SOIL: What is It?

Slide 1 of 8

Notes:

Soil. Earth. Dirt. No matter what we call it, it’s the material that constitutes the outermost solid layer of the planet. We build on it. We raise food in it. We mine mineral resources from beneath it.

Apparently unchanging and lifeless, soils are dynamic mixtures, teeming with life. One teaspoon of soil in the temperate regions can contain billions of organisms, ranging from simple bacteria and fungi to more advanced forms. Earthworms, insects, and spiders are examples. Bedrock is continually fractured, dissolved, and changed into soil, but the process occurs slowly so we usually never notice.

Soil is a naturally occurring mixture of mineral and organic ingredients with a definite form, structure, and composition. The exact composition of soil changes from one location to another. The following is the average composition by volume of the major soil ingredients:

45% mineral matter (clay, silt, sand, gravel, stones)

25% water

25% air

5% organic matter or humus (both living and dead

organisms)