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Soil Science Society of America Journal Abstract -

The Influence of Anions on Potassium Quantity-Intensity Relationships1


This article in SSSAJ

  1. Vol. 50 No. 5, p. 1182-1188
    Received: Aug 12, 1985

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  1. V. P. Evangelou2



Quantity-intensity (Q/I) relationships for potassium (K+) of soils sampled from three Maury silt loam sites (fine-silty, mixed, mesic, Typic Paleudalfs) representing different crop management practices were investigated in 10−2 molc L- each of Cl- or SO2-4 solutions. Soil samples from the three sites were (i) low in exchangeable K+ and organic matter (LK-LOM), (ii) high in exchangeable K+ with low organic matter (HK-LOM), and (iii) high in exchangeable K+ with relatively high organic matter (HK-HOM). A theoretical evaluation of K+ Q/I plots in Cl- or SO2-4 solutions employing the Gapon equation, revealed that the potential buffering capacity for K+ (APBCk), the labile K+ (AExK0) and equilibrium activity ratio for K+ (ARK0) based on single ion activities and ion pairs of CaSO04, KSO-4 and CaCl+ were independent of the type of solution anion. When considering total dissolved concentration of K+ and Ca2+, the potential buffering capacity for K+ (CPBCK) and equilibrium concentration ratio for K+ (CRK0) differed between Cl- and SO2-4 solutions. The labile K+ (CExK0) remained the same. These results were due to the differences in ion pairing abilities between Cl- and SO2-4 with K+ and Ca2+. The influences of the ion pairs are minimized at decreasing CRK0 values. The experimental data describing Q/I relationships for K+ for the three soil samples were not in full agreement with the theoretical observations. The data showed that the type of anion (Cl- or SO2-4) had an effect on the CRK0 and CExK0, but the values were not significantly different except for the HK-HOM sample. The experimental data for the soil samples HK-LOM and HK-HOM indicated agreement to some extent with the theoretical evaluation. For the same CRK values in the Cl- or SO2-4 anions, a greater change in exchangeable K+ (ΔExK) was observed in the SO2-4 system than in the Cl- system. However, for these two soil samples (HK-LOM and HK-HOM) when the Q/I plots were constructed considering single ion activities, differences remained in the Q/I plot components ARK0 and AExK0 between the two anionic systems. For the LK-LOM soil sample the Q/I plots appeared to be nearly identical for the Cl- and SO2-4 systems in terms of total cation solution concentrations and/or solution single ion activities. This agreed with the theoretical evaluation.

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