Soil Texture, Permeability, Infiltration and Groundwater

Texture decides almost everything

Soil texture is the proportion of sand ($0{,}05$–$2$ mm), silt ($0{,}002$–$0{,}05$ mm) and clay (under $0{,}002$ mm). The three fractions always sum to $100\%$, so two values give the third. Because particle size sets pore size, texture controls two properties that pull in opposite directions:

• Permeability — how fast water moves through. Large pores between sand grains drain fast; clay pores are tiny, so water moves slowly.
• Water-holding capacity — how much water is retained against gravity. Clay holds the most, sand the least.

The best agricultural soil is loam, roughly $40\%$ sand, $40\%$ silt, $20\%$ clay: enough clay to hold water and nutrients, enough sand to drain and to let roots breathe. Clay soils hold plenty of water but much of it is bound so tightly that plants cannot extract it, so plant-available water peaks in loams, not in clays.

Where the water goes

Rain that reaches the ground either infiltrates, runs off, or evaporates. Water that infiltrates past the root zone percolates down to the water table, the top of the saturated zone. A permeable saturated layer that yields useful water is an aquifer; a confined aquifer sits under an impermeable layer and is recharged only where that layer reaches the surface. A watershed is all the land draining into one river. When infiltration is reduced by compaction or paving, the same rainfall produces more runoff, more flooding and less recharge.