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).

Leaching is the process by which soluble nutrients are lost from the soil. The A horizon of the soil is the surface layer that consists largely of minerals. The A$ _2$ horizon of this layer shows the maximum leaching. 

Top soil contains detritus micro-organisms  that decompose dead plant and animals and add to the organic content of the soil. The decomposed organic matter is called humus.

• 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.

• Soil composition refers to the nutrients and various other substances present in the soil. 
  
• The basic components of soil are minerals, organic matter, water, and air. The typical soil consists of approximately 45% mineral, 5% organic matter, 20-30% water, and 20-30% air.
  
• Hence In addition to the rock particles, the soil contains Minerals, organic matter, air, and water.
• So, the correct answer is 'Minerals, organic matter, air, and water'.

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.

The top most layer of soil is said to contain and support a lot of living organisms. It also has organic manure in it that supports the growth and development of plants. 

Soil is the upper weathered humus containing part of earth's surface, which can sustain terrestrial plant life. Soil is composed of inorganic mineral particles (45 percent), water (25 percent), air (25 percent), humus (5 percent) and variable number of living organisms. Mineral particles are of four types- (i) gravel or fine pebbles, (ii) sand or silica or quartz, (iii) silt or very fine quartz, (iv) clay or hydrated silicates of aluminium. 

Sulphur is widely distributed as native deposits near volcanoes and hot springs, is a component of sulphide minerals. E.g., galena, pyrite, sphalerite, etc., and is also found in meteorites. Significant deposits exist in salt domes along the Gulf coast of the USA, and in large evaporite deposits in eastern Europe and western Asia, and are most likely the result of bacterial breakdown of sulphate minerals. The weathering of rocks will result in the production of major source of sulphur.

All living organisms have a definite life cycle.

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. 

Humus is a type of organic material. It is present inside the soil in the form of a dark-coloured substance. Humus is formed by breaking down of plant litter that includes dried and fallen leaves, branches twigs etc. along with dead bodies of animals and insects. Over a period of time, all these decompose and cause break down of their complex substances into simple ones. It contains some of the important chemicals required for the soil for its nutrient maintenance. 

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. 

• Water contains lots of impurities inside it. it includes fine sand, clay and many other materials present in it. These are carried away by the flow of running water. They are then deposited or sedimented at the bottom of the water reservoirs.
• This sedimentation forms silt. As it is present at the bottom, it is usually accumulated at the bed of the river. 
• So, option C "Silt" is the correct answer.

• The different types of loam soils each have slightly different characteristics, with some draining liquids more efficiently than others. The soil's texture, especially its ability to retain nutrients and water are crucial.
• Loam soil is a mixture of soil that is the ideal plant-growing medium. It is actually a combination soil, normally equal parts of clay, silt, and sand, which gives the benefits of each with few of the disadvantages.
  

Sandy soil is made up of constituent particles that are relatively larger in size. The larger size of sandy particles leads to their lesser absorptive surface area. As a result, their water holding capacity is less. Their particles are larger and their inter particular space is more. So, air can move easily between these particles leading to proper aeration.  

There are four major types of colloids present in 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.

A good soil needs to be deep, fine-textured, moist, free-draining, loamy, and have an organic-rich topsoil. These properties best enable plant roots to take up nutrients, water and oxygen, and get enough support for rapid growth. In clay soils with small pore spaces, water moves slowly and spreads outward from where it is applied due to capillary action. In sandy soil, irrigation water can penetrate to 72 inches below the surface within 24 hours of application. In soils with a heavy clay content, water only penetrates to roughly 36 inches in 24 hours.

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.

(b) Top soil contains decomposed and amorphous organic matter composing of sand with humus.

• A gemstone, also known as the precious stone, jewel, a gem is a piece of mineral crystal which is cut and polished and it is used to make jewellery or other accessories. Gemstones are formed from igneous rocks. Interlocking crystals grow within intrusive igneous rock and may form gemstones depending on elements present, cooling time and the environment.
  
• Hence precious stones, such as gemstones, are products of igneous rocks.
• So, the correct answer is 'Igneous rocks'.

The texture of soil is determined by the proportions of particles of different sizes. Coarse sand particles are of 0.2 to 2.00 mm in size; 0.02 to 0.2 mm particles constitute fine sand; 0.002 to 0.02 particles represent silt and particles smaller than 0.002 mm are called clay.

• Humification is a process in which organic material decomposes and breaks down. It results in the formation of humus. Humus is a dark-coloured, amorphous and acidic substance. 
• It occurs in soils and peats. It improves the quality of soil by providing nutrient substances like nitrogen.

Humus is the dark, organic material formed in the soil when plant and animal matter decays.
It is important as it retains-
1)  Moisture content in the soil.

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.

Sand can refer to a textural class of soil or soil type; i.e., a soil containing more than 85% sand-sized particles (by mass) and 15% clay.

Explanation: Humus is the end product of broken down (decayed) organic matter. Humus is an essential component of healthy soils that has many great benefits.

• Phosphorite rocks are spherical concentrations that are randomly distributed along the floor of continental shelves. It contains high amounts of phosphate minerals. 
• The size of Phosphorite nodules can range up to several tens of centimetres.
• Hence Mineral nodules present in the ocean which can meet the shortage of important soil nutrient are Phosphorite nodules.
• So, the correct answer is 'Phosphorite nodules'.