Soil is a relatively thin layer of unconsolidated mineral and organic material on the immediate surface of the earth. Fertile soil contains approximately 25% of both air and water, about 5% organic matter and about 45% mineral matter. It is important to understand something about how soils are formed to determine the best use of available soils and how to manage their fertility.
In general, soil formation starts with rocks that are pushed to the surface of the earth by geological or climactic forces. These rocks then undergo weathering – the chemical alteration and physical breakdown of rock during exposure to the atmosphere, hydrosphere, and biosphere. Through the weathering process, eventually enough essential elements become available to support lichens and other lower forms of plant life. As continuing generations of lichens grow, die, and decay, they leave increasing amounts of organic matter. Naturally-occurring organic acids further hasten decay of the rock. An increasing build-up of organic matter and formation of fine rock fragments result in more water retention in the soil and more water available for use by larger numbers of plants and animals.
Four factors determine what type of soils are formed. These are Climate, Organisms, Topography and Parent Material.
- Climate has two major components for soil formation. The first is the temperature. As the mean annual soil temperature increases, the weathering of the rocks and minerals in the soil will be faster. Along with temperature is the climate factor of precipitation or rainfall. In general, areas with more rainfall will have greater weathering and greater leaching.
- Organisms include animals living in the soil that contribute to soil development by their mixing activities. The mixing of the soil by organisms is called bioturbation. Humans also influence the soil with their activities of agriculture, urbanization, grazing, and forestry.
- Topography as a soil forming factor is related to the soil’s position on the landscape elevation, direction and depth to the water table. Topography will have a great deal to do with the soils character as different topographic locations vary in respect to water runoff, erosion, leaching and temperature.
- Parent material refers to the primary material from which the soil is formed. The type of soil that forms depends on the type of rocks available, the minerals in rocks, and how minerals react to temperature, pressure, and erosive forces. Soil parent material could be bedrock, organic material, an old soil surface, or a deposit from water, wind, glaciers, volcanoes, or material moving down a slope.
- The length of time required for a soil to form depends on the intensity of the other active soil forming factors of climate and organisms, and how topography and parent material modify their affect.
Each of the world’s soils is assigned to one of twelve taxonomic soil orders, largely on the basis of soil properties that result from the five soil forming factors acting on the parent material over time.