Use of indicators and pore volume-function characteristics to quantify soil physical quality

The Dexter “S-value” (Sgi) is a promising new indicator of soil physical quality (SPQ), but it is not well tested against established indicators, such as relative field capacity (RFC), plant-available water capacity (PAWC), air capacity (AC), macroporosity (PMAC), bulk density (BD), organic carbon content (OC), and structural stability index (SI). Furthermore, all SPQ indicators are direct or indirect expressions of pore volume and/or pore function, but optimal pore volume-function characteristics have not been identified. The objectives of this study were to: i) compare Sgi to the other seven indicators for a range of rigid to moderately expansive soils and artificial porous media; ii) use the indicators to propose an optimal pore volume distribution and soil water release curve; and iii) assess the SPQ of a compost-amended soil using indicators, pore volume distributions and water release curves.The indicators were measured in the laboratory on intact soil cores and grab samples collected from 13 soil-management combinations. Soil texture included clay loam, sandy clay loam, loam, sandy loam and sand; management included virgin soil, no-till cropping and mouldboard plough cropping. Also included were two artificial media consisting of glass beads and builders sand. Pore volume distributions and water release curves were determined by fitting the van Genuchten function to desorption data obtained from the soil cores and grab samples.The Sgi indicator gave correct SPQ designations for the structured loamy soils, but erroneous designations (Sgi > 0.035) for the structureless sands, glass beads and builders sand. The indicators suggested that four of the 13 soil-management combinations had optimal overall SPQ, and these combinations were used to define an optimal pore volume distribution and water release curve. Non-optimal soils with poor aeration (RFC > 0.7, AC < 0.14 m3 m− 3) and poor structure (PMAC < 0.07, SI < 7%, Sgi < 0.035) had greater proportions of small pores and excessive water retention relative to optimal soils, while droughty soils (RFC < 0.6, PAWC ≤ 0.15 m3 m− 3) had lower proportions of small pores and insufficient water retention. The indicators, pore volume distributions and release curves showed that adding 75 t ha− 1 compost improved the SPQ and maize yield of a clay loam soil, but addition of 300 t ha− 1 compost was required to achieve optimal SPQ and maximum measured yield.It was concluded that the Sgi indicator should be used judiciously and in concert with other indicators for assessing SPQ; and that the suite of eight indicators used in conjunction with an optimal pore volume distribution and water release curve are effective for quantifying the physical quality of rigid to moderately expansive agricultural soils.