Assessment of Critical Soil Water Content in Some Water Repellent Soils of Guilan Province

Water repellency is a soil property with short-term (or seasonal) variations and reduces or even prevents water infiltration into the soil. When soils are dry, water repellency is found to be at its most extreme. The objective of this study was to investigate variations of soil water repellencyand determine critical soil water contents in 10 forestry sites covered by different plant species (Pinus taeda, Alnus subcordata, Pterocarya fraxinifolia, Sambucus ebulus, Boxus sempervirens, Parotia persica, Quercus castanefolia, Fagus orientalis, Acer sp. and sparse grass) in Guilan province. The soils were sampled two times at autumn and summer from 0-5 cm depth. The water drop penetration time (WDPT) test was used to measure the actual water repellency at the selected sites under the natural field-moist conditions. The potential water repellency was measured in undisturbed samples dried at 25 °C taken from the same sites. Actual water repellency was determined only at forest sites in summer. A wide range of soil water repellency classes (hydrophilic to extremely water-repellent soils) were detected at the examined sites. The results show that water repellency is the largest at the sites with sandy texture. In this research, positive correlations between logWDPT and organic matter content(r=0.42) and sand percentage (r=0.27) were obtained. Correlations between Log WDPT and clay percentage was negative (r= -0.35) and no correlation was found between pH and Log WDPT. The soil water content zone at which soils became water repellent (critical water content) was different for each site and it was a function of the soil properties. Critical water content was assessed in sandy soils (0-5 -cm depth) and ranged from 4.0-6.7 %, in silty­ clay­ loam and silty­loam, between 10.1-15.8%, and in silty ­loam and clay loam with high organic matter varied from 13.3-21.3%. Apparently, soil organic matter and texture influence the critical water content.