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It is mostly made up of grains of rock (weathered by wind, rain, sun, snow) and humus. The type of soil depends on the mix of humus and on the size of the grains of the rock. The grains can be very small and smooth, such as clay, or they can be larger, like grains of sand or even a piece of gravel.  Soils are important to our ecosystem for six main reasons: first, soils are a place for plants to grow; second, soils control the speed and the purity of water that moves through them; third, soils recycle nutrients from dead animals and plants; fourth, soils change the air that surrounds the earth, called the atmosphere; fifth, soils are a place to live for animals, insects and very small living things called microorganisms; sixth, soils are the oldest and the most used building materials. The climate is very important when soil is made. Soil from different climates can have very different qualities the types of soil varies from .
- Learn Science, intermediate, grades 5 to 6 by Mike Evans and Linda Ellis
- Brady and Weil; The Nature and Properties of Soils, 14th ed. 2008.
- Climate And Man, University Press of the Pacific, p. 27,