Determination the Optimal Range of Pore Volume Distribution by Using of Soil Physical Quality Indicators and Effect of Soil Properties on Sgi Index

Soil quality is an essential concept for maximum agriculture production without environmental destruction. Studying soil physical quality indicators, that express soil structural stability and soil air-water balance in root zone, is one of the most important aspects of soil quality. So determination of optimal range for pore volume distribution is important and this research carried out to determine i) the optimal range for pore volume distribution according to soil physical quality indicators ii) the effect of soil properties on the slope of retention curve at its inflection point or Sgi index and iii) the effect of type of land use on soil physical quality indicators. The classification of studied soils was Haplocalcid, soil texture varied from sand to clay loam and organic carbon (OC) also changed between 0.2 to 2.2 percentages. In this study, Sgi index and pore volume distribution were determined by fitting the experimental soil retention curve data of 40 soil samples to van Genuchten equation in agricultural and rangeland land uses in south of Mashhad plain. Also some other soil physical quality indicators such as, percentage of organic carbon, bilk density, sodium absorption ratio (SAR), mean weight diameter of wet aggregates (MWD), relative field capacity (RFC), plant available water capacity (PAWC), air capacity (AC), and structural stability index (SI) were measured. Then the optimal range of pore volume distribution was determined by using of 8 soil physical quality indicators. Also the correlation between Sgi index and soil physical properties was determined. The positive and significant correlation between Sgi index and percentage of %OC, mean MWD, RFC, and PAWC was shown. But there was a negative and significant correlation between Sgi index and SAR in both land uses. In addition, mean comparison of indicators in two land uses showed that, among the 8 indicators, Sgi index, MWD, PAWC has been decreased significantly in agriculture land use samples.