Make your garden thrive: tips for healthier soil

An analysis of soil contamination has found the highest lead concentrations in Sydney exist in residential areas.

This research was conducted by University of Sydney PhD student, Liana Johnson, from the School of Life and Environmental Sciences in conjunction with Associate Professors Gavin Birch and Tom Bishop.

The study focused on the Sydney estuary catchment (with Parramatta River at its centre) using state-of-the-art geospatial data and statistical models.

Soil contamination in Sydney

“We found that 5 percent of this area had a predicted topsoil lead concentration exceeding the safe guideline for residential soil (300 mg/kg),” said Liana.

“The biggest area of concern was to the south of the catchment, in Sydney’s inner west.”

Other areas of concern lie closer to Sydney’s CBD where there has been significant pollution.

“This area contains older houses, which up until 1950 were coated in paint containing high amounts of lead.

“Exhaust fumes from traffic was also a large contributor to lead contamination in soil adjacent to roadways prior to the ban on leaded petrol in 2002.”

What’s causing high lead concentrations?

Soil has been significantly impacted by the human population and activities over time.

“The presence of heavy industry in parts of Sydney and other cities around the world has had a direct impact upon the soil.  Another factor is cars themselves. Emissions, brake pads and tyres all contribute,” said Liana.

It’s normal for soil to contain low concentrations of heavy metals, it’s just the high readings that communities need to be cautious of.

“There is no such thing as ‘bad soil’,” said Liana. “All soils have a purpose and can still foster growth – they might just not be as healthy for humans so it’s important to know about soil health.”

In light of contamination findings, homeowners need to be aware of how they can manage soil health to ensure a safe backyard for their families. Likewise, increased knowledge of soil conditions will give gardeners a better chance to have their plants thrive.

How to assess soil health

Become a detective. Look out for obvious indicators within your garden and assess your local surroundings.

If you are concerned, Liana recommends purchasing a lead test kit from a hardware store or online. Alternatively, you can get your soil tested by a laboratory so you know for sure.

Sydney’s soil types

“The type of soil you may encounter is often linked to the underlying bedrock. The bedrock in Sydney is mostly sandstone and shale. Areas upon sandstone are likely to have sandy soil, while areas located over shale are most likely to have clay based soil.

“Neither of these two natural soil types in the Sydney region are particularly rich in plant nutrients.”

Each soil type has pros and cons.

“Sandy soil is well drained, but it also means nutrients can be washed out, while clay soil has poor drainage and can be easily compacted. This makes it harder for plant roots and organisms to move through the soil.”

In an ideal world, a combination of these soil textures will provide optimal conditions for plants and soil organisms, along with a healthy dose of organic matter.

Ways to improve soil health

“Add compost,” said Liana, “this organic matter contains much needed nutrients for plants and soil organisms to survive and grow.

“Applying it to sandy soil can also help improve soil moisture and help retain nutrients for a longer period of time.

“If your soil is a heavy clay meaning it traps a lot of water, is hard to dig in, or plant roots can’t spread, it can help to add gypsum to help break up it up.”

Liana suggests that gardeners should be wary using pesticides as this too may negatively impact the soil. Instead you should introduce plants which encourage beneficial insects to help control the pests.

“Insects help break up the soil, and depending on the species they can help to break down organic matter (e.g. the Dung beetle), or eat other insects (e.g. lady bird eating aphids — a common garden pest).”

Provide optimal conditions for soil organisms such as this and you will help your plants thrive.

“Earthworms break down organic matter, turning it into a source of food for plants. They also break up the soil and aerate it, providing air for the millions of helpful microbes living in the soil.”

Keep your garden safe

“If your topsoil has high concentrations of heavy metals, the biggest concern is ingesting or inhaling the soil — especially if there are children playing in the yard,” said Liana.

“It’s a good idea to avoid leaving areas of exposed soil. Covering the area with turf can help.”

Plants can also be a great way to absorb metal contaminants Liana suggests, with many known species being able to withstand high metal concentrations in the soil. 

“These plants are known as ‘hyper accumulators’, examples include Pig face, which absorbs cadmium, and Bracken Fern which is able to accumulate high amounts of arsenic.”

It’s still possible to have a veggie garden in Sydney too.

“Concerned veggie gardeners should use lots of compost on the garden which can capture the metals present, preventing them from being absorbed up by the plants.”

“If metal concentrations in the soil are very high, veggies should be grown in raised garden beds filled with clean soil to prevent plant roots taking up the contaminants.”

“We also recommend rinsing veggies before use.”