The myth of P-sensitive natives

SESL Australia

Cluster roots of waratah, looking like a white dishmopMost Australian native plants are sensitive to phosphorus (P) and need special care, right?

Wrong. Keep in mind the following rules of thumb:

  • Most Australian native plants are not P-sensitive and do not need special low-P fertiliser.
  • Most Australian native plants do not need high P levels.
  • Only a minority are truly sensitive to excessive P, and great care needs to be taken when culturing these plants.
  • Excessive P has the potential to cause trace element deficiencies in any plant.

Origin of the myth

Most Australian soils are extremely low in P and other nutrients. This is a result of Australia’s long history of geological stability and P-poor geology. With no large-scale volcanism or glaciation to replace the continent’s surface periodically, what we’ve got is effectively what’s left after millions of years of erosion and sedimentation.

So it is to be expected that native plants will have evolved mechanisms to allow them to cope on limited nutrients. The sandstone soils typical of Sydney (and much of NSW) are incredibly low in P (about 30 mg/kg total P in sandstone). So the native vegetation copes by accumulating nutrients in the standing biomass. Many Australian native plant species survive by courtesy of mycorrhizae, symbiotic fungi that vastly expand the roots’ foraging power in return for carbohydrates. Plants in the family Proteaceae (waratah, Grevillea, Banksia etc.) in particular cope by producing specialised root structures called proteoid (or cluster) roots, which vastly expand the root surface area in order to pick up the traces of P in these poor soils. Other Australian natives produce other kinds of cluster roots for the same purpose, including flannel flowers and casuarinas. Other adaptations include sclerophyllous (tough) leaves and spiky or needle-like leaves to repel predators. The result is that the tissue of Australian native plant leaves is very low in P (average 0.10%–0.3%) compared with most other foliage (0.25%–0.6%).

It is a natural assumption that if native plants grow on P-poor soil, then they prefer growing there. Yet it is not a question of preference but of whether they can cope with and utilise higher P levels if presented with them. Answer: some can, some can’t.

Tolerance

It turns out that most Australian native plants are no different from other plants: up to a point, more P allows more growth. But past that point there is no further response, and the potential for trace element disorders sets in. Kevin Handreck’s work (see “Further reading” below), for example, shows that macadamia (despite being in the Proteaceae) is largely not P sensitive. We have seen P toxicity in plantation macadamias where years of chook manure was the only fertiliser source, resulting in over 600 mg/kg of available P. Perhaps this was not a true P toxicity, but simply P-induced iron deficiency?

With truly P-sensitive plants a sudden shock or highly soluble forms of P, or a slow accumulation of excessive P, kills them. These plants have no ability to prevent excessive P getting into their roots. As more and more is taken up, the plant gets poisoned, mainly because the trace elements iron, manganese, copper and zinc all bind with the P to form very insoluble phosphates. This effectively prevents the plant from using the trace elements, resulting in such severe deficiency that the plant cannot function.

On the other hand, many commercial growers of native plants are so afraid to apply P that actual deficiency is common. Kevin Handreck’s work shows a narrow “window” between deficiency and excess for many plants.

Ross Worrall, who recently retired from NSW Industry and Investment (formerly DPI) at Gosford, has spent years studying the nutrient requirements of Australian native plants. He has told Fertile Minds that plants acclimatise to P over time. In his forthcoming book Growing waratahs for cut flowers, he writes:

“Phosphorus has been implicated in the deaths of many plants, often without justification. … There is little doubt that waratahs can respond well to applied P in terms of both growth and flowering. Relatively high rates of P fertilisers are usually necessary for good growth of nursery and plantation plants. Such levels of P are within the range considered to cause toxicity problems in many P-sensitive plants.”

While waratah is not particularly P sensitive, plants in commercial production are losing P in the harvest and need to have it replaced, but many growers are not managing this replacement properly. Providing sufficient iron is also critical to how much P these plants can metabolise.

Worrall advises gradually increasing the level of available P, and using slow-release (or less soluble) forms of P fertilisers. Rapid increases with highly soluble P (e.g. superphosphate, ammonium phosphate) will not give the plant enough time to take up sufficient iron to prevent metabolic upsets.

In general landscaping work using non-P-sensitive natives and where the plants are not producing at high rates, it is remarkable how little P our native plants can live on. If they are growing adequately and look good they likely don’t need any. On the other hand, if they look starving and poor, they will almost certainly respond to a good feed like any other plant.

Exercise caution

Note, however, that some natives are P-sensitive, and P fertiliser will kill them. Click on the link below to the list by Handreck of natives and their sensitivity. This list is the best available but is by no means complete. P-sensitive plants accumulate P in their tissues to toxic levels, and the sensitivity varies. P also accumulates in the soil with repeated application and can reach toxic levels even in relatively tolerant plants. The availability of iron also affects the availability of P.

Soil and plant tissue analyses are critical to managing the narrow window between deficiency and excess in these plants.

Your own tests will also be critical. Much remains to be learned about the P tolerance of Australian natives. So exercise caution and do your own trials. Be especially careful with:

  • anything in the Proteaceae
  • many members of the Rutaceae (we all know how easy it is to get citrus “yellows” caused by iron deficiency)
  • some members of the Mimosaceae.

Final message: Enough P is enough.

Further reading

Handreck K. 1997. Phosphorus Needs of Some Australian Plants. Australian Native Plants Society (Australia).

Worrall R, Gollnow B. (in press) Growing waratahs for cut flowers. RIRDC, Canberra. (check periodically for publication)

Worrall R, Tesoriero L. 2010. Growing flannel flowers all year round. Publication 10/065. RIRDC, Canberra.