Evaluation of some effective soil physical and chemical properties on soil water retention in dominant calcareous soil series in Khuzestan province

In spite of problems in providing timely and adequate water supply, the inefficient water use efficiency in agriculture as the most important consumer of freshwater is one of the most important challenges and issues in the agriculture. The knowledge of soil physical properties and water retention have main rolled on soil water application management. This research was conducted to determine the soil physical and chemical properties affecting the soil water retention capacity in calcareous soils series in Khuzestan province.

 From the nine dominant series in the different regions, five disturbed soil samples and the same number of undisturbed soils were collected from 0-20 cm depth. Soil physical properties including soil texture, bulk density were measured. Moreover, soil water characteristic curve was measured at five points (0, 330, 1000, 5000 and 15000 cm) using pressure plate and pressure membrane devices. The van Genuchten equation was fitted on the measured soil water characteristic curve data. In addition to soil physical and hydrological properties, soil salinity as electrical conductivity (EC), pH and lime (TNV) were also measured.

 The results showed that the soil pH varied in the range of 7.1 - 7.6, EC variation range was 2.43 - 47 dS m-1, sand, silt and clay contents varied in ranges of 18 - 40%, 32 - 50% and 20 - 44 %, respectively, TNV was 48 – 60 % and the values of soil bulk density were in the range of 1.14 - 1.52 g cm-3. The studied soils were calcareous (TNV value of all studied series was more than 45 %) and had moderate to heavy texture including loam (55 % of studied soil series), silty clay loam (23 %), clay loam (11 %) and clay (3 %). The water content at field capacity (corresponding of soil suction of 330 cm) and permanent wilting point (corresponding of soil suction of 15000 cm) varied from 32 to 56 % and 16 to 26 %, respectively. There are significant correlation between clay content with the soil moisture at both field capacity and permanent wilting point. The most salinity was observed in south east of Ahvaz (loam), south Ahvaz (loam), Khoramshahr 2 (clay) and Khoramshahr 1 (loam) series. The values of soil available water in studied calcareous series were varied as Khorramshahr 2 (0.30 cm3 cm-3) > Ahvaz-Hamidiyeh (0.22 cm3 cm-3) > Khorramshahr 1 (0.189 cm3 cm-3) > Ramhormoz-Rustamabad 1 (0.177 cm3 cm-3) > Ramhormoz-Bazidi (0.165 cm3 cm-3) > South of Ahwaz (0.16 cm3 cm-3) > South east of Ahwaz (0.134 cm3 cm-3) > Ramhormoz- Rustam Abad 2 (0.119 cm3 cm-3) > Shadegan (0.102 cm3 cm-3). The results showed that van Genuchten equation had suitable fit on studied soils of dominant calcareous series. The studied soil samples from the south of Ahvaz with the highest bulk density (1.55 g cm-3) and high salinity (33.5 dS m-1) had the highest value of the van-Genuchten parameter (n) which indicates more uniform distribution of the soil pore volume. In spite of the high ability of Khorramshahr 2 series in soil water retention, based on the coefficients of van Genuchten equation and measured soil water characteristic curve, it can be argued that this soil probably has severe aeration limitation.

The result showed that salinity and bulk density were important properties that affect the ability of soil water retention and availability in the studied soil series. Extensive water retention capacity in the soil (10 to 30 %) indicates the need for different irrigation management operations in each of the studied soil series. The highest value of soil densities were observed in south of Ahvaz, Ramhormoz-Bazidi and Khorramshahr series, indicating soil degradation in studied areas due to inappropriate cultivation management practices. Moreover, the results showed that the experimental parameters of the van Genuchten equation have a physical concept and can be found useful information on the soil ability to water retention and some other soil physical properties. The results of this research confirmed that there is a significant difference in the ability of soil water retention in the dominant calcareous soils series of Khuzestan province which indicates the necessity of proper understanding of soil hydraulic properties for irrigation management in farms, increasing water use efficiency.