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On the surface, plant debris and dead animals decay and provide nutrients, and aboveground vegetation may slow rain as it falls, reducing erosion. In the upper layers of soil there may be up to five tons of living organisms (roots, animals, bacteria, fungi, and so on) per acre. Another slope consideration is the degree of slant, which affects the amount of erosion that will occur through water runoff and gravity: the steeper the slope, the less distinct the layers of the soil profile will be at the top and the wetter the soil will be at the bottom. In the Northern Hemisphere, north-facing slopes are cooler than south-facing slopes because they get less direct sunlight, while in the Southern Hemisphere the opposite is true. Aspect, or the relation of a place to sun and weather, plays a big role in the temperature of a site. In colder weather, water can get into crevices in rocks and freeze-exerting a force of 150 tons of pressure per foot and breaking the rocks into smaller pieces-then thaw, releasing the fragments and allowing water to reach deeper into the rock. Areas with high precipitation generally have thicker soil layers, especially if the rain falls when the soil is warm, leading to increased chemical weathering as organic matter decomposes and releases organic acids. Water also plays a direct role in weathering parent material. Wetter climates usually foster more plant growth, which means more organic matter to break up rocks (such as tree roots pushing through concrete) and then decaying to become part of the soil. The second factor is the climate, primarily precipitation and minimum and maximum temperatures. Then wind, water, glaciers, and gravity may move these particles long distances, so parent material in a location may not be the only or even the principal component of the soil found there. The rocks are slowly weathered by physical, biological, and chemical processes to form smaller and smaller particles. This usually comprises large igneous rocks, created from volcanic activity sediments that become solid through intense pressure or combinations of the two into metamorphic rocks through heat and pressure. Scientists who study soil formation consider five interrelated factors. The United States alone has more than twenty thousand identified soils!

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These layers, along with rocks, combine to create what has been appropriately called 'the skin of the earth.' Like skin, it is rich in variation-of color, moisture, graininess, and so on-and its multitude of types can be classified by these and other characteristics. Eventually, the soil stops at hard bedrock. Moving deeper are 'E,' 'B,' and other horizons, mostly mineral particles of increasing size. Below that is what soil scientists call the 'A' horizon, composed of fine mineral particles (sand, clay, and other inorganic matter) with some accumulated decomposed organic material.

This 'O' horizon may be only a few inches thick and is often darker than lower layers. On top, there might be a relatively thin layer of organic material made up of living, decaying, and decomposed plants and animals. Skin Deep: Soil Formation and CompositionĪs soils develop, they usually form a profile of different layers (see figure 1). They are neither permanent nor indestructible. Under most natural conditions they form at a rate nearly equal to that of their erosion, which happens naturally by means of water, wind, and gravity. They have mineral components but are teeming with life, such as fungi, beetles, worms, and gophers. Soils are incredibly dynamic-complete ecosystems that are constantly, though sometimes slowly, changing. But think about it: a substantial amount of the biological mass of a plant is in the roots, which need soil to support and nourish the parts above ground.

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Even today, many professional botanists and even horticulturists know surprisingly little about soil. When I was a botany student I was not required to know anything about soil science-I learned a bit about soil formation in an ecology class, and that was it. Many gardeners think of it mostly when they are removing it from under their fingernails after a day planting bulbs, but anyone who wants to grow plants should know something about it, and surprisingly few do. Some may use it to grow crops to support their families, some may try to stop it from washing downhill in storms, and some spend their lives studying it and mapping it. Some may call it 'dirt' and try to launder it out of their children's clothing. Land, then, is not merely soil it is a fountain of energy flowing through a circuit of soils, plants, and animals.

My zoo gardner distance