Soil Science

Soil is comprised of air spaces and organic matter, but mostly mineral particles. There are three kinds of soil minerals, sand, silt and clay. The relative percentage of each of these particles in the soil determines its texture. Sand has the largest soil particles (0.4- to 0.01-inch diameter) and the largest pores between particles. Silt soil particle size is between clay and sand (0.01- to 0.0001-inch diameter). The characteristics of silty soil are similar to those of clay, but are moderated by its larger particle size. Clay soil has the smallest soil particles (less than 0.0001-inch diameter) and the least amount of water and air spaces between particles. Humus is dark, organic material that forms in soil when plant and animal matter decays. The thick brown or black substance that remains after most of the organic litter has decomposed is called as humus. Hence, humus settles at the topmost and sand the lowermost since, its size is maximum.

Humus refers to the fraction of soil organic matter that is amorphous and without the cellular cake structure characteristic of plants, micro-organisms or animals. Humus has a characteristic black or dark brown color and is organic due to an accumulation of organic carbon found in Horizon - A. The three major horizons are: (A) surface horizon, (B) subsoil and (C) substratum. Some soils have an organic horizon (O) on the surface. Hard bedrock, which is not soil, uses the letter R.

Forests and grasslands are an excellent binding material that keeps the soil intact and healthy. They support many habitats and ecosystems, which provide innumerable feeding pathways or food chains to all species. Their loss would threaten food chains and the survival of many species.

Maize requires substantial rainfall during its growing season, ideally 650-1000mm annually. This precipitation range supports proper germination, growth, and kernel development without waterlogging or drought stress.

Halophytes are plants specifically adapted to grow in environments with high salinity, such as salt marshes or saline soils, by managing salt uptake and osmotic pressure.

The red soil is found predominantly in Sivagangai, Virudhunagar, Ramanathapuram, Madurai, Dindigul, Nagapattinam, Thoothukudi, Tirunelveli and the Nilgiris districts in Tamil Nadu. The red or brown colour of the soil is attributed to the diffusion of iron content.

The black clayey alluvium rich soil is known as black cotton soil is found in parts of Coimbatore, Madurai, Dindigul, Thoothukudi and Tirunelveli and in patches in the districts of Kancheepuram, Vellore, Salem, Dharmapuri, Ramanathapuram, Virudunagar and the Nilgiris.

In coastal regions some alluvial deposits are formed due to wave action. Coastal and deltaic areas of Tamil Nadu includes Thanjavur, Nagapattinam, Tiruchirapalli, Cuddalore, Kancheepuram, Tirunelveli, Tuticorin, Kanyakumari and Sivaganga districts have alluvial soils.

The soils of Tamil Nadu may be grouped into five types. They are:

1. Alluvial Soils:  The soils develop from the weathering material transported by rivers from their catchment areas and deposited in their basins during floods.
2. Black Soils:  The soils are residual soils derived from the rocks of Deccan Trap, Rajmahal Trap, ferruginous gneisses and schists under semi-arid conditions.
3. Red Soil: The soils have developed on crystalline rocks by prolonged weathering in hot climate.
4. Laterite Soils: The soils are residual soils developed on aluminum-rich rocks e.g. greisze, sandstone, granite and basaltic rocks in regions of high rainfall with alternating wet and dry seasons.
5. Forest, peat and organic soils: Forest soils develop in areas under heavy forest cover. Peat and organic soils develop in waterlogged, swampy or marshy areas.

• Various environmental factors can be divided majorly into climatic factors, edaphic factors and biotic factors.
  
• The factors which relate to structure and composition of soil are called edaphic factors. Soil composition, organic matter, soil water, soil air and soil organisms are examples of edaphic factors.
• Humidity, rainfall and wind velocity are climatic factors.
• Examples of biotic components include animals, plants, fungi, and bacteria.
  
• So the correct option is B.

• Alluvial Soils are formed by the deposition of silt by the rivers. 
• The black soil is formed in the Deccan region on account of climatic conditions here and the basalt rock which is the parent rock for this soil is also an important factor for the formation of black soil. Black soil is sticky and possesses high moisture retention capacity. 
• The main reason for laterite soils formation is due to intense leaching. Leaching happens due to high tropical rains and high temperatures. As a result of high rainfall, lime and silica are leached away, and soils rich in iron oxide and aluminum compound are left behind.

They prefer marshy environment and utilize dead and decaying material as food. Oligotrophic soil ( in between layers) have the above mentioned requirements in good quantities.Thus, the fungi prefer the oligotrophic soil for their survival

• As the plough is drawn through the soil, it creates long trenches of fertile soil called furrows. In modern use, a ploughed field is typically left to dry out and is then harrowed before planting. Plowing and cultivating a soil homogenizes and modifies the upper 12 to 25 cm of the soil to form a plow layer.