Soil Property- Soil Structure
The primary particles – sand, silt and clay are held together in clusters or peds of various shapes and sizes. Individual soil particles are joined together into groups or clusters by cementing agents just as bricks with cement or lime mortar to make buildings or various sizes and shapes, called as soil aggregates or peds. Natural aggregates are called as peds and artificial/aggregates by cultivation are called as clods.
The arrangement of primary particles and their aggregates (secondary) into certain pattern in the soil mass, called as Soil Structure. Soil structure influences the soil environment through its effect on the amount and size of pore spaces, water holding capacity, availability of plant nutrients and growth of micro-organisms. The size, shape and arrangement of the soil aggregates give indication of the ability of the soil to:
1. Allow air and water movements through the soil.2. Allow plant roots to move through soil and make use of soil and
3. Hold enough soil moisture in a form available for plants use.
Types of soil structure:
There are four types on the basis of shape and arrangements:
1. Plate like/Platy: Horizontally, layered, thin and flat like the plates with horizontal dimensions greater than the vertical ones.
2. Prism like: Aggregates are elongated like pillars or prism, often six sided, up to 15 cm dia. They have vertical axis greater that oriental and the length of elongated pillars varies, depending upon soil and may go up to 15 cm or more and commonly found in sub soil horizon of arid and semi-arid region soils. Further divided as: with flat tops, called as prismatic and with rounded tops, called as Columnar aggregates.
3. Blocky like: These are cubes like 3 dimensions of about same size. When the edges or size are sharp, called as Angular blocky and when rounded, called as Sub-angular blocky. These usually found in the sub-soil horizon.
4. Spheroidal like: The aggregates are rounded or like a sphere. All the axes are approximately of the same dimensions, with curved or irregular faces, not more than 1 cm dia.
Further divided into:
I) Crumb: The aggregates are small and are weakly held together and are porous like crumbs of breads, found in pasture soils or grassy lands.
II) Granular: Similar to crumb except that the aggregates are harder, less porous and the individual soil particles are more strongly held together than in the crumb structure. Commonly found in cultivated fields.
Classes of structure: Aggregates/peds classified on the basis of their sizes as: Very fine, Fine, Medium, Coarse (or thick) and Very coarse (or very thick).
Grades of structure: Depending upon the stability, distinctness, durability, strength of the ease with which they can be separated, the aggregates are classified onto the four grades as: Structure less, Weak, Moderate and Strong.