Most of the more commonly cultivated Australian plants occur naturally in nutrient deficient soils, often particularly low in phosphorus. This has led to the misconception that fertilizers should not be applied to them. However, to obtain optimum growth, all plants require the basic elements nitrogen (N), phosphorus (P) and potassium (K) in balanced proportions....it's a question of "how much" and "how often"!
General purpose fertilizers with an N:P:K composition of about 12:14:10 can be applied safely to many Australian plants, preferably in slow-release form so that the release of nutrients to the plants is gradual. However, the relatively high level of phosphorus in such fertilizers can be dangerous to phosphorus-sensitive plants. This is particularly true of many members of the Protea family such as Banksia, Grevillea, Telopea and Hakea. Most of these plants have developed specialised roots called "proteoid" roots which consist of a fine mass of many small "rootlets". These are believed to enable the plants to take up nutrients from the nutrient-deficient soils and, if fertilized to excess, they may take up nutrients (particularly phosphorus) in toxic amounts.
|Proteoid roots can often be seen by carefully removing a proteaceous plant from its pot, as shown here. Photo: Brian Walters|
There are low-phosphorus fertilizers available (N:P:K composition of about 10:3:6) which can be used on phosphorus-sensitive species. Further information on this topic is available in the article Phosphorus Needs of Australian Plants and this can be used as a guide to fertilizer application.
Fertilizer is probably best applied in spring, just prior to the active growing period. If plants are growing well, annual fertilizing may not be necessary. Avoid fertilizing just prior to winter, particularly in cold districts as this could stimulate soft growth which can be easily damaged by frost. Fertilizer should only be applied when the soil is wet and it should be well watered in.