The soil is a dynamic, complex, constantly changing part of the earth's crust, which extends from a few centimeters deep in some places to hundreds of meters deep in others. It is essential not only to our existence but also to the existence of most living organisms. It has a pronounced effect on the plants that grow in it, and they have an effect on it.
If you dig up a shovelful of soil from your yard and examine it, you will probably find a mixture of ingredients, including several grades of sand; rocks and pebbles; powdery silt; clay; humus; dead leaves and twigs; clods consisting of soil particles held together by clay and organic matter; plant roots; and small animals, such as ants, pill bugs, millipedes, and earthworms. Also present, but not visible, would be millions of microorganisms, particularly bacteria, fungi, and, of course, air and water.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.