Soil testing


Testing Vegetable Garden Soil for Common Contaminants

Testing to determine that your vege’s are growing in contaminant free soil.

About this Testing

Reasons for testing vegetable garden soil

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

Heavy Metals + Mercury

As, Cd, Cr, Cu, Ni, Pb, Zn and Hg

Organochlorine Pesticides

Includes DDT, Dieldrin, Lindane etc

Pentachlorophenol (PCP)

PCP and its main metabolite, TCP

PAH

Includes benzo[a]pyrene and a number of other PAH

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 75mm (3 inches) of soil unless you are growing root vege’s, in which case sample to  150mm (6 inches).

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 catalogue

 

Guides and Supporting Information


Contact Us

Freephone 0508 HILL LAB (44 555 22) or email Env.CSM@hill-labs.co.nz