Soil Science and Fertilizers

Presence of unwanted contaminant in soil is called soil contamination. The deposition of pesticides on fruits and vegetables lead to its accumulation in fatty tissues of the host. This phenomenon is called bio accumulation.

Xenobiotics are the unwanted chemicals or particles of compounds which are found in the regions where generally they are not present. These are the foreign particles like the herbicides, pesticides, agrochemicals like fertilizers, etc. These are the chemicals which might interact with the bacteria and other microorganisms in the soil. This might affect the absorption of nutrients by the plants.

Acidification is characterized by an increase in hydrogen ion concentration (H+), which mathematically corresponds to a decrease in the pH value (since pH is the negative log of H+ concentration).

Excessive use of artificial fertilizers leads to nutrient runoff that can contaminate water bodies (rivers) and degrade soil structure and fertility. It can also cause air quality issues through the release of gases like nitrous oxide.

While a nearly infinite variety of substances may be found in soils, they are categorized into four basic components: minerals, organic matter, air and water. Most introductory soil textbooks describe the ideal soil (ideal for the growth of most plants) as being composed of 45% minerals, 25% water, 25% air, and 5% organic matter. In reality, these percentages of the four components vary tremendously. Soil air and water are found in the pore spaces between the solid soil particles. The ratio of air-filled pore space to water-filled pore space often changes seasonally, weekly, and even daily, depending on water additions through precipitation, throughflow, groundwater discharge, and flooding. The volume of the pore space itself can be altered, one way or the other, by several processes. Organic matter content is usually much lower than 5% . Some wetland soils, however, have considerably more organic matter in them (greater than 50% of the solid portion of the soil in some cases).

• Climate affects soil formation because it determines the amount of water that is available for processes such as the weathering of minerals, the transportation of minerals and the release of elements. Climate also influences the temperature of the soil, which determines the rate of chemical weathering.
• Temperature and rainfall are the two main climatic factors responsible for soil formation. 
• Hence, Climatic factors affect soil profile and bring changes in soil structure.

Humus is dark brown amorphous gummy substance formed by partial decomposition of plant and animal matter. It constitutes the organic component of soil and is rich in lignin and cellulose. Humus is formed from organic remains through the activity of decomposer microorganism. The process of formation of humus from raw organic remains is called humification. Humus is quite resistant to microbial action. It is a reservoir of nutrients and is helpful in the maintenance of soil moisture as well as aeration. It is colloidal in nature. Nutrients are released slowly as the humus is decomposed slowly. Humus is further degraded by some microbes and release of inorganic nutrients occur by the process of mineralization.

All statements are true except the ability of roots to penetrate in sand and hummus are low as compared to clay.

Mineralization is the release of inorganic substances, both non-mineral(e.g., CO2, H2O) and mineral(e.g., Ca2+, Mg2+, K+, NH+4) from organic matter. The process is slow because of trapping of these nutrients in humus and their immobilization in decomposers/detritivores. It prevents their washing out or leaching. Nutrients immobilized in decomposer microbes and detritivores are again exposed to humification and mineralization after the death of these organisms.

Soil is the thin layer of material covering the Earth's surface and is formed from the weathering of rocks. It serves as the primary medium for plant growth.

While soil composition varies, the organic matter content in typical mineral soils is generally low, usually around 5% by volume, with the rest being minerals, water, and air.

The dark black upper layer of soil is known as humus. It is actually the dead matter of plants and animals that have been decomposed by bacteria and fungi. This humus rich soil increases the aeration capacity of soil and also helps in the penetration of roots of plants. The decomposition of organic matter helps in bringing the plant and animal residues back to soil. The soil organisms use the organic matter its food and in the process convert in into nutrient rich humus for plant growth. 

Decomposers are organisms that feed on plant and animal dead bodies. They convert their complex material into simpler materials. These simple materials are stored in the form of humus, which is a dark-coloured material present in the soil. 

Clay soil is soil that contains a high percentage of clay particles. It is slow to drain water and quick to harden leading to water logging. It blocks root absorption, which prevents plants from getting nutrients required for survival. Hence it is not suitable for plant growth. Thus the correct answer is option C.

Humus controls and regulates many soil functions, as well as plant growth. The better the humus count in the soil, the healthier and more productive soil will be. Regardless of the program one uses in farming operations, the higher the humus count, the better the crop is grown yield. Good humus count in the soil will hold soil moisture longer, thus less water is required for crop production. Also, good humus count reduces both water and wind erosion of soil.