How important are phosphorus levels in landscape fertilisers? Certainly there are some Australian native species that are sensitive to excessive amounts of available phosphorus in the soil.
The degree of sensitivity to excess phosphorus can depend on a number of factors such as the soils buffering capacity, the availability of other elements such as iron which compete with phosphorus for root uptake, the species concerned and how much phosphorus those species received when grown in the nursery. If you have any concerns you should opt for a low phosphorus fertiliser in controlled release form for maximum safety.
What does the phosphorous do to these plants? Why are these plants so sensitive? What examples can you give of phosphorous sensitive plants?
High levels of phosphorus can compete with the uptake of iron so phosphorus toxicity will show as symptoms of iron deficiency (interveinal chlorosis of the leaves). In extreme case this can progress to leaf necrosis and death of the plant. The reason some plants are susceptible is because they have developed a specialised root structure called “proteoid roots” which are highly efficient in extracting phosphorus from the soil. They exist in species which come from habitats where phosphorus is naturally very low in the soil. Link to more info on proteoid roots: http://en.wikipedia.org/wiki/Cluster_root
Examples of phosphorus sensitive plants include members of the Proteaceae family and some Australian native plants such as Banksia, Grevillea and Hakea species. It is important to remember that most Australian species are NOT phosphorus-sensitive and that those that are, still require small amounts of phosphorus to flourish.
What fertiliser would you recommend that is low P, please describe its properties?
In a landscape situation I would recommend Sierrablen Flora which is low in P (1.8%) and it is also controlled release which means you get maximum safety. Osmocote Pro Low P is also low in P (1.3%) and controlled release and this is what nurseries will use to grow phosphorus-sensitive species. It can also be used in the landscape.
What if the client has asked for a nutrient hungry understory, to be planted under mature phosphorous sensitive trees?
Speaking very generally, in most circumstances it will be fine to apply a fertiliser with moderate levels of phosphorus around mature phosphorous sensitive trees. This is due to the fact that a much larger root system which may already have “adapted” (minimised) the amount of proteoid roots it has to its current situation. Also, the rate of fertiliser you would use to suit the smaller understory plants should not be high enough to adversely affect the larger tree/s. Be aware though, that using a controlled release Low P fertiliser will be far safer than using a readily available fertiliser and of course if you are concerned always consult an agronomist.