Soil And Minerals

Soil consists of both inorganic and organic materials. The inorganic material is mostly rock particles broken off from larger rocks by the action of water and wind-a process known as weathering. Alternate freezing and thawing of water helps to crack the rock and break off pieces. Soluble minerals in the rock dissolve in water breaking the rock still further. Lichens and other soil forming organisms also act to break down rock. When these organisms die, their remains are intermingled with the rock particles, thus adding organic matter to the developing soil. Plants may take root in the thin soil and when they die, their remains add more organic matter.

The minerals present in soil depend partly on the type of rock from which the soil was formed and partly on the types of organisms living in the soil. The amount of precipitation determines the extent to which minerals will be retained in, or washed out of the soil.

As soil development proceeds, three distinct layers form (see figure). The uppermost layer, called topsoil, includes organic matter and living organisms. Plant "litter", such as fallen leaves and twigs, overlies the topsoil and gradually blends into it. The dark, rich organic matter in the topsoil is called humus. It is formed from the decay of dead plants and animals. The living organisms of the topsoil include plant roots, as well as earthworms, insects, and many other animals and protists. The organisms of decay-bacteria and fungi-are also found in this layer.

Beneath the topsoil is a layer of subsoil. The subsoil consists of rock particles mixed with inorganic compounds, including mineral nutrients. Water-soluble materials from the topsoil are constantlycarried downward into the subsoil by the downward movement of water. The bottommost layer of the soil consists of bits of rock broken off from the parent bedrock below.

There are many types of soils. They are classified accroding to their organic content, pH, and the size of the rock particles. Sandy soil has the largest particles, silt has particles of intermediate size, and clay is made up of very small particles. Water drains too quickly through sand and too slowly through clay. In general, the best soils for plants to grow in consist of a mixture of clay and larger particles. However, diferent types of plants grow best in different types of soil. Some plants require well-drained, sandy soils, while others do best in clayey soil. Also some plants thrive in acid soils but others need alkaline soils

Soil and Minerals	Biotic Organization	Authotrophic and Heterotrphic Nutrition	The Nitrogen Cycle
Temperature	Competition In Ecosystems	Symbiotic Relationships	The Carbon and Oxygen Cycles
Water		Producers, Composers and Decomposers	The Water Cycle
		Food Chains and Food Webs