Soil Composition and Properties

Soil is built from mineral particles of three sizes: sand (largest), silt (medium), and clay (smallest, finest). Particle size controls pore space. Sand grains are big and irregular, so they stack loosely, leaving large gaps between them. Water drips straight through sand because those gaps connect in wide channels: high permeability, but low water retention. Clay particles are tiny and flat, packing tightly together with almost no room between them. Water sits on top of clay or moves through at a crawl, because the pores are narrow and poorly connected: high water retention, but low permeability and poor drainage.

To name a soil, you need its percent sand, percent silt, and percent clay, which must add to 100. Plot each percentage on its own axis of the soil texture triangle and find where the three lines meet; that region gives the soil's name. A sample that is 40 percent sand, 40 percent silt, and 20 percent clay lands in the loam region: no single particle size dominates, so loam has a mix of large and small pores. That gives it decent drainage and aeration alongside decent water and nutrient holding, which is why loam is prized by farmers.

This connects to the rest of the unit because soil texture sets the stage for infiltration and runoff. Soil that drains well lets water sink down to recharge groundwater; soil that sheds water generates more surface runoff, erosion, and flooding risk. Soil texture also ties to what comes after it in this unit: soil horizons and fertility, since the pore structure here decides how roots, air, water, and nutrients move through every layer below.