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Fertilisers for native plants in gardens

Soil analysis

There are many laboratories that offer analyses for soils. They are almost all geared to providing these analyses to farmers. They are typically unable to offer interpretations for native plants. Anyway, as I have written above, most of the analytical data you would get are of no use for native plants. The only number that can be useful is one for ‘available’ phosphorus. The ‘available’ description refers to availability to crop plants such as wheat and potatoes, etc. All native plants are able to get phosphorus from soils that do not contain enough for crops.

I urge all those who want to grow phosphorus-sensitive to get their soil (or soils, if there is variation across your garden) analysed for available phosphorus. There are several analytical methods in common use. They are named Olsen, Colwell, Bray and Mehlich III, after the scientists who developed them. To keep it simple, I suggest that you ask for the Colwell test. The concentration of Colwell-extractable phosphorus should be less than 10 mg/kg for very sensitive plants and less than 30 mg/kg for those that are moderately sensitive. The many native plants that are not sensitive will manage (like grow luxuriantly) with much higher concentrations.

You need to sample your soil carefully if the analysis is to be useful. Take about 20 samples from the area for which you need information. Each could be to a depth of 150 mm. Mix together these samples and then take a subsample for sending to the laboratory. Their instructions will tell you how much they need. If you dry the sample yourself (at air temperature, and certainly at no higher than 40oC) and then sieve it through a kitchen sieve with (roughly) 2 mm holes, you will not be charged for preparation. There is no need to pay for interpretation; use the numbers above.

If you think that different areas of your garden could have different phosphorus contents, you would need to take a separate sample from each area.

I suggest either of these laboratories.

Australian Precision Ag Laboratory (APAL ), 489 The Parade, Magill 5072; Tel. 8332 0199; see their website for instructions and much information about their tests for soil phosphorus.
CSBP Laboratory, 2 Altona St, Bibra Lake, WA 6163; Tel. 08 9434 4600. See their website for price and detailed instructions.


Kevin Handreck, Netherby