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

Abstract

h laboratory procedure which ranks soils according to their affinity for Mg and K at tield pH is described. Effective relative affinity (EA) is considered to be the product of dfective cation exchange capacity and an expression of the selectivity for these two cations. In thi~ 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 tinp,]t) 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.OmM Ca, with Y [Mg]/[Ca] approximating 0.5 and aK/Ya(Ca + "g) between 0.D15 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 ~elecli\'ity 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.

Potassium in the soils of New Zealand—A review

“…The site-specific fixation of K + is made possible by the geometry of the fixation sites and the size of the K + ions. Potassium ions are entrapped in the ditrigonal holes of the facing interlayer oxygens between the unit cells of 2:1 layer silicates (Arifin et al 1973) or at specific adsorption sites located within the channels of gels (Fieldes 1958;During 1973). The extent of K + fixation is also influenced by the presence or absence and the location of wedge zones in weathered micaceous materials (Arifin et al 1973;Mengel & Kirkby 1982).…”

“…Most yellow-grey earth soils of New Zealand contain clay-vermiculite which is capable of fixing added K but such soils also contain micaceous minerals capable of supplying K + to plants (Fieldes 1958). The intrazonal soils from volcanic parent materials, where micas are low or absent, the strongly weathered zonal soils (Saunders & Metson 1959;Metson 1960;During 1973), and oxidic, ultic, and allophanic soils which contain kaolinite or allophane and small amounts of vermiculite and much organic matter (Parfitt 1992), have low K + -fixing capacities.…”

Cation-exchange capacity at field pH and its relation to the affinity of soils for individual cations

“…In an earlier paper it was shown that soils in the North Island of New Zealand differed greatly in their selectivity for magnesium and potassium (During 1973). For this reason cation-exchange capacity at field pH (effective CEC) alone cannot predict accurately the affinity of soils for these cations.…”

“…Soils were equilibrium leached (220 ml through 2.5 g soil) with two solutions to obtain estimates of TEA (During 1973). Input C contained 2.OmM CaS04, 0.5mM MgS0 1 , and 0.8mM KCl; in input D one-half of the KCl was replaced by KH2P04 in an attempt to extract additional aluminium.…”

“…However, when soils were equilibrium leached with a dilute standard solution (about 0.005M) that contained three cations, namely Ca, Mg, and K, the resulting levels of exchangeable cations could be used to rank soils according to their effective relative affinity (EA) for these cations (During 1973). Over a large number of soil samples the mean sum of effective relative affinities (TEA) thus obtained slightly exceeded the sum of exchangeable bases originally present (TEB).…”