Monday, April 26, 2010

Taking a closer look at ... leftover Gyprock

The shed received a much-needed clean-out today. With the straw bale cottage being built over the past six months or so, the shed has accumulated piles of fittings and fixtures, bits and pieces, odds, ends, and off-cuts.

I'm a pack rat from way back, so I find it very hard to throw anything away - because you just never know when it might come in handy! (I come from a family where things like string and rubber bands were always saved for re-use. And I was honestly surprised the day I learned that most people don't unwrap presents carefully in order to save the wrapping paper for another occasion).

So ... what to do with the many off-cuts of Gyprock plasterboard which have been piling up neatly (but uselessly) against a wall in the shed?

On pondering this question, my mind wandered back to an image on page 76 of Peter Bennett's book "Organic Gardening (6th Edition)". The image is really quite memorable: it shows a lady with neatly coiffed hair and a generous bosom, wearing a tight pink t-shirt, blue flares, and red sandals. Here she is:

Anyway, the significant part of the image is not the lady herself, but rather, what she's doing, which is: dressing the soil with gypsum.

What is gypsum?

Gypsum is a soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O (thanks Wikipedia).

Among gardeners, gypsum is renowned for its ability to break up clay, making it more workable and improving its drainage. And according to Peter Bennett (whose photo I borrowed, above):

Being moderately water soluble, gypsum readily dissociates into free calcium ions and sulphate ions thereby supplying two major plants nutrients in a very readily available form. [...]

There can be no doubt whatever that many of the so-called "superphosphate responses" in crops have nothing whatsoever to do with the phosphate content of "super" but are purely sulphur responses arising from the sulphate radical of the gypsum which is the "other half" of superphosphate.

What's Gyprock made of? Gypsum!

So, why not compost the scraps of Gyprock, and put them in the garden? I wasn't 100% sure if this was a good idea, so I consulted Google, and discovered this discussion over in the Gardening Australia Online Forum.

Someone on the forum simply wanted to know if it was safe to use leftover plasterboard in their garden - but, as far as I can tell, there's no such thing as a simple answer on a gardening forum.

Some people were sure it was fine, while others thought it was a very bad idea. One person believed that using plasterboard in the garden would be dangerous because (he claimed) plasterboard is allowed to contain up to 100mg/kg of lead under Australian standards, which is (he wrote) "a very high concentration of lead".

Hmmm. What to think?

My next move was to check the manufacturer's MSDS (material safety data sheet). You can find CSR's MSDS for GYPROCK Plasterboard here.

The MSDS contained no unpleasant surprises.

It stated that Gyprock is >95% gypsum, and 4-9% paper lining (don't ask me about CSR's arithmetic or how they can fit >95% + 9% of material into a product. I dunno).

The worst risk they mention in relation to Gyprock is that "repeated exposure may cause skin dryness", and, if you get hungry enough to eat the stuff, you may experience "abdominal discomfort."

The "Hazardous Decomposition Products" listed are: "None".

So what about the risk of lead contamination?

I work in the chocolate industry, where lead contamination is a very serious issue. For example, did you know that commercial cocoa powders quite commonly contain 0.2ppm of lead? That's twice the concentration considered safe by the US FDA for children to consume.

So, if more than 0.1ppm of lead is considered to be unsafe for children to consume, then a concentration of 100mg/kg (that's the same as 100ppm) in plasterboard is astronomical ... isn't it?

Well, yes and no.

First there's the fact that most plasterboard probably doesn't contain anything like that amount of lead. Even if the standards do technically allow such high levels (and I'm not convinced that they do) I can find no evidence to suggest that the average piece of Gyprock actually contains any lead whatsoever.

Then there's the fact that it's not at all uncommon for commercial fertilisers to contain all kinds of heavy metals and other nasties - including lead, cadmium, and even arsenic. This is because fertilizer often consists of various industrial by-products from environmentally dubious industries such as mining. (I nearly fell over when I read the small print on a bag of citrus fertiliser once about ten years ago, and I've never touched the stuff since then).

The third thing to keep in mind is that a particular lead level in soil doesn't produce anything near that level of lead in plants grown in that soil.

Carl J. Rosen of the University of Minnesota wrote an informative article on the subject of Lead in the Home Garden and Urban Soil Environment.

He explains that surface agricultural soil in the USA contains an average lead concentration of 10ppm, and that dangerous exposure to lead comes from two major sources:

1) lead-based paint, and
2) auto emissions.

Furthermore, Rosen explains:

The most serious source of exposure to soil lead is through direct ingestion (eating) of contaminated soil or dust. In general, plants do not absorb or accumulate lead. However, in soils testing high in lead, it is possible for some lead to be taken up. Studies have shown that lead does not readily accumulate in the fruiting parts of vegetable and fruit crops (e.g., corn, beans, squash, tomatoes, strawberries, apples). Higher concentrations are more likely to be found in leafy vegetables (e.g., lettuce) and on the surface of root crops (e.g., carrots).

Since plants do not take up large quantities of soil lead, the lead levels in soil considered safe for plants will be much higher than soil lead levels where eating of soil is a concern (pica). Generally, it has been considered safe to use garden produce grown in soils with total lead levels less than 300 ppm.

We live in a toxic world where even breast milk is almost universally tainted with toxic chemicals. Sad but true (and just one more reason why I'll be remaining childless by choice).

If I had to choose between having a fertile garden with a few grams of lead in it, or an un-fertile and unproductive garden, I'd take the fertile garden every time.

So, it looks like my scraps of Gyprock will be going into the compost - and I'll be sure to let you know if I die of lead poisoning any time soon.


  1. I note that the Wikipedia article on Gyproc mentions that on average 17% of this material is typically wasted on any building project.

    As a result there is a push in the industry to re-use it on-site for clay soil remediation and also to use it in composting ...

  2. I love the picture
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  3. Thanks, read the gardening Australia blog just before yours. I agree considering the toxins in fertilisers and inherrent in suburban life, it a very minimal risk.

    j & a

  4. Plaster board would have paint on it and most houses being renovated will have atleast one Layer of lead paint