Soil Structure: Types, Grades and Its Improvement

After reading this article you will learn about:- 1. Types of Soil Structure 2. Classes of Soil Structure 3. Grades 4. Improvement 5. Importance.

Types of Soil Structure:

The type of soil structure refers to the general shape of peds. The general shape of the peds is as described below:

(a) Platy Structure:

The horizontal azes of peds are much larger than their Vertical axis. This means that the peds are in the form of thin plates.

(b) Prismatic Structure:

The vertical axes of peds are larger than their horizontal azes. Tall peds are bounded by plane faces intersecting each other at sharp angles.

Prismatic structure has been subdivided into two sub types:

(i) Prismatic:

The top of the ped is flat. s

(ii) Columner:

The top of the ped is rounded.

(c) Blocky Structure:

Horizontal and vertical axes of peds are roughly equal, which means that the peds are roughly in the form of cubes.

Blocky structure has been subdivided into two types:

(i) Angular blocky:

Peds are bounded by plane faces intersecting each other at sharp angles.

(ii) Sub-angular blocky:

Peds are bounded by both plane faces intersect­ing each other at sharp angles and round faces. In the case of platy, prismatic and blocky structure peds are in close contact with each other so that there is less air space between them.

(d) Spheroidal structure:

The horizontal and vertical axes of peds are more or less equal; peds are not in close contact with each other, so there is plenty of air space between them because they are irregular in shape.

Spheroidal structure has been subdivided into the following two types:

(i) Granules are relatively less porous.

(ii) Crumbs are very porous.

Classes of Soil Structure:

This refers to the average size of ped as shown in Table 4.2 (Diameter in millimeters)

Grades of Soil Structure:

A grade of soil structure refers to the durability of peds and distinctness with which they can be observed in undisturbed soil, as described below:

(i) Structure-less:

No peds are observed in undisturbed soil.

(ii) Weak:

Poorly formed indistinct peds are found in undisturbed soil. When the soil structure is disturbed, it breaks down into a mixture of a few entire peds, many broken peds and a large amount of un-aggregated material.

(iii) Moderate:

Fairly well formed peds are found in undisturbed, soils that are not very distinct in undisturbed soil. When the soil structure is disturbed it breaks down into a mixture of many entire peds, some broken peds and a little un-aggregated material.

Strong:

Well-formed distinct peds are bounded by planes of weakness when the soil structure is disturbed, then it breaks down into almost entire peds, and a little broken peds and no un-aggregated material.

Formation of Soil Aggregates under Natural Conditions:

Water molecules are dipolar. One side of a water molecule is positively charged, while the opposite side is negatively charged. The negative end of one water molecule attracts the positive end of a second water molecule, the negative end of which attracts the positive and of third water molecule, and-so on. In this way, long chains of water molecules are formed in nature.

Positive cations are hydrated. They attract the negative ends of a chain of dipolar water molecules, the positive end of which are attracted by a clay particle. Hence clay particles are held by bridges of oriented water molecules and positive cations as shown in Fig. 4.3.

When the soil dries up, the chain of water molecule shortens more and more, so the clay particles are ultimately united. They are united together by some, organic compounds formed during the decomposition of soil organic matter.

Actually several clay particles are united in this way to form compound particles which, in turn, are bound together by cementing agents like clay, iron and aluminum oxides to form the soil aggregates.

Soil micro-organism decomposes soil organic matter to form dark, gummy substances which bind the clay and sand and silt particles to form the soil aggregate. Root hairs and fungal hyphae also bind the compound particles of the soil as well as sand and silt particles to form soil aggregates.

Destruction of Soil Structure under Natural Conditions:

Wet soil aggregate break down when the negative clay micelle is saturated with highly hydrated cations like sodium ions. Clay is deflocculated. This may be corrected by adding gypsum when calcium ions replace sodium ions.

Soil aggregates are rapidly wetted by heavy rains. The outer layer of the soil aggregates are detached from their main body. This process is repeated several times and the soil structure ultimately breaks down to mud.

The fast falling raindrops strike the soil aggregates with great force and break them up. Therefore the surface of the soil should be kept covered with organic matter. Ploughing a wet clayey soil also destroys soil structure.

Therefore, clayey soils should be ploughed to an intermediate soil moisture content, when they are dry enough to lose their stickiness yet moist enough not to form hard clods.

Improvement of Soil Structure:

Sandy soils should be worked with a little pressure when the soil pores are just filled with water. Consequently, smaller peds are united with each other to form bigger peds. Large clods of sandy soils are easily broken down to smaller ones.

The large clods of clayey soils are broken down when they are slowly wetted by a prolonged drizzle. Larger clods of clayey soils can also be broken down to smaller ones when water slowly freezes to form ice. Within the pore space and exert pressure on the moist soil, Large clods of clayey soils are broken down by repeated freezing and thawing.

Addition of lime to acidic soils improves their structure because lime stimulates the growth of micro-organisms which are probably responsible for improvement of soil structure. Addition of fertilizers to bare uncultivated soil reduces the stability of soil aggregates.

Addition of sodium nitrate cause maximum damage to the soil structure. Whereas addition of calcium nitrate-causes the least damage.

Addition of large doses of partially decomposed organic manures like far­myard manure or compost, or ploughing in of green manuring crops like sun- hemp, (crotalaria juncea) or Dhaincha, (Sesbania bispinosa) greatly improves soil structure in about three weeks. Lime increases the speed of decomposition of organic matter by micro-organisms, thus improving the soil structure.

The surface of the land should be kept covered with waste organic materials like grasses, leaves, straw etc., which are known as mulches. They keep the surface soil moist and cool, where micro-organisms multiply rapidly and improve soil structure. Mulches also protect the surface from the beating action of the raindrops.

Roots of grasses bind the soil particles to form the soil aggregates. It legumes like horse gram Kudzu etc. are grown with grasses the soil structure is further improved, because the micro-organisms get the additional nitrogen which has been fixed by the symbiotic bacteria living in the root nodules of the legumes.

Importance of Soil Structure:

A good soil structure is essential for the circulation of a proper proportion of air and water in the soil which, in turn, is essential for the growth and functioning and root system. A good soil structure is especially essential for certain crops like potato, sugar-beet maize etc. which need better soil aeration than other crops.