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• Soil is the outermost fertile layer of earth in which plants can grow.
• Soil formation –
  • Soil takes long time to form, even 2cm layer of soil takes several years for formation.
  • Soil is formed naturally from bedrock
  • When it is exposed to earth’s atmosphere, factors such as air, water, rain, temperature, chemicals such as acid rain, biological factors such as roots of trees, starts acting on it.
  • Due to hot temperature rock expand, due to cold temperature rock shrinks, this process breaks rocks into smaller pieces. Slowly they get converted into stones, and small rock particles.
  • During this period there is also mixing or organic matter in uppermost part. It is decomposed by organisms thus making humus.
  • The rock particles along with the humus is called the soil.
  • The slow process of breaking of parent rock by various environmental factor so as to produce soil is called weathering. Weathering can be physical or chemical.
• Properties of soil –
  • Soil texture – soil particles are of different sizes
    Sandy soil – size between 0.05 – 2 mm
    Clayey soil – size < 0.002 mm
    Silt soil – (size between 0.002 – 0.05 mm
    Loamy soil – mixture of sand silt and clay
  • Soil color – soil can be of different color black, red, brown, yellow or gray.
  • Soil Structure- While making the soil the particles of sand, silt or clay joins up together forming smaller clumps called peds or aggregates. The aggregate can have specific shape such as granular, crumb, blocky, platy, columnar, Prismatic etc. The arrangement of these aggregates form soil structure.
  • Percolation of water by Soil: The ability of soil to allow passage of water through it is called percolation. Percolation rate = Amount of water (mL)/Percolation time (min)
  • Absorption of Water by Soil: It is the ability of soil to hold water. Clayey soil have highest ability to hold water while sandy soil have lowest ability to hold water. By holding water the soil ensures its availability to plants, therefore we can see more vegetation in clayey soil as compared to sandy soil.
  • Moisture in Soil: Amount of water in the soil is called soil moisture.Usually water is present between the pores in the soil particles and aggregates. It helps the plants to obtain water.
  • Air in Soil: Like water soil also contain air. If we put small part of soil in water then we can see air coming out in the form of bubbles. Air in the soil is used by roots and other microorganisms to breathe.
  • Soil pH: Soil could be acidic, basic or neutral
    pH of soil < 7 ⟶ acidic pH of soil > 7 ⟶ basic
    pH of soil = 7 ⟶ neutral
  • Soil Testing: Color, texture and organic matter in the soil is examined for getting the information of missing nutrient from the soil. Primarily pH and electrical conductivity tests are carried out during soil testing. Based on the test it is possible to take measures for removing the deficiencies in soil.
• Soil and crops
  

  Soil	Crops
  Sandy soil	Cactus, coconut, watermelon, etc
  Clayey soil	Wheat, paddy, gram, etc
  Silt soil	rose, tomato, pine, etc
  Loamy soil	wheat, cotton, jute, sugarcane, pulses, vegetables, oilseeds

• Soil Profile
  • Topsoil (A horizon) – The depth of this layer varies between 2 inches to 8 inches. It consists of decomposed organic material called humus.Humus gives the dark color to soil and makes the soil fertile. Topsoil is generally soft and porous. Therefore the soil have good air and water holding capacity. Seeds germinate in this layer, organism like worms, beetles, etc live in this layer.
  • Subsoil (B horizon) – It has depth ranging from 10 – 30 feets.It contain less humus but it is rich in clay and minerals like iron and aluminium. Color of soil is not dark. the color of subsoil is brownish or reddish due to higher amount of iron oxide. It provides shelter for many organisms such as worms, and insects and plant roots may extend in this layer in search of water and minerals.
  • Regolith horizon ( C horizon) – It has depth ranging from 30 – 48 feets. Generally it is made from broken rocks. It is less affected by weathering as it is far below the surface of earth. This layer does not contain humus.
  • Bedrock (R horizon) – . The depth of this layer is greater than 48 feets. It is very hard. This layer is not at all affected by the weathering process.