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 cmolc 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.