The 0.02 M SrCl 2 ‐0.05 M citric acid solution (Sr‐citrate) was developed for the simultaneous determination of availability indices for P and other plant nutrients from soils. The objective of this study was to compare the Sr‐citrate extracting solution with the water, NaHCO 3 , Bray 1, Bray 2, Mehlich 3, and 0.1 M HCl soil tests as extractants for soil P, and as predictors of plant‐available P for oat ( Avena sativa L.) and corn ( Zea mays L.) in the glasshouse. The Srcitrate solution extracted more P than did water, or the Olsen, Bray 1, and Mehlich 3 solutions in acidic and neutral to calcareous soils but less than the Bray 2 and 0.1 M HCl solutions. The amounts of P extracted from 34 soils by the Sr‐citrate procedure were very closely related to that extracted by the six other solutions and, in particular, to the amounts extracted by Olsen ( r = 0.97** [significant at P = 0.01]) and Mehlich ( r = 0.96**) solutions. The Sr‐citrate solution was among the best in predicting uptake of P and the relative dry‐matter accumulation by oat and corn from both acidic (pH < 6.5) and higher pH soils. The relationships between the amounts of soil P extracted by the seven procedures and plant uptake were particularly improved when the clay content or the effective cation‐exchange capacity and, to a lesser extent, the maximum buffering capacity were included in multiple‐regression equations. The Sr‐citrate extractant can simultaneously extract NO 3 ‐N, P, K, SO 4 ‐S, and micronutrients from acid and calcareous soils. The results of this study show that Sr‐citrate procedure has definite advantages over standard extraction procedures in the evaluation of the amount of plant‐available P in soils.
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Simard et al. (1991) studied this question.