Importance of soil structure:
Structure is very important in plant growth relationship as it chiefly influences the amount and nature of porosity and regulates the moisture air regime in the soil. The best structures are mechanically stable and strong but when they absorb moisture and are wet they become soft and loose their shape and size. Soils high in water table aggregates are fewer bends to puddle. Structure is one of the properties, which is easily liable to change under different management practices such as ploughing, draining, liming, fertilizing and manuring. Addition of organic matter and its proper decomposition are important for building up and maintenance of soil structure. Grasses are most effective in promoting granulation.
The arrangement of primary soil particles in the compound particles of clusters, that are separated from adjoining aggregates and have properties unlike those of an equal mass of unaggregated primary soil particles. Aggregates are secondary units or granules composed of many soil particles bound or cemented together by organic substance iron oxides and carbonates, Natural aggregates are called peds and vary in their water stability.
Fragment consists pieces of broken ped. Concretion is formed within the soil by the precipitation of salts dissolved in percolating water. Structure is best studied in the field under natural condition and it is described under three categories.
¶ Type: Shape and forms and arangement pattern of ped.
· Class: Size of peds.
¸ Grade: Degree of distinctness of ped.
Types of structures
Following are the principle geometric forms of soil structure.
1. Platee or platelike:
The horizontal dimensions are much developed than the vartical giving a flattened, compressed or lens like appearance to the peds. When the units are thick then they are called platy and when the units are thin they are called laminar plate often overlap and impair permeability. Generally this type of structure is found in forest and clay pan soils.
2. Block or Blocklike: (Blocky)
All 3 dimensions are of about the same size and the peds are cube like with flat or rounded faces. When the faces and edges are mainly rounded it is called subangular blocky found in B-horizon. Either some of the block like structure were called nutty or nuciform.
In case of spheriodal structures, the aggregates are rounded but when curved or irregular and less porous the structure is called granular. Granular structures are not fitted to adjoining aggregates. It is found in horizon A.
It is pillar like ped, which is rounded at the top structure, is known as columnar. It is laterally bounded by columnar aggregates, which forms the cost for the peds.
5. Prism like or prismatic:
The vertical axis is more developed than other with flattened sides giving a pillar like shape, when the top of such ped is rounded the structure is termed as columnar and when flat it is termed as prismatic. Prismatic aggregates form the cast for the peds. Some prismatic aggregates break into smaller blocky peds.
6. Crumb structure:
Spheroidal structure, the aggregates are rounded and relatively porous, it is called crumb structure. Small and spheroidal peds not filled to adjoining aggregates. Amongst all kind of structures described above the crumb structure is the most ideal for crops. It is closely related to various soil properties like
i. Good drainage, aeration and removal of excess water.
ii. Spread of plant roots
iii. Resistant to erosion etc.
Classes of Structure
There are following classes based on their size they are:
1) Very fine or very thin (2) Fine or thin (3) Medium (4) Coarse or thick
Grades of structure
Grade indicates the degree of distinctness of the individual peds. Four terms commonly used to describe the grade of the soil structure are:
1. Structureless: There are no noticeable peds such as conditions exhibited by loose sand or cement like conditions of some clay soils.
2. Weak: Instinct formation of peds, which are barely durable.
3. Moderate: Moderately well developed peds, which are fairly durable and distinct.
4. Strong: Strong well developed peds are durable and distinct.