The major nutrients in fertilisers are nitrogen (N), phosphorous (P) and potassium (K). These are usually represented as a ratio of N:P:K The percentage of these elements marked on a packet of fertiliser gives you a way of comparing one fertiliser with another but the secondary and trace (minor) elements are also important, as is the availability of the nutrients to the plants. A complete fertiliser should be one that supplies a well balanced source of all the necessary elements.
Secondary elements are calcium (CA), magnesium (MG) and sulphur (S). The trace elements are important but are only required in minute quantities and will usually be present in any good soil. Be careful when applying fertilisers not to put them too close to the plant as this can cause fertiliser burn. Always water in well.
Some manufactured fertilisers are tailor made to suit particular species or types of plants with the ratio of elements apportioned for maximum benefit. For example, fertiliser for grass, being a leafy plant, is high in nitrogen – but it should be slow or controlled release to avoid spikes in growth which will encourage lawn grubs. Fertiliser for flowering and fruiting plants will have increased levels of potassium. Native fertilisers will be light on phosphorous, and so on. Manufactured fertilisers are available as organic and inorganic forms, as well as a mix of both. The organic products usually have a lower N:P:K: than the inorganic and are slower acting. For this reason, you are less likely to over fertilise and harm your plants. There will be a better long-term effect on your soil, plants, and the environment.
Manufactured fertilisers, used responsibly, can make quite a difference to the performance of your garden plants, however, what they don’t do is improve your soil structure in the way organic matter does. There is also no increase in your soil materials, and these fertilisers are not supporting soil organisms. For these reasons a combination of adding organic matter to your soil (see Getting to Know Garden Soil at www.tanbygardencentre.com.au) and organic fertiliser application will provide the best long term outcome.