Use of water retention data and soil physical quality index S to quantify hard-setting and degree of soil compactness indices of gypsiferous soils

Hard-setting gypsiferous soils become hard and compact during drying without the impact of soil compaction caused by agricultural field traffic. Thus, these soils are difficult or impossible to cultivate unless they are re-wetted. Moreover, it has previously been shown that Dexter soil physical quality index (S-index) negatively affected by compaction. This study was done to quantify the hard-setting as well as the soil compactness (expressed in terms of the degree of compactness, DC, which is relative density expressed as the ratio of bulk density to a reference density) using water retention data and soil physical quality index of fifteen soils with a wide range of gypsum content (30−301 g kg⁻¹). Repacked soil cores were prepared with less disturbance of soil aggregates ≤ 4 mm). Water retention curves of the soils were measured in the matric suction range 10−15000 hPa. A single-porosity van Genuchten-Mualem model was fitted to water retention data. S-index and Dexter hard-setting (HDₑₓₜₑᵣ) were calculated using the van Genuchten-Mualem parameters. The relationships between S-index and both DC and HDₑₓₜₑᵣ were investigated for the 15 gypsiferous soils. A strong positive polynomial and power correlation were found between DC and HDₑₓₜₑᵣ and S (i.e. 1/S) respectively, and that are valid across different soil gypsum content. Significant negative relationships between HDₑₓₜₑᵣ and soil gypsum content were observed. Positive polynomial relations were derived between S and bulk density and polynomial relation was obtained between HDₑₓₜₑᵣ and bulk density. Positive relationships were observed between HDₑₓₜₑᵣ, DC, and relative bulk density. These findings confirmed that the new proposed HDₑₓₜₑᵣ index enables the prediction of soil hard-setting behavior to quantify the gypsiferous soil physical quality using only water retention data with no need to measure soil mechanical strength.