Question

why do farmer normally use a mixture of manure and fertilizer in their field

Answer 1

While manures are plant and animal residues which are used as source of plant nutrient. They contain very less concentration of primay nutrients . ... So manures are added along withfertilisers to add to the soil the complete nutrient basket it needs for successful crop production

Answer 2

as we know different vegetables and fruits have different nutrients contained in them similarly Manures ( in the market ) are of different nutrients so they make a mixture SO as to give all nutrients to the plant...
Hope this helps