An idealized healthy soil contains about 45% mineral matter, the result of perhaps millions of years of breaking down (weathering) of underlying rock, 5% organic material including organisms and their remains, and 50% pore spaces that contain both air and water. The mineral matter in the soil determines the soil texture, meaning the relative fineness or coarseness of the soil. Texture is the result of the relative amounts of sand, silt, and clay found in the soil and plays a key role in determining the ability of the soil to hold water and to provide oxygen to trees. This idealized composition of soil is likely more common to the open forest and meadow undisturbed "natural" areas. As we shall discuss later, the situation in urban settings is often radically different.
Another key factor affecting soiL health is its relative acidity or alkalinity, or pH. On the pH scale, a measurement of 7 means that the soil is chemically neutral, less than 7 becomes progressively more acidic and more than 7 becomes more basic (alkaline). The pH of the soil is important because as the acidity or alkalinity change, essential nutrients can become bound in chemical compounds that make them unavailable to plants. Different species of trees have different pH tolerances, but generally, a range of 6.0 to 6.5 is favorable to most plant growth.
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