Least limiting water and matric potential ranges of agricultural soils with calculated physical restriction thresholds

The least limiting water range (LLWR) is a modern and widely used soil physical quality indicator based on predefined limits of water availability, aeration, and penetration resistance, providing a range of soil water contents in which their limitations for plant growth are minimized. However, to set up the upper and lower limits for a range of soil physical properties is a challenge for LLWR computation and hence for adequate water management. Moreover, the usual LLWR is given in terms of the soil water content in which only for field capacity and permanent wilting point, the matric potential range is known. In this paper, we present a procedure for calculating LLWR using Genuchten’s water retention curve parameters and introducing the least limiting matric potential ranges of agricultural soils, which we named LLMPR, defined as the range of matric potential for which soil aeration, water availability, and mechanical resistance would not be restrictive to plant growth. Additionally, we calculated the minimal air-filled porosity, field capacity, permanent wilting point, and limiting soil penetration resistance thresholds which define the upper and lower limits of LLWR and LLMPR. Finally, we present some application examples using experimental data (from cultivated and forest soils) and developed an algorithm for their calculation in the R software. The calculated soil physical restriction thresholds were sensitive to changes in soil structure and clay content and were changeable rather than fixed. Based on experimental data, our calculations with the calculated parameters showed that an increase in LLWR and its corresponding LLMPR could be achieved with improvements in soil structure. Higher water content at field capacity, as well as a larger soil penetration resistance threshold to a given root elongation rate were observed in the structured in comparison to the cultivated soil. The LLWR and LLMPR as presented in this study was computationally implemented as an R function (R software), named llwr_llmpr, and in an interactive web page, both available in the R package soilphysics, version 4.0 or later, available from https://arsilva87.github.io/soilphysics/ or CRAN (http://cran.r-project.org/web/packages/soilphysics/index.html).