Theory:

Continuous cultivation of crops makes the soil poor in nutrients.
In order to overcome this loss of nutrients in the soil, farmers add manure and fertilisers  to the soil in the form of nutrients for the healthy growth of plants.
The process of adding manure and fertilizers to replenish the soil in the fields is called manuring.
Manure and fertilizers:
 
Manure is an organic substance obtained from the decomposition of plant and/or animal wastes.
Farmers keep the plant and animal waste in the pit called compost pit that allows the materials to decompose. Some microorganisms does the process of decomposition. The decomposed organic material is used as manure. Farmers also get the help of earthworms in digesting the organic waste by putting them in the compost pit.
  Manure formed by earthworms is known as Vermicompost.
 
compost pit1.jpg

In addition to adding nutrient to soil, the use of manure also improves soil texture as well as its retaining water capacity.
 
On the other hand, fertilisers are chemicals which are rich in a particular nutrient. Fertilisers are produced in factories.
Example:
Urea, Ammonium sulphate, Superphosphate, potash, NPK (Nitrogen, Phosphorus, Potassium).
The use of fertilisers has helped farmers to get better yield of crops.
 
Fertilisers also causes water pollution. Excessive use of fertilizers makes the soil infertile. Thus farmers are advised to use more manure and less fertilisers which are required to the particular farm or the plant being cultivated.
The organic manure is considered better than fertilisers. This is because, manure not only enhances the water retaining capacity of the soil, it also makes the soil porous making the flow of gases easy. Further, it improves the texture of the soil, which help in the growth of the plant..
Crop rotation and other methods:
 
Other methods of replenishing the soil with nutrients is through crop rotation and leaving the soil fallow.
We do crop rotation by growing different crops  alternately. In crop rotation, the farmers  used to grow legumes as fodder in one season and wheat in the next season. It helps in the replenishment of the soil with nitrogen.
Rhizobium bacteria is present in the nodules of roots of leguminous plants. They fix
atmospheric nitrogen in the root nodules.
Some times leaving the field uncultivated (fallow) in between two crops will increase soil fertility.
Reference:
Image courtesy :
Compost pit : Image by Susan A, https://www.flickr.com/photos/gtzecosan/8436201718 (free image)