Fertilizers are used to improve plant growth. The faster growing the plant, the more it will benefit from fertilizer application. If you have a healthy soil, it is often not necessary to use fertilizers, but using them may produce a showier display of blooms or a higher yield of produce from edible crops. Fertilizers are also used where plants are showing signs of nutrient deficiency, usually shown by leaf yellowing or discoloration in varying patterns for different nutrients.
Remember that healthy soil structure and pH are just as important as fertilizer application in the prevention of plant nutrient deficiencies. Soil conditioners such as manure and compost help the soil to form into crumbs with spaces for air and water between them, making nutrients, water and air all more available to plant roots. Lime is added to remedy acidity.