Soil forms continuously, but slowly, from the gradual breakdown of rocks through weathering. Weathering can be a physical, chemical or biological process:.
The accumulation of material through the action of water, wind and gravity also contributes to soil formation. These processes can be very slow, taking many tens of thousands of years. Five main interacting factors affect the formation of soil:. Soil minerals form the basis of soil. They are produced from rocks parent material through the processes of weathering and natural erosion. Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all help break down parent material.
The types of parent materials and the conditions under which they break down will influence the properties of the soil formed. For example, soils formed from granite are often sandy and infertile whereas basalt under moist conditions breaks down to form fertile, clay soils.
Soil formation is influenced by organisms such as plants , micro-organisms such as bacteria or fungi , burrowing insects, animals and humans. As soil forms, plants begin to grow in it. The plants mature, die and new ones take their place. Their leaves and roots are added to the soil. Animals eat plants and their wastes and eventually their bodies are added to the soil. This begins to change the soil. Bacteria, fungi, worms and other burrowers break down plant litter and animal wastes and remains, to eventually become organic matter.
This may take the form of peat, humus or charcoal. Temperature affects the rate of weathering and organic decomposition. With a colder and drier climate, these processes can be slow but, with heat and moisture, they are relatively rapid.
Rainfall dissolves some of the soil materials and holds others in suspension. The water carries or leaches these materials down through the soil. Clays have a large surface area, which is important for soil chemistry and water-holding capacity. The texture of a soil is based on the percentage of sand, silt, and clay found in that soil.
The identification of sand, silt, and clay are made based on size. The following is used in the United States:.
Sand 0. Figure 2. The U. Department of Agriculture Soil Texture Triangle is used to determine the overall texture of soil based on the percentage of sand, silt, and clay. The texture of a soil can be determined from its sand, silt, and clay content using a textural triangle. The triangle above Figure 2 is the one created by the U. Percent clay in this triangle is read on the lefthand side of the triangle, the percent silt is read on the righthand side, and the percent sand is on the bottom.
Water Water is the second basic component of soil. Water is important for transporting nutrients to growing plants and soil organisms and for facilitating both biological and chemical decomposition. Soil water availability is the capacity of a particular soil to hold water that is available for plant use. The capacity of a soil to hold water is largely dependent on soil texture. Any type of soil consists of three particle sizes namely sand, clay, and silt even though in some rare cases a soil sample may have only one type of particle.
Among these three particles, the largest particles are sand while clay particles are the smallest. The texture of the soil will be determined by the ratio of these three particles. The organic matter of soil includes microbial material, animals, and plants, which can be either dead or alive. The organic matter also includes a small part of living cells that work to disintegrate dead organic matter, which is primarily from dead trees and animals. Soil structure refers to the organization of soil particles into lumps known as pedoliths or peds.
Depending on what is contained in the peds, their shapes vary. Other conditions that affect the shapes of the peds include the conditions present during their formation and human activity. The peds have shapes resembling balls, columns, plates, or even blocks.
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