Assessment of available phosphorus and potassium in soils by the calcium chloride extraction method

The availability of phosphorus (P) and potassium (K) of 30 soils representative of the main great soil groups of northern Greece was estimated by the 0.01M CaCl2 extraction method, and the values obtained were correlated with short- and long-term uptake and dry matter yields of ryegrass grown in pots as well as with the soil Olsen-P and NH4OAc-K values. Coefficients of correlation for P and K values of soils with total P uptake of six cuttings and K uptake of ten cuttings were calculated for groups of soils, classified on the basis of pH, texture, cation exchange capacity, and percent base saturation. The amounts of P extracted by 0.01M CaCl2 solution were extremely small, ranging from 0.15 to 1.50 ppm P and in certain cases lead to considerable analytical difficulties. The correlations between P and K obtained by ryegrass and P and K values determined by the 0.01M CaCl2 method were statistically significant (r = 0.750*** for P and r = 0.700*** for K). Very close relationships were also found between Olsen-P and CaCl2-P (r = 0.845***), and NK4OAc-K and CaCl2-K values (r = 0.750***). The correlations between total P uptake by ryegrass plants and soil P analysis values were generally better for the fine-textured soils than for the coarse-textured soils as well as on the soils with base saturation levels less than 60%.