Alluvium is loose, unconsolidated soil or sediment that has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it is called an alluvial deposit.

 Factors which lead to soil degradation are deforestation, overgrazing, overuse of chemical feritilisers or pesticides, rain wash, landslides and floods.

Agricultural inputs are classified into two types.

1. Human inputs: Labour (workers), Machinery (tractors, combine harvesters, etc.), Buildings (barns, silos), seed to grow crops, animal feed, fertilisers and pesticides, calves, chicks, piglets, etc. (small animals brought to rear and later sell).

2. Natural inputs: Soil, precipitation, sun, alluvium, relief, drainage.

Alluvial soils are formed mainly due to silt deposited by Indo-Gangetic-Brahmaputra rivers. They are the largest soil group covering about 15 lakh sq km or about 6 percent of the total area. They support more than 40% of India’s population by providing the most productive agricultural lands.

Black soil is ideal for growing cotton and is also known as black cotton soil. These soils are made up of extremely fine i.e. clayey material. They are well-known for their capacity to hold moisture.  

Alluvial soils as a whole are very fertile. Mostly these soils contain an adequate proportion of potash, phosphoric acid and lime which are ideal for the growth of sugarcane, paddy, wheat and other cereal and pulse crops. Due to its high fertility, regions of alluvial soils are intensively cultivated and densely populated.

Fertile soil has the following characteristics: 

Colluvial soil is made of materials of all sizes (from boulders to clay), that accumulate at the bottom of steep slopes and cliffs without the help of streams. 

Soil fertility is the ability of the soil to supply essential plant nutrients and soil water in adequate amounts for plant growth and reproduction, thereby support life. Thus the correct answer is option D.

Fertile soil contains sufficient minerals and nutrients needed for plant growth. Most minerals and nutrients are more soluble in slightly acidic soil than in neutral or slightly alkaline soils. The pH range of 5.5-7 is highly suitable for plant growth and hence such a soil is considered fertile. Thus the correct answer is option C.

Loam soil is a soil that has a good combination of minerals, rock, sand, silt water and organic matter. The soil contains microorganisms, insects and worms that replenish soil fertility. Because of this good composition, it is the healthiest type of soil. Thus the correct answer is option B.

The soil needs to be aerated and watered properly to allow proper growth and development of the plants. When the plants are watered frequently or the water is not drained properly, then the water logging takes place. Due to the accumulation of the water in the soil, the oxygen cannot enter into the soil and in the roots of the plants. This results in the death of the plants. 

Intensive agriculture is the commercial agriculture where chemical pesticides and fertilizers are used to increase the crop production for more earning. By the use of the chemical in large amount soil fertility gets reduced. Thus, the correct answer is option D.

Fertility of the soil is not only decided by its ability to hold water or organic materials or its ability to retain nutrients but it is decided by all the factors combined and how well can it support life. If a soil has a huge thriving of plants, microorganism, etc with the availability of adequate resources then it is considered as very fertile soil.

Soil pH is important because it influences several soil factors affecting plant growth, such as (1) soil bacteria, (2) nutrient leaching, (3) nutrient availability, (4) toxic elements, and (5) soil structure. Bacterial activity that releases nitrogen from organic matter and certain fertilizers is particularly affected by soil pH, because bacteria operate best in the pH range of 5.5 to 7.0. Plant nutrients leach out of soils with a pH below 5.0 much more rapidly than from soils with values between 5.0 and 7.0. Plant nutrients are generally most available to plants in the pH range 5.5 to 6.5. Aluminum may become toxic to plant growth in certain soils with a pH below 5.0. The structure of the soil, especially of clay, is affected by pH. In the optimum pH range (5.5 to 7.0) clay soils are granular and are easily worked, whereas if the soil pH is either extremely acid or extremely alkaline, clays tend to become sticky and hard to cultivate

It is not true that soil $pH$ is in the range $6$ to $8$, the soil fertility is diminished

During soil testing, the soil is examined for colour, texture and the proportion of organic matter in it.