Prediction of moisture at inflection point of retention curve by using of readily measurable soil properties

Tillage properties at optimum moisture after irrigation or rainfall have a great influence on prevention of soil structure destruction. Soil sampling was carried out on spring of 1384 from Karaj and Varamin plains of Iran. The water content at inflection point of retention curve (θi) was used to determine the optimum water content for tillage. So, the soil moisture was measured in matric pressure heads 0, 1.5, 2.5, 5.5 kPa by hanging water column and in 10, 20, 30, 50, 100, 200, 300, 500, and 1000 kPa by pressure plate for 140 samples. The particle size distribution (clay, silt and sand) and bulk density data of soils with measured moisture contents in above suctions were input to RETC software and parameters of Van Genuchten equation (1980) and θi were determined for any samples. The results of this study showed that the range of θi was 0.17-0.77 and its mean was 0.52 cm3cm-3 in all 140 soil samples. Also there was a positive and significant correlation between θi and percentage of Clay, Calcium carbonate and saturated water content (P)