Gypsum Amendment Improves Saturated Hydraulic Conductivity and Plant-Available Water in Heavy Clay Soil

Soil hydrophysical properties play a key role in processes such as water movement through soil and also affect the amount of water available to plants, thus influencing the sustainability of water management in lowland agricultural landscapes. This study investigated whether the application of calcium sulfate dihydrate (gypsum, CaSO 4 ·2H 2 O) can improve selected hydrophysical properties of a heavy clay agricultural soil from the Eastern Slovak Lowland (Slovakia). In a controlled laboratory experiment, topsoil samples (0–15 cm depth) were treated with four rates of gypsum application (0.5, 1, 2.5 and 10 g core −1 ; ≈2–40 t ha −1 equivalents) and then repacked in 100 cm 3 cores. Gypsum caused a marked apparent shift from “clay” to “silt” in the particle-size analysis, consistent with flocculation and incomplete dispersion rather than a real textural change. Increasing the gypsum dose also led to a gradual increase in saturated hydraulic conductivity (from 0.68 ± 0.21 to 2.00 ± 0.66 cm d −1 ). Water retention near saturation changed little, but water content at the wilting point decreased at higher doses, increasing plant-available water (maximum ~59% at 2.5 g core −1 ). Under laboratory conditions, gypsum improved the hydraulic function of the soil, and, at selected doses, increased water availability related to drought, supporting its potential as a structural amendment for enhancing the sustainable management of heavy clay soils.