Introduction:

Soil classification system, called soil taxonomy, gives the hierarchical grouping of natural soil bodies. Classification based on soil properties reduces the probability of controversy over the categorization of the given soil that can occur when scientists deal with systems based on presumed mechanism of soil formation.

Bases of Soil Classification:

Soil taxonomy based on visible properties of soils as they are found today. Such properties for instance, comprise moisture, soil color, temperature status, texture and structure of soil. Chemical and mineralogical properties, like the contents of clay, organic matter, iron and aluminum oxides, salts, silicate clays, the pH, the percentage base saturation (cation exchange capacity), and soil depth are other significant criteria for categorization. Precise measurements are also used in defining soil horizons, the presence or absence of help to find out the place of the soil in classification system.

Diagnostic Surface Horizons:

Epipedon comprises upper part of the soil darkened by organic matter, the upper eluvial horizons, or both. Seven epipedons are recognized, but only five (comollic, umbric, ochric, melanic, histic) take place naturally over wide areas. Other two, anthropic and plaggen, are the result of intensive human use. They are common in parts of Europe and Asia where soils have been utilized for several centuries.

The mollicepipedon (Latin mollis, soft) is the mineral surface horizon noted for its dark color because of accumulated organic matter. Umbricepipedon (Latin umbra, shade; hence, dark) has same general features as the mollicepipedon except that cation exchange capacity is less than 50%. Umbricepipedon commonly grows in areas with fairly higher rainfall and where parent material has lower content of calcium and magnesium.

Ochricepipedon (Greek ochris, pale) is the mineral horizon which is too thin, too light in color, or too low in organic matter to be either a mollic or umbric horizon. It is generally not as deep as mollic or umbricepipedon.

Melanicepipedon (Greek melas, black) is the mineral horizon which is very black in color because of its high organic matter content (organic carbon > 6%), it is features of soils developed from volcanic ash. And histicepipedon (Greek histos, tissue), a 20- to 60-cm-thick layer of organic solid materials overlaying the mineral soil which is formed in wet areas, is the layer of peat or muck with a black to dark brown color and very low in density.

Subsurface Horizons:

Several subsurface horizons (18) are utilized to characterize different soils in soil taxonomy. Each horizon gives the characteristic which assists place a soil in its proper class in system. For instance argillic horizon is the subsurface accumulation of high-activity silicate clays which have moved downward from upper horizons or have formed in place. The natric horizon similarly has silicate clay accumulation, but clays are accompanied by more than 15% exchangeable sodium on colloidal complex and by columnar or prismatic soil structural units.

Kandic horizon has the accumulation of Fe and Al oxides and low activity silicate clays. The oxic horizon is the highly weathered subsurface horizon which is very high in Fe and Al oxides, and in low-activity silicate clays. Spodic horizon is the alluvial horizon which is characterized by accumulation of colloidal organic matter and aluminum oxide (with or without iron oxide).

The sombic horizon is also the illuvial horizon, dark in color due to high organic matter accumulation. It has a low degree of cation exchange capacity. Albic horizon is the light-colored eluvial horizon which is low in clay and oxides of Fe and Al.

Calcic horizons include the addition of carbonates (CaCo₃) which frequently appear as white chalklike nodules. Gypsic horizons have the accumulation of gypsum (CaSQ₄.2H₂O), and salic horizons an accumulation of soluble salts. These are found generally in soils of arid and semiarid regions. In few subsurface horizons, materials are cemented or compactly packed, resulting in relatively impermeable layers known as pans (duripan, fragipan and placic horizons). These can resist water movement and penetration of plant roots. These pans hold back plant growth and may support water runoff and erosion as rainwater can't move readily downward through the soil.

Classification Based on Soil Moisture Regime:

The soil moisture regime refers to presence or absence of either water saturated conditions (generally groundwater) or plant-available soil water. Soil moisture regime classes are utilized to characterize soils.

• Aquic - Soil is saturated with water and almost free of gaseous oxygen.
• Udic - Soil moisture is adequately high year-round in most years to meet plant requirements.
• Ustic - Soil moisture is intermediate between udic and aridic regimes.
• Aridic - Soil is dry for at least half of growing season and moist for less than 90 consecutive days. This regime is feature of arid regimes.
• Xeric - This soil moisture regime is found in typical Mediterranean type climates.

Classification Based on Soil Temperature Regime:

Soil temperature regimes, like frigid, mesic, and thermic, are utilized to categorize soils at some of the lower levels in soil taxonomy.

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