Components of Soil
Soil consists of four major components. They are:
(i) Mineral matter,
(ii) Organic matter,
(iii) water, and
Physically, soil consists of stones, large pebbles, dead plant twigs, roots, leaves and other parts of the plant, fine sand, silt, clay and humus derived from the decomposition of organic matter. In the organic matter portion of the soil, about half of the organic matter comprised of the dead remains of the soil life in all stages of decomposition and the remaining half of the organic matter in the soil is alive. The living part of the organic matter consists of plant roots, bacteria, earthworms, algae, fungi, nematodes actinomycetes and many other living organisms.
Soil contains about 50% solid space and 50% pore space. Mineral matter and organic matter occupy the total solid space of the soil by about 45% and 5% respectively. The total pore space of the soil is occupied and shared by air and water on roughly equal basis. The proportion of air and water will vary depending upon the weather and environmental factors.
(a) Soil mineral matter (SMM) – Size and composition of mineral matter in soils are variable due to nature of parent rock from which it has been derived. The rock fragments are disintegrated and broken portion of the massive rocks, from which regolith through weathering, the soil has been formed.
These materials are usually very coarse and the minerals are extremely variable in size. The primary minerals viz., quartz, biotite, muscovite (dominates coarse fractions of the soil) and the secondary minerals viz., silicate clays and hydrous oxides clays of iron and aluminium (as very fine fraction) are present.
(b) Soil organic matter (SOM) – Soil organic matter exists as partly decayed and partially resynthesized plant and animal residues. These are continuously being broken down as a result of microbial activity in soil. Due to constant change, it must be replenished to maintain soil productivity.
The organic matter content in a soil is very small and varies from only about 3–5% by weight in topsoil. In addition to partly decayed plant and animal residues, soil organic matter contains living and dead microbial cells, microbiologically synthesized compounds and derivatives.
• Organic matter is a storehouse of nutrients in soil. It is responsible to get the most desirable soil structure.
• It promotes greater proportion of large pore sizes, improves water holding capacity and aeration status of soil.
• It is a main source of N, 5-6% of P, and 80% of S. It also supplies different trace elements like boron, molybdenum etc.
• It acts as a chelate, due to chelate formation between organic matter and various metals; the availability of these metallic elements will be increased.
• It contributes to cation exchange capacity in soils.
• It reduces soil erosion; shades the soil and keeps the soil cooler.
(c) Soil water – Soil water plays a very significant role in soil-plant growth relationship. Water is held within the soil pores with varying degree of forces depending upon the amount of water present. With the increasing amount of water in soil, the forces of retention of water by the soil will be low and vice-versa.
The movement and retention of water in the soil is primarily influenced by the characteristics of the soil viz., texture, nature of inorganic and organic colloids, type and amount of exchangeable cations, size and total amount of pore spaces etc.
Water held by soil with high force of attraction is not available to the plants. Soil water along with dissolved salts makes up the soil solution. These soil solution acts as an important medium for supplying different nutrient elements through exchange phenomena between soil solid surface and soil solution and the plant roots.
(d) Soil air – Pore spaces in soil consist of that portion of the soil volume not occupied by soil solids, either mineral or organic. Under field condition, pore spaces are occupied by air and water; the more the water the less the room for air and vice-versa.
The relative amounts of air and water in the pore space fluctuate continuously. During rainy season, water replaces air from the soil pore spaces, but as soon as water leaves by downward movement, surface evaporation, and transpiration etc., air gradually replaces the water, as it is lost form the pore spaces. Soil air contains various gases like CO2, very small amounts of O2 and N etc.
Generally, soil air contains much more CO2 and small amount of O2 than that of atmospheric air due to microbial respiration when large amounts of CO2 releases into the air and O2 is taken up by soil microorganisms. Well-aggregated soil having large pore spaces offers less mechanical impedance to root developments and shoot emergence and do not form crusts easily. Good aeration occurs in well-drained soils, which have sufficient proportion of their volume occupied by pores.
Cultural practices affect soil aeration and plant growth through modification of different soil physical properties like bulk density, porosity, aggregation etc. Soil air also influences beneficial microorganisms in soil.
- Soil Science One Liner For Competitive exam-3
- Important Agriculture MCQ With Explanation for Competitive exam
- Soil Science One Liner For Competitive exam -4
- Agriculture Meteorology One Liner for Competitive exam-2
- Agriculture MCQ Important for IBPS AFO, NABARD and Competitive exam