4.3 Soil Composition and Properties

Soil texture is the percentage of sand, silt and clay in a soil. Gravel and larger stones aren't counted. Texture affects almost everything about how a soil behaves.

Porosity is the amount of open space (pores) between soil particles. Permeability is how easily water flows through the soil, which depends on how large and connected the pores are. Water holding capacity is how much water the soil can hold against gravity for plants to use.

These don't all go together. Clay has many tiny pores, so its total porosity is high, but the pores are so small that water moves through very slowly: low permeability. Clay holds a lot of water, but it can become waterlogged and lack air for roots. Sand has larger pores that drain fast: high permeability but low water holding capacity, so sandy soils dry out quickly.

Organic matter (humus) acts like a sponge. Adding it raises water holding capacity in sandy soils and loosens clay soils.

The soil texture triangle is a chart for naming a soil from its percentages of sand, silt and clay. Each side of the triangle is a scale from 0 to 100% for one particle type. In the standard version, clay is on the left side with its lines running horizontally, silt is on the right side with lines running down and to the left, and sand is on the bottom with lines running up and to the left.

To use it, find a soil's clay percentage and follow that line across, then do the same for one of the others. The point where the lines cross falls in a labeled region, like loam, clay or sandy loam. The third percentage should cross at the same point, which is a good check. The three percentages must add up to 100.

Loam is a fairly even mix, roughly 40% sand, 40% silt and 20% clay. It's ideal for most crops because it drains well enough for roots to get air but holds enough water and nutrients.

Soil tests help farmers decide how much to irrigate and fertilize. Chemical tests measure pH and nutrients like nitrogen, phosphorus and potassium. Physical tests measure texture, permeability and water holding capacity. Biological tests look at soil life, like earthworm counts or microbial activity.

• Nutrients: plants need nitrogen, phosphorus and potassium in the largest amounts, plus calcium, magnesium and others.
• Organic matter: humus releases nutrients as it breaks down and helps soil hold water and nutrients.
• Ability to hold nutrients: clay particles and humus have negative charges that hold positively charged nutrients, like potassium (K⁺), calcium (Ca²⁺) and ammonium (NH₄⁺), so they aren't washed away. Sandy soils hold nutrients poorly.
• pH: most crops grow best in slightly acidic to neutral soil (about pH 6–7). Very acidic soil makes some nutrients unavailable and releases toxic aluminum. Acid rain can lower soil pH, and farmers add lime (crushed limestone) to raise it.