Stuart Rodda on soil


Stuart Rodda is an avid gardener who lives in Eltham.

Soil safety

In November 2018, Stuart Rodda submitted his soil for testing by VegeSafe and has just received his results. “I am relieved to say that my results are all well within what is considered as ‘safe’ for food growing by the Australian Government’s published standards.” The results from VegeSafe are set out in the table below, together with the Australian standards that Stuart and I have agreed are the best ones to use (VegeSafe sent Stuart a table with seven different sets of standards from around the world but it is a real dog’s dinner with the different standards being for completely different things and it has taken Stuart and I a considerable amount of time to work out that the most appropriate standards to use are the so-called ‘Australian NEPC Health-based investigation levels (Residential A).) The numbers are all in mg/kg (milligrams per kilogram), which is the same as ppm (parts per million).

  Mg / Kg
  As Cd Cr Cu Mn Pb Ni Zn
West veggie patch 4.9 0 51 22 210 24 0 55
East veggie patch 15.5 0 66 40 277 69 0 152
Raised bed 7.5 0 45 22 201 17 0 76
Old orchard 8.0 0 57 15 304 51 0 28
North end 5.2 0 42 24 261 19 0 347
Health-based investigation levels
(Residential A)
100 20 100 6,000 3,800 300 400 7,400


Here’s Stuart’s commentary: “In thinking about the results, I have found this paper from Cornell University very helpful. The elements of most interest from a human health point of view are arsenic (As), cadmium (Cd), chromium (Cr) and lead (Pb). Copper (Cu), manganese (Mn), nickel (Ni) and zinc (Zn) are all essential for plant growth and human nutrition so, unless exceptionally high, are not generally a concern. My land was previously used as an orchard (around 40 years ago) so there was the potential for residues of toxic materials in the soil from sprays, etc. The test included samples of the ‘original’ orchard soil and two vegetable growing beds which I have heavily modified with imported manures, mulches, lime, gypsum, and more recently coffee and lignite (brown coal). There was no cadmium detected. The ‘East veggie patch’ (which is at the bottom of the valley) had consistently higher levels of As, Cr and Pb than the ‘West veggie patch’ (which is part way up the hill), which makes sense if the elements present leach downhill over time or tend to accumulate in the bottom of the valley. Note that chromium and arsenic are two of the elements which are used in ‘treated pine’ as a preservative and these products are no longer recommended for use where there is human contact.

We talked to Kara Fry, from VegSafe. She has confirmed that the most appropriate standards to use when judging whether your soil is safe are the so-called ‘Australian NEPC Health-based investigation levels (Residential A). She also added: “The guidelines are not necessarily ‘safe’ levels; for example, when the NEPC set their guide of 300 mg/kg for lead (Pb), they also said that no level of lead is safe. Rather, the 300mg/kg mark is the level at which intervention is recommended.

The VegeSafe program is run by Environmental Science staff at Macquarie University and offers a very cheap ($20 donation) say of getting your soil tested for metal and metalloid contaminants. Read more. Submit sample.

Coffee composting

In 2017, we posted about Reground, saying that if you want lots of coffee grounds at zero cost and you live in Melbourne, then Reground may well be the organisation for you. They will deliver to your door(!) and as often as you want, but with (large) minimum delivery quantities: 2 bins if you live in inner Melbourne or 7 bins if you live in outer Melbourne.

In response, Stuart wrote in: “I am exceptionally pleased with how it has worked out with Reground. They have brought regular deliveries of bulk amounts of coffee grounds, virtually free of any rubbish, and I am already starting to see beneficial effects in my garden. Where I have placed pure coffee grounds in the garden (but not directly into soil where I am about to plant because of known inhibitory properties of fresh grounds on plant growth), the coffee has become a seething mass of earthworms. By the time the worms are finished with it, it will be that ‘black gold’ of gardening, worm castings.

On 15th August 2018, Stuart was interviewed about his coffee composting by Gardening Australia:

Where to find finely ground rock dust

Stuart wrote: “An ‘organic’ source of many minerals, including potassium, is finely ground rock dust, particularly basalt rock. There are various suppliers of rock dust-based soil additives, such as Munash, but most of them have other additives to enhance the overall effect and are thus quite expensive (upwards of $2/Kg). If one just wants basalt rock dust, it can be bought at some local garden suppliers, but it is often quite coarse and would take a long time (decades) for the larger grit to break down in the soil and release its nutrients. Thus it is quite important that it is finely ground. I have one source of quite fine basalt rock dust and, while it is dearer (per cubic metre) than a lot of other garden soil additives, it is much cheaper than specially manufactured and packaged ‘rock dusts’. It is from Mercuri Garden and Building Supplies at 2, The Concord, Bundoora, and costs about $105 per cubic metre. This equates to around 10 cents per litre and, because it is quite dense (heavy), this is less than 5 cents per Kg, or thus a tiny fraction of the cost of packaged rock dusts. I am not sure what they would charge for small quantities but, for people like me with larger gardens who buy loose materials by the trailerload, it is a bargain.


In 2017, we posted a link to an article entitled Why you should pee in your garden. Stuart wrote in about the article’s inaccuracies:

  • Urine is not a rich source of uric acid, but urea. Bird droppings are full of uric acid, which is what makes them white. If we have too much uric acid in our bodies, we get gout, due to uric acid crystals in the joints.
  • “Urine is not ‘acidic’ as quoted in that article. Indeed, if it stands too long then it becomes alkaline because urea breaks down to ammonia and CO2 (e.g. the ammonia smell of baby nappies left on too long).
  • “The Romans knew about this and used stale urine for washing clothes because alkalis are good at removing stains (which is why we use ammonia, or sodium carbonate / washing soda). I’m not sure if stale urine is a weedkiller, maybe worth testing.”

Marina Bistrin also wrote in: “Thanks for the urine article link – I enjoyed reading it and thought it brought up lots of interesting points. I use undiluted urine around my citrus tree’s drip line as much as possible and they are doing well (I have been doing it for 8 years, ever since I moved into my current home). I think if the soil were really dry, I’d water it in, but I don’t need to most of the time. Another benefit is that you don’t waste water by flushing that valuable substance down the toilet. ‘Humanure’ article next time? (not that I use it but I am interested). There is a man at the Sustainable Living Festival that has had a caravan there for at least 3 festivals, near the food area, and he collects it in wheelie bins below the toilet and keeps it for a year, and then it’s ok to use. This year he separated the wee out by putting a metal partition in the front part of toilet and asked the guys to sit to wee. Some people have composting toilets and I feel that’s a great water saving and use of valuable fertiliser resources.

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