Assessment of the S-index using three models of the soil water retention curve in five different Land uses in south of Kerman

One of the ways to express physical behavior of soil is to describe the soil structure. Since physical soil could not be measured directly, therefore, the special properties defined as indicators, are used to express the soil physical quality. of different indicators, S-index is used to express the physical quality of the soil which is the slope of the soil water retention curve at inflection point. It seems that little research has been done on the effect of landuse on the S-index, then The purpose of this study was to compare the results of S-index of three different soil water retention curve models of the moisture characteristic of the soil (Van Genuchten, Brooks-Corey and Grant-Groenevelt Models) in 5 different land uses in Jiroft city. For this reason, 350 disturbed and 350 undisturbed soil samples of different landuse (citrus, palm, arable, forest and combinatory) were analyzed. Some of the physical and chemical features of the soil, including sand, silt, and clay percentage, the electronic conductivity at saturation, Bulk Density, the percentage of the total porosity and the percentage of the Organic Matter were measured. Also, the amount of the soil moisture in suctions 0, 10, 30, 50, 100, 300, 500, 1000 and 1500 KPa was determined using pressure plates. The results showed in the study region the best physical quality of the soil was seen in palm land use. All landuses, except forest, had high physical quality. High value of S index in citrus, palm, and arable land uses can indicate better soil management at the start of farming. In combinatory land use, due to Simultaneous cropping of alfalfa and palm, alfalfa causes more organic matter to be returned to the soil and as a result, the soil structure improves. Which is displayed at high value of S index. In general, The S index was higher in landuses where the soil had better structure or management operations were improved. because of the distinction between the soils with closely physical quality, Van Genuchten model was better and more appropriate than the other two models in assessing the physical quality. In general, despite the slight differences in citrus, palm and cultivate landuses, palm landuse had a higher mean of S index. So that their values in Van Genuchten, Brooks-Corey and Grant-Groenevelt models were 0.06, 0.06 and 0.04 respectively. The results of this study showed that the Van Genuchten model accuracy was better in estimating and evaluating the soil quality due to the greater differentiation between the soils with close physical quality. Because the fitting of Soil water characteristic curve had a higher R2 (R2=0.92). However, due to low differences with two other models, in particular the Brooks and Corey model (R2 =0.87), It seems that, in different landuses, a suitable model for assessing soil quality varies. Therefore, it is imperative that this index and other indicators of soil physical quality assessment in different climates are further examined.