Soil: Physical, Chemical, and Biological properties for Class 10th and 12th
Different physical, chemical, and biological characteristics exist in soils.
These include colour, texture, structure, depth, and moisture under the heading of physical qualities. High organic content is indicated by the brownish-black and dark brown colours in the A horizon. Dark brown to blackish colours in the B horizon indicate poorly drained soil, whereas very pale brown to reddish colours in the B horizon indicate well-drained soil. Iron oxides are what give red soils their colour. Good drainage and aeration are indicated by the vibrant colours. Colours that are red and yellow get brighter as you get closer to the equator. Permanently saturated soils with ferrous iron are identified by their greyish colour. Munsell colour charts, which are standardised colour swatches, are used to determine the hues of soils.
Texture: The proportional amount of mineral particles, such as gravel, sand, silt, and clay, in a given soil is what gives the soil its particular texture. Particles greater than 2.0 mm in diameter are considered to be gravel. Between 0.5 and 2.0 mm in thickness, sand has a gritty feel to it. Silt is made up of particles ranging in size from 0.002 to 0.5 mm, is invisible to the unaided eye, and has a flour-like consistency. The exchange of ions between soil particles and soil solution is controlled by clay particles, which are less than 0.002 mm in size. Soils are divided into various textural classifications according to the amounts of sand, silt, and clay in them. The pore space determined by texture affects root penetration as well as the flow of air and water. Large pore spaces in coarse-textured soils favour quick water filtering and quick drainage. Clay makes up a large component of fine-textured soils, which are also weakly aerated.
The aggregates or peds, which come in a variety of sizes and forms, are what hold soil particles together. Soil structure refers to how these particles are arranged. Granular, crumb-like, plant-like, blocky, prismatic, and columnar are the different types of soil aggregates.
Depth: Depending on the slope, parent material weathering, and vegetation, the depth of the soil strata varies. Beneath native grasslands, soils are several metres deep, whereas they are only slightly shallower under forests.
Moisture: There are different types of water in the soil, including hygroscopic water, capillary water, and gravity water. Field capacity is the greatest volume of water that the soil can store after gravity water has been drained. Hygroscopic and capillary water are both present. Between 0.1 and 0.3 bars is the range of the soil’s metric tension at field capacity. The primary supply of water for plants is capillary water, which is held in place by capillary action against the pull of gravity in microscopic soil pores. The surface of soil particles has hygroscopic water adsorbed on it. The amount of water in soil when plants growing in it are irrevocably wilted is known as the permanent wilting percentage. Except for xerophytes, the majority of plants irreversibly wilt at a soil metric tension value of roughly 15 bars. Additionally, soil’s ability to retain water is improved by the presence of organic matter.
The soil contains dissolved chemical components that are part of the organic matter and are adsorbed on the soil particles. Anions are dissolved in the soil water, whereas cations, which are positively charged minerals, are kept on the surface of the particles. Hydrogen ions are used by plant roots to absorb cations. Cation exchange capacity is the total number of negatively charged exchange sites on clay and humus particles that draw in positively charged cations (CEC). The soil’s negative charges make it possible to stop cations from leaching. Calcium (Ca2+), magnesium (Mg2+), potassium (K+), sodium (Na+), and hydrogen (H+) all occupy exchange sites. The term “per cent base saturation” refers to the proportion of sites filled by ions other than hydrogen. One of the most significant chemical properties of soil is its acidity. A typical soil’s pH ranges from 3 (severely acidic) to 9 (basic alkalinity) (strongly alkaline).
Soil has a wide range of species. Bacteria, fungi, protozoans, and nematodes are the dominant organisms. The habitat of soil creatures includes gaps within a surface litter, cavities in soil aggregates, pore spaces inside individual soil particles, and root canals. These organisms get their food from living plants’ roots and organic substances found in the pore spaces. Mites are the most prevalent animals in the soil. The earthworms are a part of the wider fauna. Millipedes that feed on the litter’s surface break it down mechanically, making it more susceptible to fungus-induced decomposition. Millipedes eat mushrooms in the trash to survive. Along with millipedes, snails and slugs aid in the hydrolysis of lignins and even very indigestible cellulose. The breakdown of the cellulose in wood is caused by bigger occupants like termites and some dipterans. The fauna of tropical soil is dominated by termites.
The development of O and A layers is significantly influenced by soil organic matter, which results from biological processes during decomposition. Humus, a dark-coloured, chemically complex organic compound with distinctive components like fulvic acid and humic acid, is the portion of organic matter that survives after decomposition. Physical, chemical, and biological forces interact to generate two different forms of humus;
Mor: The organic layer is dense and clearly defined, sitting on mineral soil. Horizons O and A are distinct from one another. The primary decomposition organisms are fungi, which also create acid and reduce soil animal activity.
Mull: The mineral soil has a high level of organic content, and there is only a small covering of litter on the surface. There is a lot of animal activity. The O and A horizons don’t abruptly diverge. In this soil, bacteria are the primary decomposers.