If we have planted your trees (or you have planted them according to our instructions), they will not need fertilising in the first one or two growing seasons, as slow-release fertiliser tablets that last 14 months will have been added to the soil at time of planting. Thereafter, provided they have been planted in a good quality soil, most trees will enjoy good health and make satisfactory growth without the need to fertilise (particularly if you are applying organic mulch as mentioned earlier). This is not to say that a healthy tree wouldn't enjoy an annual top-dress of fertiliser if it was offered, especially a good autumn feed with high potassium! Top-dress slow-release fertilisers formulated specifically for trees and shrubs are easy to apply and available from most garden centres. Be sure to follow instructions carefully.
A final note on fertilisers: it is important to be aware that many lawn fertilisers contain weed and feed formulations that may be harmful to your trees. The same systemic herbicide that kills broadleaf weeds in your lawn is picked up by tree roots and may harm or kill your broadleaf trees if applied incorrectly. Understanding the actual size and extent of a tree's root system, before you fertilise, is necessary to determine how much, what type, and where to best apply fertiliser.