Soil Profile and Infiltration: How Water Moves Through Horizons

Horizons: a soil is not one material

Dig a vertical face into undisturbed ground and you see layers, not a uniform block. The O horizon is loose organic litter; the A horizon (topsoil) mixes that organic matter with mineral grains and holds most of the roots and biology; the B horizon (subsoil) is where clay, iron oxides and dissolved minerals accumulate as water passes through and evaporates; the C horizon is weathered parent rock; R is unweathered bedrock. Each horizon has its own texture, bulk density and pore structure, so water does not move through a soil profile at one constant speed — it moves through a stack of different pipes.

The wetting front and Richards' equation

Below field capacity, water in soil is unsaturated: it fills only some of the pore space, held against gravity by capillary suction. The physics is Richards' equation, a nonlinear diffusion equation for the volumetric water content θ:

∂θ/∂t = ∂/∂z [ K(θ) · ( ∂ψ(θ)/∂z + 1 ) ]

θ  = volumetric water content
K(θ) = unsaturated hydraulic conductivity (rises steeply with θ)
ψ(θ) = matric suction (capillary potential, negative, rises toward 0 as θ→saturation)
z  = depth (positive downward)

K(θ) can vary by four or five orders of magnitude between dry and saturated soil, which is why infiltration fronts are so sharp: dry soil ahead of the front conducts almost nothing, so water piles up behind a nearly vertical wetting boundary and that boundary advances almost as a single sheet. Sandy soils have large, well-connected pores, so K is high and the front is fast and diffuse; clay soils have tiny pores and high suction, so K is low and the front is slow and knife-sharp.

Green-Ampt: the engineer's shortcut

Solving Richards' equation numerically is expensive, so hydrologists often use the Green-Ampt model, which idealises the wetting front as a step: fully saturated soil above depth L(t), untouched dry soil below it. Mass balance and Darcy's law then give a simple ODE for how fast the front descends, and integrating it gives cumulative infiltration as a function of time, matric suction at the front and the saturated/initial moisture difference. It reproduces the two hallmark infiltration behaviours: a high initial rate when the whole soil column is offering little resistance, decaying toward the saturated hydraulic conductivity Ksat as the wetted column lengthens and gravity becomes the only remaining driver.

Why nutrients leach

Dissolved nutrients — nitrate above all, since it carries a negative charge that most soil colloids do not attract — travel largely at the same speed as the water molecules around them, a process called advection. A fast, deep wetting front after heavy rain or over-irrigation can carry nitrate past the root zone entirely before a crop has a chance to take it up, which is both a fertiliser loss and a groundwater contamination pathway. Phosphate and ammonium behave differently: they bind to clay and organic matter (adsorption), so they leach far more slowly and instead accumulate in the B horizon, which is exactly why B horizons are described as the profile's zone of illuviation.

What texture changes in practice

Texture sets the whole shape of the curve. Sand has large grains, big pores, low suction and a high Ksat, so water infiltrates fast and drains fast, giving low water-holding capacity and a high leaching risk. Clay has tiny grains and pores, very high suction and a low Ksat, so infiltration is slow, ponding and runoff are common, but once wetted the profile holds water and nutrients far longer. Loam sits between the two and is agronomically prized for exactly that balance — infiltrating quickly enough to avoid runoff, but retaining enough water and nutrients between rain events to support root growth.

Frequently asked questions

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