The noun soil is derived through old French from the Latin solum, which means floor or ground. What a soil scientist calls soil, a geologist may call fragmented rock, an engineer may call earth, and an economist mat call land.
There are two basic concepts of soil that have already evolved through two centuries of scientific study. The first one considered soil as a natural body, a biochemically weathered and synthesized product of nature and the second one considers soil as a natural habitat for plants and other living organisms and justifies soil studies primarily on that basis.
The two approaches to study soil are:
Pedologist: which includes the study of origin of the soil, its classification, and its description
Edaphologist: which covers the study of soil in relation to growth, nutrition and yield of crops
There are four components of soil:
Mineral matter
Organic matter
Water
Air
All these components can't be separated with much satisfaction because they are present very intimately mixed with each other. The mineral matter makes the bulk of soil solids and a very small amount of the soil solids are occupied by organic matter.
Physically, the soil consists of stones, large pebbles, dead plant twigs, roots, leaves and other parts of the plants, 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 total organic matter comprise of the dead remains of the soil life in all the 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.
The total solid space of the soil occupied by mineral matter is about 45% and by organic matter is 5%.
The total pore space of soil is occupied by 25% air and 25% water. The proportion of air and water will vary under natural conditions depending upon the weather and environmental factors.
Thus, these four components exist in an intimately mixed condition which encourages various reactions within and between the groups and gives optimum conditions for the crop growth.
Soil Mineral Matter:
The size and composition of mineral matter in soils are variable due to nature of parent rock from which it has been derived. In general, the primary minerals viz. quartz, biotite, muscovite etc. dominate the coarser fractions of the soil, on the other hand, the secondary minerals viz. silicate clays and hydrous oxide clays of iron and aluminium etc. are present as very finer fractions, clay in the soil.
Soil Organic Matter:
Soil Organic Matter (SOM) exists as partly decayed and partially synthesized plant and animal residues. Such organic residues are continually being broken down as a result of microbial activity in soil and 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 to 5 percent by weight in the top soil. In addition to partly decayed plant and animal residues, soil organic matter contains living and dead microbial cells, microbially synthesized compounds and derivatives of these materials produces as a result of microbial decay.
Organic matter is a storehouse of nutrients in soil. Besides these, organic matter is responsible for most desirable surface soil structure, promotes a greater proportion of larger pore sizes, improves water holding capacity and also aeration status of the soil.
It is a major source of Nitrogen, Phosphorus (5%-60%) and Sulphur (~80%). Besides these it can also supply different trace elements like Boron, Molybdenum etc. to soil which are essential for the plant growth. Organic matter is the main source of energy for soil-microorganisms.
Organic matter also acts as a chelate. A chelate is any organic compound that can bound to a metal by more than one bond and form a ring or cyclic structure by that bonding. Due to chelate formation between organic matter and various metals, the availability of those metallic elements will be increased to the plants through increasing their mobility in soils, organic matter contributes to the cation-exchange capacity in the soils. It also reduces soil erosion, shades the soil, and keep soil cooler in very hot weather and warmer in winter.
Soil Water:
Water is held within the soil pores with varying degrees of forces depending on the amount of water present. With increasing amount of water in soil, the forces of retention of water by the soil will be low and vice-versa. So, the movement and retention of water in the soil is primarily influenced by the characteristics of the soil viz. texture, structure, nature of inorganic and organic colloids, type and amount of exchangeable cations and size and total amount of pore spaces etc.
Soil water also presents along with dissolved salts and makes up the soil solution. This soil solution (water plus dissolved salts) acts as an important medium for supplying different essential nutrient elements to growing plants through exchange phenomena between soil solid surfaces and soil solution and finally between soil solution and the plant roots.
Soil Air:
Air spaces or pore spaces (voids) in a soil consists of that portion of the soil volume not occupied by soil solids, either mineral or organic. Under field conditions, pore spaces are occupied all the times 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.
Soil contains various gases like carbon dioxide, very small amounts of oxygen and nitrogen etc. The amount of carbon dioxide is more than atmospheric air due to microbial respiration where large amounts of carbon dioxide is released in the soil and the soil is taken up by the soil microorganisms.
Good aeration can only happen in well aerated soils which have sufficient proportion (atleast 10%) of their volume occupied by pores. So cultural and other soil-management practices affect soil aeration and plant growth through modification of different soil physical properties like bulk densities, porosity, aggregation etc.
The dynamic nature of soil air is apparent and it definitely influences the growth of the plants as well as the activity of different beneficial microorganisms present in the soil.
Difference between Surface Soil and Sub-Soil:
Surface Soil: It is defined as the immediate uppermost loose layer of the earth consisting of organic matter and soil organisms suitable for plant growth. It is generally called furrow slice soil layer (0-15cm depth) and fertile soil.
Sub-Soil: It is defined as the compact soil below the furrow slice (below 15cm) soil layer which can't be cultivated by tillage operation. It is generally poor in nutrient status and organic matter and hence it is less fertile than that of surface soil.
Sometimes in the sub-soil there is a formation of hard pan that cannot allow water to move down below the hard layered formed either due to accumulation of clay particle or formation of insoluble CaCO₃ layer resulting stagnation of water on the soil surface that affects plant growth.
Soil Profile:
Soil profile may be defined as a vertical section through a soil. It represents the sequence of horizons or layers differentiated from one another but genetically related and included to the parent material from which the soil profile is developed.
Practically, the soil profile is an important tool for soil classification, nutrient management and assessing soil fertility. As soil weathers or organic matter decomposes, the profile of the soil changes.
That's all for this brief introduction to what we call soil.
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