Structural stability of sodic soils in sugarcane production as influenced by gypsum and molasses

Abstract

The aim of this work was to determine whether molasses, a by-product of sugar manufacture, alone or combined with gypsum, could improve the structural stability of sodic soils used for sugarcane production. A Burdekin sandy clay loam with an exchangeable sodium percentage (ESP) of 7.9, and a Proserpine loamy sand with an ESP of 18.8 were incubated with molasses (0 and 10 t/ha) and gypsum (0 and 10 t/ha) for 12 weeks, during which time they were leached 5 times with water (0.5 pore volumes each time). In the Burdekin soil, molasses and gypsum, either alone or combined, decreased spontaneous clay dispersion from 2.6 to <0.2 g/kg soil. Mechanical dispersion was reduced from 21.2 to <0.2 g/kg soil by gypsum alone, and to 14.9 g/kg soil by molasses alone. Molasses and gypsum both increased wet aggregate stability, with the combined effect being greatest; the proportion of aggregates >250 μm was 31% in the control and 71% with molasses + gypsum. Electrical conductivity (EC 1:5) was 0.1 and 1.9 dS/m, pH1:5 in water was 7.7 and 7.1, and ESP was 4.1 and 0.2 in the control and molasses + gypsum treatments respectively. In the Proserpine soil, the amounts of dispersible clay were much less than in the Burdekin soil. The effects of molasses and gypsum in decreasing spontaneous and mechanical clay dispersion were similar to those in the Burdekin soil, but less pronounced. Molasses and gypsum, either alone or combined, improved the structural stability of both soils by decreasing dispersion and/or slaking. An implication of this work is that molasses may be a useful ameliorant for sodic soils, either alone or combined with gypsum.