Using Fertilisers in the Native Garden

Kevin Mulqueeny

I live on a steeply-sloping north-facing acre block on the eastern slopes of Mt. Ousley (north of Wollongong, New South Wales). Bought in 1975, I terraced the block with railway "timbers" (double-thickness sleepers), planted a few natives, and left them to sink or swim. By 2004 the timbers had been eaten away by termites, and a whole rebuild was undertaken, using treated pine. This was done during 2004/05, and mass plantings took place during April-May 2006. By now we had a tank installed which irrigates the lower half of the garden (with mains pressure for the higher areas) so the plants have survived well.

Fired by enthusiasm, I am determined to keep the progress flowing and plan to fertilise some 300 plants in early spring. Here is the problem. My ancient Austraflora catalogue states: "Australian plants prefer low levels of phosphorus, and it is advisable to use those fertilisers low in this component. Fertilisers are rated by their NPK content, that is Nitrogen (N), Phosphorus (P) and Potassium (K) levels. Selecting one with ratios of approx N 14%, P 4%, and K 8% is safe as long as it is used according to manufacturers specification" although Simon Leake recommends an "NPK ratio something like 8:1:5" and "as a general rule avoid fowl manure and mushroom compost altogether" (see Phosphorus and Iron Nutrition in Australian Native Plants).

Those with higher P levels are not safe. The Proteaceae family, with their highly specialised root systems and ability to re-absorb phosphorus from damaged leaves/stems, living in poor, low-rainfall areas, always rate first priority in this discussion. But that is not the whole answer. Many rainforest plants also belong to this family. . Phosphorus tolerance also increases as the nitrogen content increases (and that is the rationale in having significantly higher N than P in the fertiliser for natives plants) and as the Iron content increases. Phosphorus tolerance generally decreases as the Calcium content increases. It is best to have the phosphorus present in insoluble form or as slow release (e.g. Native 'Osmocote' or blood and bone, although the Calcium content can be high in the latter).

Phosphorus sensitive plants

Goodwin (1981) presented a list of phosphorus sensitive plants as follows:

• Acacia baileyana, A. iteaphylla, A. obtusata, A. suaveolens, A. verticillata
• Banksia aemula, B. ericifolia, B. oblongifolia, B. robur
• Beaufortia squarrosa
• Boronia megastigma
• Callistemon citrinus
• Grevillea aquifolium, G. glabella, G. "Poorinda Firebird"
• Hakea laurina
• Pultenaea pedunculata
• Telopea speciosissima

However, it is probably safe to assume that all Proteaceae genera are sensitive, viz.

Acidonia, Adenanthos, Agastachys, Alloxylon, Athertonia, Austronnuellera, Banksia, Bellendena, Bleasdalea, Buckinghamia, Cardwellia, Carnarvonia, Catarrhine, Conospermum, Darlingia, Dryandra, Floydia, Franklandia, Grevillea, Hakea, Helicia, Hicksbeachia, Hollandeae, Isopogon, Lambertia, Lomatia, Macadam/a, Musgravea, Neorites, Opistheolepsis, Oreocallis, Orites, Persoonia, Petrophile, Placospermum, Pycnonia, Stenocarpus, Stirlingia, Strangea, Symphionema, Synaphea, Telopea, Triunia, Virotia and Xylomelum.

From the newsletter of the Sutherland Group of the Australian Plants Society, July 2006.

Australian Plants online - 2007
Association of Societies for Growing Australian Plants