Soil Profile, Structure, and Properties: A Comprehensive Guide, Study notes of Analytical Chemistry

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Soil profile, Structure, and properties;

Determination of soil properties;

Fertility of the soil

Soil profile

Soil is that part of the earth's crust in which the roots of plants

grow. It is a vital, living component of the environment–a

component that can be manipulated to affect crop performance.

When soil is misused, crops become less productive; when

handled with due consideration for its biological and physical

nature, it can continue to yield crops throughout countless

generations of cultivation and use.

i. Support growth of higher plants

ii. Primary factor controlling fate of water in hydrologic system

iii. Nature’s recycling system

iv. Habitat for organisms

v. Engineering medium

Soil has 3 primary functions in sustaining plant life:

1. It supplies mineral elements, serving both as a medium of

exchange and as a place of storage.

2. It supplies water and serves as a storage reservoir.

3. It serves as a medium within which the roots of terrestrial

plants, as well as those of many aquatic plants, attach

themselves.

Functions of soil

Soil horizons

• Soils consist of one or more distinct layers called horizons.

If a vertical cut is made in a soil, several layers, or horizons, can be

viewed.

• The differentiation of horizons is the result of many soil-forming

processes. Most importantly:

(1) physical breakdown of particles,

(2) leaching of salts that are more or less soluble,

(3) accumulation of organic matter,

(4) chemical weathering of primary minerals and the formation of

silicate clay minerals

(5) translocation of silicate clay minerals from one horizon to another

by percolating water, and

(6) accumulation of some iron colloids.

Some of these processes take place in all the soils, but the number of

active processes and the degree of their activity vary from one soil

to another.

E: Horizon characterized by eluviations (removal of materials

such as silicate clay, iron, aluminum, or organic matter), if

distinct from the A horizon. Frequently not present (usually not

present in grassland/agricultural soils). Usually more pale

colored than the A horizon.

B: This is dominated by destruction of all or much of the original

rock structure and which shows evidence of soil formation such

as illuvial (moved down from an above horizon) concentration of

silicate clay, iron, aluminum, humus, carbonates, gypsum, or

silica; development of soil color or structure; or brittleness, etc.

Material from the A (or E) horizon, such as clay and carbonates,

leach downward and accumulate in the B horizon.

C: Horizons or layers, excluding hard bedrock, which is little,

affected by pedogenic (soil forming) processes and that lack

properties of O, A, E or B horizons.

R: Hard bed rock

Soil profile

Soil consists of four major components i.e., inorganic or mineral materials, organic matter, soil water and soil air. Soil water and soil air are held in pore space In a representative loam surface soil, the solid mineral particles comprise about 45 % of the soil volume and organic matter 5 %. At optimum moisture for plant growth, the pore space is divided roughly in half, 25 %, of volume being water space and 25 % air.

Soil components

b) Organic Matter ( 0 – 10 %) i. Flora and fauna ii. Live and dead (organic matter) iii. Macroscopic and microscopic It comprises disintegrated and decomposed plant and animal residues and other organic compounds synthesized by soil microbes as the decay occurs. It is a transitory soil constituent as it is continuously broken down by soil organisms and lasts from few hours to several hundred years. It requires maintenance by regular addition to the soil of plant and/ or animal residues. Organic matter content varies from 1. 0 to 6. 0 % by weight in top soil and very less in sub soil. In respect of soil productivity organic matter plays an indispensable role.

Soil components

c) Pore space ~ 50 %

Pore space consists of the “empty” spaces in the soil. The pore

space is a very important part of the soil. Pore space might be

filled with one of two things:

i. Air

ii. Water

Soil components

Soil Air The content and composition of soil air are determined largely by the water content of the soil, since the air occupies those soil pores not filled with water. Soil air always differs from atmosphere air in composition because of moisture content, root and microbial activities.  Relative humidity may approach 100 % at optimum soil moisture  CO 2 content is often several hundred times higher than 0. 033 %  Oxygen content usually < 20 %. In cases of low soil air, the diffusion rate of air in to and out of the soil would be slow and this leads to unsatisfactory conditions for optimum plant growth.

Soil components

Physical properties of soil

1. Colour Colour gives a ready clue to soil conditions and some important properties. It is either due to mineral or organic matter and mostly to both. Red, yellow or brown colors are usually related to the different degrees of oxidation, hydration and diffusion of iron oxides in the soil. Dark colors of a soil are due to: Poor drainage conditions, content and state of decomposition of organic matter, the presence of titaniferous magnetite etc. Uniformity in nomenclature of colors is possible by comparing the soils with charts containing standard colours. One such chart is the Munsell Colour Chart. It consists of coloured chips arranged according to hue, value and chroma, the three simple variables that combine to give the colour. The hue refers to the dominant spectral colour, the value to relative lightness of colour and chroma to relative purity.

2. Density Soils having larger particles are usually heavier in weight per unit volume than those having smaller particles. True density of a soil is based on the individual densities of soil constituents and according to their proportionate contribution. The bulk density or apparent density is the weight per unit volume of dry soil as a whole i.e. particle and pore space and hence it is lower than the true density. The relationship between the true density (T) and the apparent density (A) and the pore space (P) is as follows: P % = (T-A) x 100 /T In most mineral soils the true density varies within narrow limits of about

2. 5 to 2. 7 and the apparent density between 1. 4 and 1. 8.

3. Pore space The pore space of soil is the portion occupied by air and water and it is determined largely by structural conditions. Sands have low pore space of about 30 %, whereas clays may have as much as 50 - 60 %. Although clays possess greater total porosity than the sands the pore spaces in the latter being individually larger are more conducive to good drainage and aeration. 4. Plasticity and cohesion Plasticity is the property that enables a moist soil to change shape on the application of force and retain this shape even when the force is withdrawn. On this basis, sandy soils may be considered to be non- plastic and clayey soils to be plastic. Cohesion is the tendency of the particles to stick to one another. Plastic soils are cohesive. Plasticity and cohesion reflect the soil consistency and workability of the soils.

Soil profile

Structure, and properties;

Determination of soil properties;

Fertility of the soil

Soil Structure

• Soil Structure may be defined as ‘the arrangement of

primary particles (sand, silt and clay), secondary

particles (aggregates) and voids (pores) in to a certain

definite pattern under field conditions’.

• Soil Structure denotes:
• a) the size, shape and arrangement of particles and

aggregates;

• b) the size, shape and arrangement of the voids or spaces

separating the particles and aggregates;

• c) the combination of voids and aggregates in to various

types of structures.