Busted: The I-hate-phosphorus myth

[SteveW]

A surprising number of people believe that most Australian native plants are sensitive to phosphorus (P) and need special care.

The truth is that:
• 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;
• A minority are truly sensitive to excessive P; and
• Excessive P can cause trace element deficiencies in any plant.

Most Australian soils are very low in P (and other nutrients). The plants have evolved mechanisms to cope with these limited nutrients. Many have symbiotic relationships with soil fungi (mycorrhizae); others have specialised root structures (e.g. waratah, grevillea, banksia, and casuarina). This does not mean that they can't handle, nor prefer higher phosphorus levels.

Cultural practices can also assist a plant manage higher P levels. For example, banksias grown with normal P level fertilisers do not develop proteoid roots, the specialist root clusters that gather P. Or, if any proteoid roots are removed at repotting, the plant does not get overwhelmed with normal P-level fertilisers afterwards.

About 20 years ago Kevin Handreck tested a wide range of species for their P sensitivity (see anspa.org.au link below for details regarding specific species). Of the 810 species tested, about 82% did not show any of the symptoms that are typical of phosphorus toxicity.

The results of his research are summarised below for species commonly used for bonsai.

Phosphorus sensitivity (rated 1-7)
1. Plants were healthy at all levels of phosphorus addition. No growth without added P.
Eucalypts
Melaleucas (63 species)
Acacias (66species), including howittii
Allocasuarina
Banksia (8 species)
Callistemon
Callitris
Casuarina glauca
Leptospermum (7), including laevigatum

2. Plants were healthy at all levels of phosphorus addition, some growth without added P.
Acacias (6)
Banksia (11)

3. All plants were healthy. The largest plants tended to be in the mix with the second-highest phosphorus addition rate.
Acacias (35)
Allocasuarina (2)
Casuarina (1)
Banksia (6), including marginata

4. There were slight toxicity symptoms at the highest P addition rate. The largest healthy plants were at the second-highest rate.
Acacias (24)
Banksia (9), including integrifolia
Melaleuca (3)

5. There were severe symptoms of P toxicity at the highest rate of addition, and some toxicity at the second-highest rate.
Acacia (36)
Banksia (5)
Grevillea (2)

6. There was considerable P toxicity at the two highest rates. The best plants were in the mixes with the two lowest rates of amendment with superphosphate. Plants were smaller in the mix without added phosphorus.
Acacia (36)
Banksia (16)

7. Plants in the mix without added phosphorus were the only ones that grew well.
Acacia polystachya

Two experienced bonsai growers tested banksia seedlings with low and normal phosphorus level fertilisers. The batch with normal levels of phosphorus did better than the low P ones.

If you are swapping from a low-P fertilising regime, gradually increase the level of P using slow release fertilisers. This allows the plant time to take up enough iron to prevent metabolic upsets. Exercise caution as there is a narrow range between P deficiency and excess, and phosphorus accumulates in the soil. That said, if the plant looks starved, it will respond to a good feed like any other plant.

References:
http://anpsa.org.au/APOL8/dec97-4.html
http://sesl.com.au/blog/the-myth-of-p-s ... e-natives/

[treeman]

Lept. laevigatum is listed in both ''score 1'' and score 4.
Which is it?
B. integrifolia score 4. Seeing they grow together, I'm going with 4.

[SteveW]

Lept. laevigatum was listed in both score 1 and score 4 in Handreck's paper. I didn't change it. Either way, it does not need a low-P diet.

[treeman]

Lept. laevigatum was listed in both score 1 and score 4 in Handreck's paper. I didn't change it. Either way, it does not need a low-P diet.

No, but it may very well show slight toxicity at 0.9g/L superphosphate which would make it somewhat sensitive to P.