Effect of root refuse of liquorice on different characteristics of soils from southern regions of Iran. III. Structure and hydraulic conductivity of soils under saline sodic conditions

Abstract

ABSTRACT Population increase and increasing demand for food have extended agriculture to troublesome lands with different problems, e.g., salinity and deficiency of organic matter. Although, even the application of a little amount of organic matter may have great effects in improving microbial, chemical, and physical properties of soil, but organic matter management in all soils, especially saline–sodic soils is difficult, and its mismanagement may deteriorate soil conditions which leads to low soil productivity. Root refuse of liquorice is added to soils as a soil amendment in some parts of Iran, but its effects on structure and hydraulic conductivity of salt-affected soils has not been studied yet. The present study was conducted to evaluate the effect of root refuse of liquorice on structure and hydraulic conductivity (HC) of soils under saline–sodic conditions. To 6 samples of surface soils (0–15 cm) of southern Iran, equivalent rates of 0, 10, 20, and 30 tons ha−1 of root refuse of liquorice were added, and the samples were incubated under room temperature and moisture content of field capacity for 50 days. At the end of incubation time, the soils were air dried, ground, and passed through a 2-mm sieve. Columns of soil samples were saturated with solutions of 100 meq (NaCl+CaCl2)L−1 with sodium adsorption ratios (SARs) of 5, 10, 15, and 20, and subsequently leached with distilled water. The “Sensitivity Index” or SI concept was used to obtain a very general index of the magnitude of structural deterioration and reduction in the HC of the tested soils. In general, (1) the SI values, swelling, and dispersion of the soil columns leached with different saline–sodic solutions, were different for tested soils; and increases in SAR level resulted in decreases in the SI values and initial height of soil columns, and increases in the percentage of dispersion. (2) The R2 for the regression equation between SI and the initial organic matter contents of the tested soils was not statistically significant, but application of the root refuse of liquorice by modifying the suppressing effects of salinity and sodicity improved the SI values of the tested soils, and the effect was more pronounced for higher rates of up to 20 tons ha−1, and application of 30 tons ha−1 resulted in the same effect as 10 tons ha−1. The multiple regression equation between SI values and swelling/dispersion processes, as well as the mineralogy of tested soils, all suggested that the slaking of the structure was responsible for the decrease in the height of soil column and consequently reduction in the SI values (i.e., HC) of the tested soils. Decrease in recovery effect of organic matter on structural deterioration and reduction in HC, associated with increase in the rate of applied root refuse of liquorice, may be due to the formation of sodium humates and increase in the percentage of dispersion.