The requirement for fertiliser vary with your soil type and condition, and with the type of plants being grown. In general plants which are harvested (vegetables or fruit) often or being pruned (hedges or lawns), or which are fast growing and short lived (annuals) will require more fertilising than trees and shrubs.
Most gardeners are familiar with the need to fertilise but significant confusion exists with regard to which product to use and when.
A fertiliser provides the major nutrients that a plant requires for growth. The quantities of these nutrients is always given on the product packaging to assist you to decide on the most appropriate fertiliser for your situation. Nitrogen is the major element required for growth of green leaves. It is highly soluble in soil and for this reason requires replenishing by using fertilisers. Fertilisers which are high in nitrogen (over 10%) tend to be most suited to plants which require additional leaf growth.