This simulation drives water infiltration through a soil column using the van Genuchten (1980) water-retention curve, θ(ψ) = θr + (θs−θr)/[1+(α|ψ|)^n]^(1−1/n), letting you pick a soil texture and watch the wetting front and nutrient leaching move down through topsoil, subsoil and bedrock horizons as rainfall soaks in.
A cross-section of soil horizons with an advancing wetting front driven by Darcy flux, plus live readouts of surface moisture, field capacity, wilting point and nutrient leaching.
Pick Sand, Loam or Clay texture, adjust Rainfall and Vegetation cover and Organic matter sliders, then use ↺ Reset or ⏸ Pause to control the simulation.
Clay soils hold far more water at their wilting point than sandy soils do — meaning a clay soil can look "wet" while still withholding much of that water from plant roots, unlike free-draining sand.
Sand has large, well-connected pores so water infiltrates quickly and the wetting front advances fast; clay has tiny pores with strong capillary forces that slow water movement dramatically, which is why switching texture changes the Wetting front stat's pace.
Field capacity is the water content soil retains against gravity after free drainage stops; wilting point is the much lower water content at which plant roots can no longer extract water. The gap between them is the water actually available to plants.
Higher vegetation cover reduces surface runoff and increases infiltration by protecting soil structure and adding root channels, which is why raising the Vegetation cover slider tends to slow nutrient leaching for a given rainfall rate.
Organic matter increases soil's capacity to hold onto water and nutrients (cation exchange capacity), so higher Organic matter percentages reduce the Nutrient leach. stat by binding nutrients that would otherwise wash down with the wetting front.
It converts soil water potential (ψ, the suction pulling water into the pores) into volumetric water content (θ), giving the retention curve shape that determines how much water each texture holds at any given moisture tension.