Soiled soils…heavy metal contamination in garden soils

Healthy soil

Healthy soil bursting with life

The health benefits of growing your own vegetables are well researched and many people dig up their lawns in favour of a backyard vegetable garden. There is nothing nicer than doing your vegetable shopping in your own backyard and nothing beats the taste of vegetables picked moments before you eat them…but have you ever wondered about the health of the soil you are growing them in?

The soil in your backyard has a long history and you don’t always know how it was used and cared for before you become its custodian. Many soils, particularly those in older urban and industrial areas and those close to main roads, are contaminated with chemical residues from past activities on that land or from pollutants from the surrounding area. Of particular concern to backyard vegetable growers and gardeners are high levels of heavy metals in your soil. The heavy metals and metalloids which pose the greatest risk to human health when found in soil are Arsenic, Cadmium, Chromium, Copper, Manganese, Nickel, Lead and Zinc. These heavy metals when ingested have been linked to cancers, kidney, bone and nerve damage. As gardeners, our exposure to heavy metals comes less from eating the vegetables we grow and more from the accidental ingestion and inhalation of soil and dust while gardening or from soil dust brought into our houses on gardening clothes and shoes.

Digging in the garden

In urban areas, the heavy metal of most concern is lead, which may have accumulated in the soil due to the past use of lead-based paints on old houses, sheds and fences and from the emissions of cars run on leaded petrol before it was phased out. Soil naturally contains small amounts of lead in the range of 15-40 parts of lead to one million parts soil (ppm). In some urban areas however, lead levels in the soil may be as high as 500-10,000 ppm which is way above the level of 300ppm which is considered safe in Australia.

The good news however is that here is no need to stop growing vegetables in your backyard. If you are concerned about the health of your soil, there are two things you can do. The first is to have your soil tested for heavy metals and the second, is not to test it but assume it could be contaminated and take the relevant precautions. Since 2013 Macquarie University has been offering free soil testing for vegetable gardeners in Australia via their VegeSafe program, although they welcome donations to be able to continue to offer this service. For further information on how to get your soil tested for free go to the VegeSafe web link at the end of this article.

Raised beds

Grow your vegetables in raised beds with new soil if you are unsure of your soils health

If you find your soil is contaminated you can still grow vegetables by using a raised garden bed with soil brought in from a clean source. Make your raised beds from white cypress and not from treated pine or recycled railway sleepers which can both leach chemicals into your soil. When growing vegetables, root crops are the most likely to contain high levels of lead as they grow in the soil and soil can get trapped in their skin. Peeling and washing these vegetables before consuming is recommended to reduce lead ingestion. Leafy vegetables may also contain higher levels of lead, but mostly due to soil particles on their leaves, so it is advisable to also wash leafy vegetables thoroughly. Fruiting vegetables have a very low uptake of lead making tomatoes, capsicums, peas and beans the safest to eat if you are unsure of the soil they are grown in.

washing carrots

Washing leafy and root vegetables well, can minimise heavy metal ingestion

As mentioned before, most ingestion of lead actually comes from ingesting soil dust particles and not from the eating of home grown vegetables themselves. To reduce soil dust from the garden being brought into your house where it can become airborne and easily breathed in, leave your garden shoes outside and wash your hands and vegetables outside before coming inside. Avoid breathing in dust particles from your soil when the weather it is dry and windy by covering bare soil areas with grass or mulch. Having your vegetable garden soil at a pH of 6.5-7 (which is the optimal pH for vegetables anyway) and adding lots of organic matter such as compost (also great for vegetable growing) means that any lead in your soil will be bound to soil particles in a way that it is less likely to be taken up by your vegetables. So its best to be informed and get to know your garden soil. Growing your own organic vegetables in clean soil and harvesting just before eating, is still hard to beat for quality, taste and nutrition.

Winter salad leaves

For information on getting your soil tested visit VegeSafe Soil Testing:


Urban Gardening, Managing the Risks of Contaminated Soil, Environmental Health Perspectives • volume 121 | number 11-12 | November-December 2013

How to make compost


These compost bays were one of the first things we made when we moved into our house. They have removable timber slats on the front which make accessing the compost and turning the heap easier.

About 42% of all Australian household waste that ends up in landfill sites is food or garden waste. There it is buried with all the other rubbish and decomposes anaerobically, resulting in the production of methane (adding to global warming at 25 times the rate of carbon dioxide) and leachates that have the potential to contaminate groundwater and pollute waterways if not contained.

Most of this food and garden waste is valuable organic matter that instead of going to landfill can be composted at home and used to improve your soil and grow amazing vegetables. Recycling your own organic waste into compost is great for the planet and easy to do. Every home should have a compost bin; so here is some advice on how to make compost…

Firstly it helps to have something to make your compost in and contain it. There are countless compost bins and tumblers available on the market that can all make great compost if used correctly. As we have a lot of organic waste to process with all the food we grow, I prefer to use homemade composting bays made from timber and corrugated iron offcuts. We have two bays at home, but you can have three or four for a large household; at work we have twelve bays to process all the organic waste from the kitchen and garden.

