Effect of Agricultural Sulfur on Characteristics of Different Calcareous Soils from Dry Regions of Iran. II. Reclaiming Effects on Structure and Hydraulic Conductivity of the Soils Under Saline-Sodic Conditions

Abstract

The use of elemental sulfur (S) is of special interest in Iran since most soils are calcareous and elemental S, as a byproduct of the petroleum industries is readily available. Recently, the National Petrochemical Company (NPC) of Iran has produced an agricultural sulfur (A.S.) that contains 90% pure S and 10% bentonite, but its effects on structure and hydraulic conductivity of salt-affected calcareous soils has not been studied yet. The present study was conducted to evaluate the effect of A.S. on structure and hydraulic conductivity (HC) of soils under saline-sodic conditions. To six samples of surface (10–15 cm) calcareous soils of a dry region of southern Iran, equivalent rates of 0, 2, 4, and 6 t ha−1 of A.S. were added, and the samples were incubated under room temperature and moisture content of field capacity. 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) regardless of SAR level and A.S. rate, the SI values, the percentages of expansion and dispersion of the soil columns leached with different saline-sodic solutions, were different for tested soils. Increases in SAR level resulted in decreases in the SI values and initial height of soil columns; but although, increase in the SAR level from 5 to 10 resulted in decreases in the percentage of dispersion, there was no significant difference between the effect of SAR 10 and SARs 15 and 20. (2) Application of A.S. resulted in no significant (statistical) effect on the SI values of the tested soils, but was associated with (somehow) a modifying effect on the degrees of expansion and dispersion of the soil columns, which indicated the improving effect of A.S. application on the structure of tested soils under saline-sodic conditions. 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 (due to both swelling and dispersion processes) was responsible for reduction in the SI values (i.e., HC) of the tested soils.