The topmost layer of the earth is called soil . It is great importance to us and to the entire world because it supports the growth of plants . Soil is formed by the weathering or breaking down of rocks . Natural factors like changes in temperature rain and wind help to break down rocks and form soils . The process takes a million of years . By the way the same factors can also erode or destroy soil . 
     
The daily and seasonal changes in temperature have a big role to pay in the formation of soil . When rocks are heated they expand . When they cool they constract . This constract expansion and contraction weakens them and makes them brake. 
       
               Rain water loosens peices of rock and carries them with it . As the peices tumble and knock against each other they break into smaller peices . The flowing water carries these peices and deposits them over land when it slows down . Rainwater also enters cracks in rocks and freezes in winter . When water freezes it expands . This makes the rocks crack further or break . 
                
                          The oxygen present in air reacts with some minerals in rocks and makes the rocks crumble . This is somewhat the way iron crumble when it rust in the presence of moist air . Like air rainwater wind too Carrie's away losse peices of rock . 
What soil contains 
Since soil is formed by the breaking down of rocks a large part is made up of minerals derived from the rocks . It also contains humus or organic matter derived mostly from fallen leaves and other decaying plant and matters . 
       So soil contains mineral water air and humus . It also has bacteria fungi algae and tiny organism called protozoan.  These organisms help improve the fertility of soil by breaking down organic matter and converting the nitrogen of the air into nitrogenous compounds that the plants can use .   
              Worms and insects also live in the soil.  These too help to improve the fertility of the soil . One of the most useful of these organism is the earthworm.  It ingests soil digests the organic matter present in the soil and excretes soil full of plant nutrient .
 The extracted soil looks like small piles of coiled earth and is called worm casts . Earthworms and other worms and insects also help by borrowing into the soil and thus tilling or aerating it. All the organism living in the soil are often referred to as biota.