So what can you put in a compost heap? Basically any organic material can be used to make compost. That said it is best not to add meat or dairy products to the compost as they can attract vermin. The best things to compost are; weeds (not with bulbs or runners), grass clippings, non-woody garden clippings, kitchen scraps, leaves, animal manures (sheep, cow, goat, chook, horse, but not dog or cat), shredded newspaper, hay, straw and even hair, vacuum cleaner dust and old cotton clothing. Other things you may want to add in small amounts are lime, blood and bone and rock dust.

A good mix of ingredients will have a carbon to nitrogen ratio of 30:1, but what does this mean? I like to think of this ratio as equal parts in volume of dry (usually brown) things to fresh (usually green) things.


Nitrogen rich ingredients


Carbon rich ingredients







Nitrogen rich ingredients include; fresh grass clippings, vegetable scraps, fresh animal manure, lucerne hay, weeds, garden clippings etc. While carbon rich ingredients include; shredded paper, dry grass, straw, autumn leaves, sawdust, shredded cardboard etc. As all the nitrogen rich ingredients also contain carbon, adding the ingredients from the two boxes in the photos above in in equal volumes, will give you a pretty good carbon to nitrogen ratio for making compost.

Hundreds of different types of microorganisms (mainly bacteria and fungi) go to work in your compost heap to break down your organic waste if the conditions are to their liking.   You don’t have to add them to the heap as their spores are everywhere. Their main requirements are the right C:N ratio, oxygen and water. If you have a compost heap of at least one cubic meter, the microbes will thrive and you will notice your heap getting warmer from the heat generated by the microorganisms as they feed. Ideally your heap will reach 60ºC at which point many weed seeds and plant pathogens die. You may need to turn your heap as it cools to kick start the microbial process again if your ingredients are not composted yet. You will also notice the volume of your compost heap decreasing by as much as 30-60% which is caused by the microorganisms consuming much of the carbon in the heap. If your compost heap gets too dry, add more wet ingredients or water your heap (particularly during summer). If your compost heap gets too wet (and it smells), mix through some dry ingredients and get the air back into the heap, it may also help to cover the compost heap with a tarp to keep the rain off.

When the microorganisms have done their thing your heap will start to cool and the worms will move in to finish off the process. Once your compost is ready it should have a pleasant earthy smell and you shouldn’t be able to distinguish the original ingredients. It is now ready to add to your garden beds. I love adding compost to my garden…it improves difficult clay soils and dry sandy soils, helps retain moisture and nutrients in your soil, improves soil structure, adds lots of beneficial microorganisms, it can stabilise the pH of your soil, add trace elements and combat harmful pathogens in your soil…what’s not to love!


Spinach plants love a compost rich soil.





Testing the pH of your soil

Why is soil pH important?

pH testing kit

Example of a soil pH test kit which you can buy from garden centres for around $20

Many gardeners may not be aware of how the pH of their soil can affect the growth and health of their plants. Without becoming too scientific, the pH (potential hydrogen) of your soil is a measure of how acidic or alkaline your soil is.

This balance affects the plant’s ability to access and benefit from nutrients which are present in the soil. In many instances, what appear to be signs of nutrient deficiency in plants (yellowing leaves, poor growth) can directly be attributed to an incorrect soil pH for that type of plant. Adding more fertiliser has no effect, as some nutrients become ‘locked up’ in the soil and not available to the plant.


When soil has a pH of between 6.5 and 7 most nutrients are readily available to plants

Most plants prefer a near neutral soil, which has a pH of between 6.5 and 7. When the pH falls below this, conditions are said to be ‘acidic’ when above they are said to be ‘alkaline’.

How to test your soil’s pH

soil pH kit contents

Contents of a soil pH test kit

To test your soil pH you will need to take a teaspoons of soil from just below the surface of the soil in various parts of the garden bed you want to test. Mix this soil together and then take one teaspoon full and place it on the white plastic sheet from your testing kit. Add a few drops of the indicator liquid to form a paste. Sprinkle some of the Barium Sulphate on the paste and wait a minute or so. Hold the pH chart from your kit next to your sample and compare the colour to find your soil’s pH.

checking soil pH

Reading your soil’s pH

So how do you change your soil pH?
If your soil is a little on the acidic side, adding lime (either dolomite or garden lime) will raise the soil pH. It is best to do this a little at a time so as not to shock your plants and to prevent adding too much and making the soil too alkaline. Should your soil be alkaline, it is a little more difficult to fix the problem, but not impossible. One of the fastest ways is to add sulphur. However, adding sulphur can be a temporary fix. Adding lots of organic matter or compost tends to be helpful long term, particularly when sourced from things of a naturally acidic nature such as pine needles and sawdust.

An alternative to changing the pH of your soil is to grow the things that don’t mind a slightly high or low pH. Below are some examples:

Vegetables/fruit that tolerate acidic soils down to a pH 5.5
Apple, berries (strawberry, blueberry, currants, raspberry, gooseberry), carrot, cauliflower, chicory, corn, cucumber, garlic, melons, parsnip, peppers, pomegranate, potato, pumpkin, rhubarb, shallots, swede, tomato, turnip.

Vegetables/fruit that tolerates alkaline soils with a pH up to 7.5
Artichoke, asparagus, beans, beetroot, Brussels sprouts, cabbage, cauliflower, corn, currants (white and black), garlic, grapefruit, kale, leek, mulberry, nectarine, parsnip, peach, pear, plum, pumpkin, spinach, strawberries.