Fundamentals of Soil Science (1+1)

Grades of Soil Structure

Grades of Soil Structure

• The grade of structure is the degree of aggregation, expressing the differential between cohesion within aggregates and adhesion between aggregates. In other words, it indicates the degree of distinctness of the individual peds.
• Grade of structure should be determined when the soil is neither unusually moist nor unusually dry.
• There are four major grades of structure rated from 0 to 3 as follows:
• Structure-less (0): It represents the conditions in which there is no observable aggregation or no definite orderly arrangement of natural lines of weakness, such as:
• Massive structure (coherent): where the entire soil horizon appears cemented in one great mass;
• Single-grain structure (non-coherent): where the individual soil particles show no tendency to cling together such as pure sand.
• Weak structure (1): These structures are poorly formed from indistinct aggregates. When removed from the profile, the soil material breaks down into a mixture of very few entire aggregates, many broken aggregates and much un-aggregated material.
• Moderate structure (2): The structures are well formed from distinct aggregates, moderately durable and evident but not distinct in undisturbed soil. When removed from the profile, the soil material breaks down into a mixture of many distinct entire aggregates, some broken aggregates and little un-aggregated material;
• Strong structure (3): These are well formed from distinct aggregates that are durable and quite evident in undisturbed soil. When removed from the profile, the soil material consists very largely of entire aggregates and includes few broken ones and little or no non-aggregated material.

Factors Influencing Genesis of Soil Structure:

• The genesis of soil structure refers to bonding of soil particles into structural units or 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 colloids.
• The cementing materials taking part in aggregate formation are colloidal clay, iron and aluminium hydroxides and decomposing organic matter.

1. Climate:

• Climate has considerable influence on the degree of aggregation as well as on the type of structure. In arid regions there is very little aggregation of primary particles.
• In semi arid regions, the degree of aggregation is greater.

2. Wetting and drying:

• It is a well known fact that alternate wetting and drying produce aggregation. When a dry soil is wetted, the soil colloids swell on absorbing water.
• On drying, shrinkage produces strains in the soil mass gives rise to cracks, which break it up into clods and granules of various sizes.
• The breaking of aggregates is due to unequal swelling and compression of the entrapped air.

3. Organic matter:

• Organic matter improves the structure of a sandy soil as well as of a clay soil. In case of a sandy soil, the decomposing organic matter and the associated microorganism cement the sand particles together to form aggregates.

4. Tillage:

• Cultivation implements break down the large clods into smaller fragments and aggregates.
• For obtaining good granular and crumby structure, optimum moisture content in the soil is necessary. If the moisture content is too high, it will form large clods on drying.
• If it is too low, some of the existing aggregates will be broken down.

5. Plants, Roots and Residues:

• Excretion of gelatinous organic compounds and exudates from roots serve as binding agents. Root hairs make soil particles to cling together. In grassland soils more granulation is observed. Pressure exerted by the roots hold the particles together.
• Plant tops and residues – shade the soil – prevent it from extreme and sudden temperature and moisture changes and also from rain drop impedance. Plant residues – serve as a food to microbes which are the prime aggregate builders.

6. Microbes:

• Growth of micro-organisms is responsible for binding together of soil particles and thus increases the soil aggregation.
• Algae, fungi and actinomycetes keep the soil particles together by the products of decomposition. Mycelial growth of fungi bind the particles together.

7. Animals:

• Among the soil fauna small animals like earthworms, moles and insects etc., that burrow in the soil are the chief agents that take part in the aggregation of finer particles.