Modelling of soil water retention properties for soil physical quality assessment.

Because the soil water retention curve assessment requires (in terms of standardized European methodology) long time for measuring all levels of the soil water suction applied, we have used an indirect estimation methodology (deterministic) in order to have an overview of the soil capacity to retain water at different levels, which correspond to the main soil hidrophysical indicators, particularly important in assessing soil fertility status, as characterized by the ability of soil to maintain water in soil, and release the water when there are needs and / or requirements for crop plants. In this paper we present the model of Arya - Paris, which assess the water retention curve using soil texture and bulk density data. This model is based on the analogy between the soil water retention curve and the cumulative distribution curve by soil particles size. Parameter α introduced by Arya and Paris is an effective way to assess the length of capillaries in natural soils using as a measure spherical particles associated with the class in which was divided the solid fraction of soil. Arya-Paris model with α values calculated was applied to evaluate pairs of θ (soil water content) - ψ (matric potential). The van Genuchten equation was then used to estimate the water retention curve in the analyzed soils. We used in the paper three soil types (Calcic Chernozem, Rendzic Leptosol and Calcaro-calcic Kastanozem) located in the Dobrogea area from SE of Romania. Parameters of the van Genuchten equation, θr, θs, α, n were evaluated using the pairs of values θ - ψ calculated by Arya-Paris model using the Solver tool from Microsoft Excel software package. Van Genuchten equation was then used to assess some parameters that have agronomical significance. Optimum soil water content and soil matric potential for soil workability correspond to the inflection point of the soil water retention curve and were calculated using the van Genuchten parameters. Depending on the characteristics of the soil water retention curve at the inflection point the soil physical quality index, S, was calculated.