SOiL cONTAINS PuLVERizED rock, organic matter, and microorganisms, which convert the organic matter to humus. Soils range from loosely-packed sandy soil, to finely packed, sticky clay soil. Between these two extremes is loam, whose high content of organic matter makes it easy to cultivate. Soils are thin in the Mediterranean basin, and thick in northern Europe, Russia, and the American Midwest and Mississippi delta. As the basis of agriculture and as a factor in the formation of climate, soils have shaped the destiny of humans.

Soils warm when absorbing sunlight, and cool when reflecting sunlight. Soil temperature coincides with an area's climate. In tundra, for example, the air warms in spring and summer much faster than the soil does. Only the outermost layer of the soil warms enough to thaw, and then for only a few months, before winter returns. Not all soils absorb and reflect sunlight at the same rate. Wet soil darkens, and absorbs the most sunlight and gains the most heat. Dark soil absorbs as much as 86 percent of sunlight, gray soil absorbs 80 percent, and light soil absorbs only 20 percent of sunlight, reflecting the rest into the atmosphere. When water saturates

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soil, however, its color lightens. Being light, saturated soil reflects sunlight, yet the large amount of water that soil can hold increases the capacity of soil to absorb heat. Saturated soil absorbs and radiates heat slowly and in large quantities. Water evaporating from soil liberates heat, warming the surrounding air. Dry soil reflects more sunlight and heat than wet soil. Yet dry soil, because it contains little moisture, warms quickly on a cloudless day and becomes hot. At night, dry soil quickly cools, radiating heat back into the atmosphere.

The term albedo is the ratio of the amount of sunlight an object reflects to the amount it absorbs. A solid black object absorbs all sunlight and reflects none, and so has an albedo of zero. A white object absorbs no sunlight, reflects all of it, and has an albedo of one. The mean albedo of earth is 0.36. Dark soil has an albedo between 0.1 and 0.2. Clay soil has an albedo between 0.15 and 0.35. Sandy soil has an albedo between 0.25 and 0.45, and light soil has an albedo between 0.4 and 0.5.

At most sunlight penetrates only a few feet of soil. Infrared light, for example, penetrates to a depth of 3 ft (.9 m.). Because the amount of sunlight that soil receives varies during the day, soil temperatures vary. Soil temperatures also vary throughout the year. In Bridgewa-ter, Massachusetts, for example, soil at a depth of one inch varied from a low of 35.2 degrees F (1.7 degrees C) in February to a high of 63.5 degrees F (17 degrees C) in August. Soil temperature therefore varies less than the temperature of air. Sunlight penetrates more deeply in rocky and wet sandy soil than in wet clay. Sunlight penetrates least in dry sandy soil.

Vegetation affects the capacity of soils to absorb sunlight. Plant cover decreases the albedo of light soil and increases the albedo of dark soil. Dense foliage blocks sunlight from reaching the soil, and so lessen the soil's absorption of sunlight and heat. Before the evolution of plants, soils must have been, along with the oceans, the major reservoirs of heat by absorbing sunlight. Like water, soil absorbs the most heat when the sun is overhead. Soil reflects increasing amounts of light as dusk approaches. Along with the oceans, the soil shapes the climate and influences the course of life.

SEE ALSo: Albedo; Climate; Plants.

BIBLioGRAPHY. Endre Dobos, "Albedo," http:kyclim.wku. edu/bolder/Lawrence&Chase.ppt; D.F. Post, A. Fimbres, A.D. Matthias, E.E. Sano, L. Accioly, A.K. Batchily and L.G.

Ferreira, "Predicting Soil Albedo from Soil Color and Spectral Reflectance Data,"

Christopher Cumo Independent Scholar

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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