Vegetable garden soil may become contaminated for many reasons. Some gardens were essentially used as rubbish tips in the past and so may have become contaminated by deliberate burial of unwanted waste. Some gardens may have had contaminated soil brought on to the site, while others may have had contaminated water or fertiliser applied. An example of the latter is chicken manure which may contain arsenic due to the addition of Roxarsone (controls coccidial intestinal parasites) into chicken feed.
One of the main reasons for growing your own veges is to make sure you are eating contaminant free food. However if your vege garden soil is contaminated, this may be uptake by the plants and you may be ingesting harmful chemicals.
The following table lists common contaminants, their source and guideline values:
Contaminant
Chemical Class
Biogro Guideline for Soil
(mg/kg dry wt)
Comment
Arsenic
Heavy Metal
<20
Arsenic sprays were used to control sheep parasites, but can be found naturally at high levels. Also used as a timber preservative.
Cadmium
Heavy Metal
<2
Contaminant in imported Phosphate rock which is used as a fertiliser.
Chromium
Heavy Metal
<150
Used as a timber preservative.
Copper
Heavy Metal
<60
Used in horticultural sprays and as a timber preservative.
Lead
Heavy Metal
<100
High levels in Lead based paints, also used as an anti-knock additive in petrol.
Mercury
Heavy Metal
<1
Previously used in temperature thermometers.
Nickel
Heavy Metal
<35
Zinc
Heavy Metal
<300
Used extensively in industry.
DDT
Organochlorine Pesticide
<0.2
Used to control grass grub and other pests.
Lindane
Organochlorine Pesticide
<2.0
Here are some other possible contaminants in vegetable garden soil
Contaminant
Chemical Class
Comment
Boron
Heavy Metal
Used as a timber preservative.
Dieldrin
Organochlorine Pesticide
Previously used in sheep dips.
Chlordane
Organochlorine Pesticide
Insecticide, as well as timber treatment chemical.
Pentachlorophenol (PCP)
Chlorinated Phenol
Previously used as a timber preservative
Benzo[a]pyrene (BaP)
Polycyclic Aromatic Hydrocarbon (PAH)
High levels found in creosote (used for timber treatment), bitumen and tar oil (roading). PAH shouldn’t be a concern for high water content plants such as lettuce, however plants with high oil content may be susceptible to uptake.
Here are the following testing options that we can deliver to someone wishing to test their soils in a vegetable garden:
Profile
Compounds in Suite
Cost (all prices excl. GST)
Heavy Metals + Mercury
As, Cd, Cr, Cu, Ni, Pb, Zn and Hg
$90
Organochlorine Pesticides
Includes DDT, Dieldrin, Lindane etc
$110
Pentachlorophenol (PCP)
PCP and its main metabolite, TCP
$120
PAH
Includes benzo[a]pyrene and a number of other PAH
$110
Taking a Sample
Make sure that your sample is representative of the soil in your garden. The best way to achieve this is to take a number of small samples from different places in the garden using a small hand trowel, and put them into a resealable plastic bag. Mix well, remove any weeds and rocks, and send to the lab along with a request form.
In terms of depth, sample the top 3 inches of soil unless you are growing root vege’s, in which case sample to 6inches.
Sampling equipment:
A small hand trowel and some resealable plastic bags.
Unsure about the correct container to use to submit a sample? Consult our container guide
