24.13 SOIL

Soils are a thin layer on top of most of Earth’s land surface. This thin layer is a basic natural resource and deeply affects every other part of the ecosystem. Soils are composed of three main ingredients: minerals of different sizes, organic materials from the remains of dead plants and animals, and open space that can be filled with water or air. A good soil for growing most plants should have about 45% minerals (with a mixture of sand, silt, and clay), 5% organic matter, 25% air, and 25% water.

Soils develop in response to several factors:

1. Parent material: This refers to the rock and minerals from which the soil derives. The natural of the parent rock, which can be either native to the area or transported to the area by wind, water, or glacier, has a direct effect on the ultimate soil profile.

2. Climate: This is measured by precipitation and temperature. It results in partial weathering of the parent material, which forms the substrate for soil.

3. Living organisms: These include the nitrogen-fixing bacterial Rbizobium, fungi, insects, worms, snails, etc., that help to decompose litter and recycle nutrients.

4. Topography: This refers to the physical characteristics of the location where the soil is formed. Topographic factors that affect a soil’s profile include drainage, slope direction, elevation and wind

exposure.

With sufficient time, a mature soil profile reaches a state of equilbrium. Feedback mechanisms involving both abiotic and biotic factors work to preserve the mature soil profile. The relative abundance of sand, silt, and clay is called the soil texture.