A handful of topsoilSoil is a mixture of broken down rocks, decaying organic matter, gases, liquids and countless micro-organisms. How much of each is in the soil depends on the type of rock, how long weathering (the process by which rock is reduced to fragments) has been occurring, the local climate (whether it is wet or dry, hot or cold), the abundance of living things, and human activities such as farming, grazing, earth-moving and gardening. An average soil will consist of about 45% rock fragments, 25% water, 25% air and 5% organic material.
The rocky part of soil is called the soil matrix. It is a mixture of sand (coarse fragments), silt (finer fragments) and clay (very fine fragments), eroded from rocks, the "parent material". The fragments are made up of different kinds of minerals. Soil can form from the rocks below, but in some instances the rock fragments may have been eroded from rocks by wind, water or glaciers, and deposited some distance away. Loess, or example, is a soil from windblown deposits.
Lancashire loam: the rich, black, fertile soil of south Lancashire, England
A handful of soil may contain over 10 billion bacteria—more than there are people living in the world.
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