2 x 0.008) [R-2 0.9 4**]; CaCl2-P = 1.48 + (Olsen-P x - 0.088) + (Olsen-P-2 x 0.002) [R-2 0.93**]. Fifty-two sites with a range of soil P concentrations were sampled to depths of 7.5 cm and 10 cm. In the comparison of these two sampling depths, there tended to be higher soil P concentrations in th e samples taken to a depth of 7.5 cm compared to samples taken to a de pth of 10 cm for each of the three soil tests, although differences be tween the two sampling depths were very variable. Fertiliser P recomme ndations for grassland in the Republic of Ireland generally exceeded t hose of Northern Ireland at similar levels of soil phosphorus.