If it is too low, some of the existing aggregates will be broken down. If the moisture content is too high it will form large clods on drying. For obtaining good granular and crumby structure, an optimum moisture content in the soil is necessary. This helps making clay more crumby.Ĭultivation implements break down of large clods into smaller fragments and aggregates. In the case of clayey soil, it modifies the properties of clay by reducing its cohesive power. In a case of sandy soil, the sticky and slimy material produced by the decomposing organic matter and the associated microorganism cement the sand particles to form aggregates. Organic matter improves the structure of a sandy soil as well as of a clay soil. In semi- arid regions, the degree of aggregation is greater than arid regions. In arid region, there is very little aggregation of primary particles. The development of structure in arable soil depends on the following factors:Ĭlimate has considerable influence on the degree of aggregation as well as 011 the type of structure. In normal soil, calcium is the predominant cation and forms calcium humate in combination with humus. It seems that humus absorbs both cations and anions. Some fungi and bacteria taking part in the decomposition have also been found to have a cementing effect.Īnother view of structure formation is that clay particles adsorbed by humus forming a clay-humus complex. During decomposition of organic matter, humic acid and other sticky materials are produced which helps to form aggregate. Organic matter plays an important part in forming soil aggregates. These are also responsible for forming aggregates by cementing sand and silt particles. Iron and aluminium hydroxides act as cementing agent is binding the soil particles together. So also clay minerals that have high base exchange capacity form aggregate more readily than those which have a low base exchange capacity. Clay particles smaller than 0.001 mm aggregate very readily. The greater the amount of clay in a soil, the greater is the tendency to form aggregates. The amount and nature of colloidal clay influence the formation of aggregates. Sand and silt particles cannot form aggregates as they do not possess the power of adhesion and cohesion. The mineral Colloids (colloidal clay) by virtue of their properties of adhesion and cohesion, stick together to form aggregates. In aggregate formation a number of primary particles such as sand, silt and clay are brought together by the cementing or binding effect of soil colloidal clay, iron and aluminium hydroxides and organic matter. The mechanism of structure (aggregate) formation is quite complex. For example, large prismatic types may break into medium blocky structure, constitute the compound structure. Often compound structures are met within the soil under natural conditions. For naming a soil structure the sequence followed is grade, class and type for example, strong coarse angular blocky (soil structure). Very well-formed peds which are quite durable and distinct. Moderately well-developed peds which are fairly distinct. Indistinct formation of peds which are not durable. There are no noticeable peds, such as conditions exhibited by loose sand or a cement-like condition of some clay soils. Grade also depends on organic matter, texture etc.įour terms commonly used to describe the grade of soil structure are: Grade of structure is influenced by the moisture content of the soil. It is determined by the stability of the aggregates. Grades indicate the degree of distinctness of the individual peds. The terms thin and thick are used for platy types, while the terms fine and coarse are used for other structural types. The terms commonly used for the size classes are: The aggregates of this group are usually termed as granular which are relatively less porous when the granules are very porous, the term used is crumby.Įach primary structural type of soil is differentiated into 5 size-classes depending upon the size of the individual peds. Therefore in sphere-like structure infiltration, percolation and aeration are not affected by wetting of soil. When wetted, the intervening spaces generally are not closed so readily by swelling as may be the case with a blocky structural condition. Those rounded complexes usually lie loosely and separately. All rounded aggregates (peds) may be placed in this category, although the term more properly refers to those not over 0.5 inch in diameter.
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