An evaluation of the basic cation saturation ratio concept in sandy soils of the Free State Province, South Africa

Abstract

Results of the first plantings of field trials conducted at six localities on sandy soils in the Free State Province of South Africa are presented. The effective cation exchange capacity (ECEC) and Ca²⁺, Mg²⁺, K⁺ as a percentage of the soils ECEC (Cation%ECEC) and the ratios amongst these cations were calculated. The average relative yields (RY) of maize were determined for the reference treatments (Ca+Mg+K) which received all essential nutrients and for the other three treatments which received the same nutrients except that Ca²⁺ or Mg²⁺ or K⁺were omitted. The ECEC values of the topsoil samples ranged from 1.3 to 4.2 cmol_c kg⁻¹. The cations as a percentage of the ECEC ranged from 33–69% for Ca²⁺, 14–32% for Mg²⁺ and from 6–22% for K⁺. The relative proportions in which the three individual cations were present changed as the ECEC of the soils increased. The Ca²⁺ increased in relation to the Mg²⁺ and K⁺ and the K%ECEC diminished as the ECEC increased. Results supported the basic cation saturation ratio (BCSR) concept as far as that Ca²⁺ should make up at least 65% of the ECEC to assure RY's of more than 80%. This is not mainly caused by the increase in Ca²⁺ as a plant nutrient, but rather by making adsorbed Mg²⁺and K⁺ more available to plants. Under these conditions it is important that Mg²⁺ and K⁺ must be available in sufficient quantities to meet the requirements of the crop.