1. Soil is a mixture of weathered rock and organic matter
Soil is a mixture of weathered rock and organic matter. It also contains mineral fragments, water, and air.

2. Soil Formation
Soil starts as weathered rock. Plants grow in weathered rock. Worms, insects, bacteria and fungus live among the plant roots and help soil evolve by adding organic matter.

3. Humus
Humus is dark-colored organic matter made of pieces of decaying plants and animals
Worms and insects mix the humus with the fragments of rock.

4. Soil Profiles AB&C Horizon
The O horizon is filled with organic matter (leaf litter). The A horizon, known as topsoil, it is the most fully evolved layer in a soil profile. It is dark & contains more humus & smaller sediments.

5. B - Horizon
The B layer is lighter in color & contains little or no humus & contains minerals that were washed down (leaching) from the topsoil.

6. C-Horizon & Below
The C horizon is the bottom layer in a soil profile, it contains partly weathered rock, but no humus. Below the C-horizon is parent rock.

7. Types of Soil
The thickness of the horizons & the soil composition depend on many conditions.
Rate of weathering
Hot & Moist = increased chemical weathering & more vegetation
Cool & Moist = increased mechanical weathering
Climate (humidity and temperature)
The type of rock the soil has evolved from.
Slope of the land & the amount of humus in the soil.
Grassland vs. Mountain slope

8. Soil Loss Desertification or desert formation
occur where soils are damaged by overgrazing & poor farming practices in areas that receive little or no rain.
Overgrazing strips plants from the land. This results in erosion of topsoil & slows soil evolution.

9. The Dust Bowl
In the 1930s, the “Dust Bowl” was created by poor farming practices, overgrazing, & a drought in the Great Plains. It caused soil to be carried away by water & wind, creating dust storms and widespread famine.

10. Overgrazing in the Tropics creates Desertification
Deforestation, clear-cutting, and slash & burn farming removes all trees & many nutrients, it also gives a very short growing period before the existing nutrients in the soil are used up.

11. Preventing Soil Loss
Crop rotation – Rotating crops so nutrients are not used up.
Cover crops -
Keeps soil in
place.
Barriers –
reduce wind and
water erosion