Kevin Handreck

Over the past several years a number of SGAP members (listed below) assisted me with a CSIRO trial that tested the phosphorus-sensitivity of a considerable number of Australian plants. The plants were grown from seed in small tubes, each of 200 ml capacity. The potting mix was composted bark and sand to which had been added single superphosphate at rates of zero, 0.06, 0.25, 0.40 and 0.90 grams per litre of mix. The mix contained ample supplies of all trace elements. Nitrogen and potassium were supplied via a zero-phosphorus Osmocote. Sowing was direct onto the surface of the mix in the tubes. The seed was covered with washed 3 mm gravel. Seedlings were thinned to one per tube and grown until the largest were considered to be ready for planting out. Deaths of seedlings were noted and the growth and quality of survivors was recorded.

We started off with seeds of nearly 2000 species, but lack of germination reduced the number of species for which data were collected to 810. Eucalypts were deliberately not included in the trial, because most are quite tolerant of phosphorus. Results with them would have been similar to those obtained with melaleucas. The species were assigned scores according to their growth, the appearance of toxicity symptoms and general pattern of response to phosphorus.

Score 1

Plants were healthy at all levels of phosphorus addition, with a grading from essentially no growth without phosphorus addition to the greatest growth at the highest rate of phosphorus addition.

Score 2

Plants were healthy at all levels of phosphorus addition, but there was some growth in the mix that had not received any phosphorus. Growth was greatest at the highest rate of P addition.

Score 3

All plants were healthy, with some growth without added phosphorus. The largest plants tended to be in the mix with the second-highest phosphorus addition rate, with those in mix with the highest rate of addition being somewhat smaller, but without any unusual foliar symptoms.

Score 4

There were slight toxicity symptoms at the highest P addition rate. The largest healthy plants were at the second-highest rate.

Score 5

There were severe symptoms of P toxicity at the highest rate of addition, and some toxicity at the second-highest rate.

Score 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.

Score 7

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

An obvious thing about the list in that the majority of the species were assigned to the first three score categories. In fact, of the 810 species for which data were eventually collected, about 82% did not show any of the symptoms that are typical of phosphorus toxicity. They grew best at the two highest rates of single superphosphate addition (0.4 to 0.9 kg/m3).

The amount of phosphorus in a 200 ml tube from an addition of 0.25 and 0.4 kg/m3 of single superphosphate was 4.3 and 6.8 mg, respectively. The lower amount is not quite enough to produce a seedling of planting-out size, but the higher amount is sufficient as long as little of it is lost by being leached from the container.

The more usual method of supplying phosphorus to plants in containers is via a controlled-release fertiliser (CRF) such as Nutricote, Osmocote, Green Jacket or Apex. At its simplest, no superphosphate is added to the mix and the CRF supplies all that is needed. The following table lists a few controlled-release fertilisers of low water-soluble phosphorus content that will provide enough phosphorus for a whole production period for most Australian plants.

Phosphorus Release of Some Controlled Release Fertilisers
Note: these products may have been superseded since this trial was carried out or may now have different specifications

While these CRFs can provide all the P requirements of Australian plants that are being grown in a mix that has essentially no soluble phosphorus, there is one situation that makes it desirable to include some soluble phosphorus in the mix. This is in the production of seedlings of myrtaceous species (eucalypts, melaleucas, callistemons, etc). Phosphorus reserves in the often tiny seeds of these species are very low. In mixes with essentially no soluble phosphorus, growth will be very slow until seedling roots make contact with CRF prills. Early slow growth is avoided if the mix contains about 0.2 kg/m3 of single superphosphate.

Healthy phosphorus-sensitive plants typically have much lower phosphorus concentrations than do plants of less sensitive species. Such plants are successfully produced in tubes that contain any of CRFs listed above. However, if such a plant were being produced in a 130 mm pot with low-phosphorus CRFs as listed above, the 30-40 mg phosphorus being released could be enough to cause toxicity. The larger the pot, the lower should be the actual percentage of phosphorus in the CRF used. Blends of zero-phosphorus and low-phosphorus products should be used. This is likely to apply to all species that are listed in the 6-7 score categories in the table, and possibly to some of those in the score 5 category.

Phosphorus Sensitivity Lists – Least Sensitive (Score 1) to Most Sensitive (Score 7)

I would like to sincerely thank the following SGAP members for the considerable amount time and effort they put into this project: Lloyd and Lorraine Carman, John and Tracy Early, Wendy and Brian Fopp, Mick Freeborn, Barbara Godfrey, Ian Graham, Karen Lane, Ben Lethbridge, John Maddern, Jamie Mugridge, Karen Parry, Arthur Pratt, Warrick Pybus (and his employees Eddie Bos, Mark Castine, Martin Giles and John Rievel), Jeff Read, Dean Reid, Jeff Reid, Ian Roberts and Diedre Stewart.

This article is reproduced from the February 1997 issue of the “SGAP Journal”, the newsletter of the South Australian Region of the Society for Growing Australian Plants.

At the time of this trial, Kevin Handreck was Senior Research Chemist with the CSIRO (Commonwealth Scientific and Industrial Research Organisation) Division of Soils, Adelaide.