They occupy nearly 20 percent of the earth surface. It covers around 13000 million hectares of the area. The houses, roads and factories occupy nearly one third of the land. The forests occupy another one third of the land. The rest of land is used for ploughing and for meadows and pastures. The soil forms the surface layer of land which covers more than the 80 percent of land. The soil is defined as a natural body which keeps on changing and allows the plants to grow. It is made up of organic and inorganic materials. This definition is given by the Buckman and Brady. The branch of science which deals with the formation and distribution of soil in the different parts of the world is referred as a pedology. The professional which deals with the soil is known as the pedologist. The inorganic component in the soil is 45 percent and the organic component in the soil is 5 percent. The water component in the soil is 25 percent and the air component in the soil is 25 percent. The soil particles have fine spaces which are known as the pore spaces. These are also known as the interstices. They contain air and water along with the dissolved substances. The water and air content in the soil is inversely related to each other. The more is the water content lesser is the space for air to exist. The soil has both the plants and animals. The micro flora consists of the heterotrophic and autotrophic bacteria. It also contains the fungi and algae. The heterotrophic bacterium consists of the nitrogen and non nitrogen fixing bacteria. The nitrogen fixing bacteria can be symbiotic, non symbiotic, aerobic and anaerobic. The non nitrogen fixing bacteria can be aerobic or anaerobic. The fungus includes the yeast and mushrooms. The algae can be red or brown or green. The fauna can be micro or macro. The micro fauna includes the protozoa and nematodes. The macro fauna includes the earthworm, mites, termites, snails and mice. The soil has different types of soil particles. The mineral composition of the rock determines them along with the size of particles. It includes the gravel particles, sand, silt and clay particles. The gravel particles are mainly small stones and have a few sand particles and are used to make roads. The sand particles have pores and are aerated. They can hold little bit of water and are made up of large quartz. The silt particles are moved by the help of water. They are left at the bank of river. They are inert and are made up of large quartz. The clay particles have nutritive salts and have ability to retain water. They are not inert and react chemically. Some of their pure forms are not suitable for the growth of plants as they form a non penetrable mass. The other components of the soil mix with the clay particle and form a granular soil. This type of soil is ideal for the cultivation. It has pores as well as has the ability to hold water. It also contains the nutritive salts.
The loamy soil is made up of clay, silt and sand. The proportion of the clay is least and is half as compared to the silt and sand. The silt and sand are twice and equal in the proportion. It is also a good soil for the growth of plants as it has pores as well as has the ability to hold water. It also contains some nutritive salts. There are many factors which control the nature of soil. They are porosity, water holding capacity and the texture. They come under the physical nature of soil. The chemical nature of soil is governed by the salt content, inorganic and organic content includes certain metals. The topography, climate and the organisms also play a vital role in deciding the nature of soil. The half decayed and half synthesized part of organic material in the soil forms humus. It contains the nutrients and help in growth. It makes the soil granular by its porosity and water holding capacity. It has the ability to absorb the heat and warm the soil.