The effect of applied 'high magnesium' bitterns upon the physical and chemical properties of soils from Wakool-Tullakool Irrigation District, New South Wales, Australia

This study was concerned with determining the effect upon soil structure when the drainage scheme evaporation pond product "high magnesium bitterns" was applied to soils from the Wakool-Tullakool irrigation district, N.S.W, Australia. Two soils were chosen covering the extremes of soil types found in the district. They were; 1. Burraboi Sandy Loam (Gn 2.63) a Calcic Aqiuc Haplustult and, 2. Moulmein Clay (Ug5.24) a Typic Chromoxerert. Laboratory tests were carried out to determine the basic physical and chemical properties of the two soils. Clay mineralogy for the two soils was determined using X- ray diffractometry. Both Na/Ca and Na/Mg cation exchange isotherms were constructed from data obtained using a sequential dialysis equilibration on the clay fraction of both soils. The final solution concentration was set at 0.006 mmol(+)L-1 thus approximating the actual soil solution at field capacity. Both soil clays showed exchange preference for the divalent cations over the monovalent cation Na. The values of K(VAN) show that Na selectivity at very low values of ENa decreases rapidly with increasing ENa to a minimum value after which Na selectivity increases. The values of Kex, the thermodynamic exchange constant, for Burraboi Sandy Loam were 0.746 for the Na/Ca system and 1.193 for the Na/Mg system. Thus the Ca system shows a decrease in Na selectivity relative to the Mg system. There was no significant difference in between the values of Kex for the Na/Ca and Na/Mg systems in Moulmein Clay. The calculated values of the rational activity coefficients /Na and /M2+ were not equal to unity over the range of Na saturation, and Kex did not remain constant, Thus non-ideal exchange occurred. It was also evident that some heterogeneity of exchange sites existed. An infiltration trial using diluted bitterns was carried out on Moulmein clay to assess its effect on the infiltration characteristic of the soil. No significant difference between treatments could be detected and it is thought that soil variability masked any treatment effect. A separate experiment on the same site where the infiltered soil was sampled for moisture content in a vertical grid pattern, showed that the downward movement of infiltering solute, when the soil is at the peak of its drying cycle, is governed by vertical movement through conducting macropores. Little lateral movement was seen and the solute did not penetrate the soil matrix to any great extent until redistribution occurred. Upon wetting soil swelling occurred and infiltration effectively ceased. Laboratory examination of samples taken from the infiltration trial also showed no detectable treatment differences. The total concentration of the immiscibly displaced soil solution increased with depth in all treatments as inferred by electrical conductivity measurements. It is thought that the infiltering solution moved rapidly down the profile carrying salts dissolved from the macropore periphery stopping when the pores became constricted in a region of increased soil moisture and subsequent soil swelling. Emerson structural stability tests carried out on the infiltration samples showed that the soils treated with the diluted bitterns (0.1 M and 0.01M) had slightly improved structural stability moving from class 1 and 2 to class 3, as defined by Emerson. This could be due to displacement of the Na by Mg or due to an increase in ionic strength in the soil solution