Abstract taken from 'Agricultural Research', 1973, p. 169:
Reduction of soil exchangeable sodium percentage to low levels was possible using low electrolyte water to leach gypsum-ameliorated soil, provided sufficient gypsum was applied to replace most of the exchangeable sodium present. However, although there was little reduction in relative hydraulic conductivity, absolute hydraulic conductivity values were very low owing to the low cation concentration subtended by dissolving gypsum (less than 28 me calcium/l). In addition the naturally occurring high-salt water of Ovambo was not suitable for reclamation owing to its low content of divalent cations. Improvement could only be obtained by using gypsum as a source of divalent cations while retaining high-salt water as a means of ensuring high hydraulic conductivity values during reclamation
A comparative study was carried out to determine the best method of combining gypsum and high-salt water. Two methods were proposed for improving the divalent cation status of either the high-salt water or the low electrolyte water used for diluting high-salt water (HSWD method and constant calcium HSWD method respectively). In addition two methods involving application of gypsum to soil and leaching with either water comparable to Cunene River water or high-salt water were tested (gypsum plus low-salt water and gypsum plus high-salt water respectively). All methods were equally effective in reducing exchangeable sodium percentage, but the HSWD methods showed very low exchange efficiencies (quantity of exchangeable sodium removed by unit quantity of calcium) and thus required large amounts of gypsum. The method involving leaching of gypsum- treated soil with low electrolyte water gave high exchange efficiency but was inferior to the other methods with respect to hydraulic conductivity. One method, the gypsum plus high-salt water method, was superior to the others in both reclamation efficiency and hydraulic conductivity, but its success was critically dependent on synchronizing depths and concentrations of high-salt water with rate of gypsum dissolution
Hydraulic conductivity data gathered during the course of a large number of reclamation studies indicated that the total cation concentration of the initial reclamation solution was optimal at between 50 and 60 me/l. Final exchangeable sodium percentage of reclaimed soil would have to be less than 5 percent in order to avoid large hydraulic conductivity reductions at commencement of irrigation using the low electrolyte water of the Cunene River. In addition, dissolution of alkaline-earth carbonates in the soil had a marked and favourable effect on reclamation
