![]() The high moisture holding-capacity of organic matter, its potential for retaining nutrients, and its ability to maintain a high microbial community makes soils with moderately high organic content very conducive to growing crops. In temperate climates, an organic layer can easily form a thick, dark, organic horizon at the surface of the soil. Notably, the parent material is immediately below this horizon, making it a relatively shallow, poorly formed (or new) soil. The 'A' horizon, denoted in the adjacent figure, is an example of a high organic soil horizon. The organic content of soils is therefore as important as the inorganic content. In addition the organic matter can greatly improve the water holding properties of the soil. These living things are dependent on the dead organic matter for some of the nutrients they require to survive and grow. ![]() Some organic matter will be incorporated deeper into a soil over time through the action of earthworms, roots, animals, and water.Īs well as dead organic material, soil will contain living organisms and plant roots. decaying plant and animal material, is incorporated into the soil from the surface, and can form a very obvious layer, horizon, within the soil. So far we’ve only considered the mineral (i.e., inorganic) content of soils, although we have noted that biological processes influence soil formation and that soil provides a medium for plant growth. Loam soils are well suited for most types of agriculture, as they provide a balance of good water retention along with the capacity to drain and not get water logged. The range of soil textures can be used as a quick means of assessing possible land uses, for example, a soil that is very high in sand would be excellent for a dune habitat, but retain insufficient moisture for many types of vegetation. A high clay soil might be a good place to site a reservoir because of its low water permeability, as well as a possible source of raw materials for the pottery and brick industry. This classification system uses the term loam to describe a particular combination of the three components. As can be seen in the soil triangle below, various combinations of sand, silt and clay have been given names. The combination of different percentages of sand, silt and clay particles is known as the soil texture. Soils are rarely made up of particles of a uniform size, but instead contain a mixture of particle sizes. Most are familiar with the size of sand particles, but by way of comparison, silt particles are roughly the size of a typical cell in our body or the thickness of a piece of paper, and clay particles are typically the size of micro-organisms - so extremely small! Silt particles are the next smallest particle, typically ~2 to ~50 micrometres and clay particles are all those particles that are smaller than 2 micrometres. The largest particles are sand which range from ~50 to 2000 micrometres (1000 micrometres = 1 millimetre). The size of the mineral particles is a key soil property and provides the basis for classification of soils. The adjacent figure shows how the soil transitions from parent material at the base to well weathered soil at the surface. These layers within a soil profile are termed horizons. In a soil derived from the underlying parent material, a vertical profile down into the ground will reveal this transition from bedrock through unconsolidated rock containing coarse material and stones, to a well weathered soil. As this freeze-thaw cycle continues, the rock is fractured into smaller and smaller pieces. Of particular note is the freeze-thaw process in which water contained in fissures in the rock freezes and expands causing further fissures to develop. Water plays a major role in this process of soil formation through mechanical fracturing, the dissolution of bedrock, and by supporting biological activity. As bedrock varies greatly from place to place, so do the soils derived from it. ![]() The mineral content of a soil is derived from a parent material, generally bedrock, that has been modified through time through the action of physical, chemical and biological processes. Soil is therefore vital to life on Earth. Many processes take place in soils including the recycling of nutrients, purification of water, and exchange of gases with the atmosphere. It provides a medium in which plants grow, a habitat for animals, and storage for water. Soil is a mixture of mineral and organic matter that contains air, water, and micro-organisms.
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