The effective relative affinity for magnesium and potassium of some soils from the North Island, New Zealand

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Abstract
A laboratory procedure which ranks soils according to their affinity for Mg and K at field pH is described. Effective relative affinity (EA) is considered to be the product of effective cation exchange capacity and an expression of the selectivity for these two cations. In this paper EA for a cation was defined as the quantity of this cation in exchangeable form which is retained by the soil after leaching with a suitable unbuffered standard solution (input) until the cation concentrations of the leachate (output) equals that of the input. An input solution suggested as suitable for the soils of the North Island contains 2.0 to 3.0mM Ca, with √[Mg]/[Ca] approximating 0.5 and aK/√a(ca + (mg) between 0.015 and 0.020. Sulphate and chloride were chosen as the anions, and 220 ml solution leached through 2.5 g air-dry soil. Among the soils examined there was a large range of EA for K and Mg. Great differences also occurred in their ratios, suggesting wide differences in the selectivity for these two cations. The allophanic yellow-brown pumice soils commonly had very low EA for magnesium, yet within this group some high values were also found. Though the recovery of applied K by a perennial ryegrass was highly significantly related to EA for K, it was greater on deeply friable soils than on slowly draining soils.