Prominent soil scientist Dr Peter May of The University of Melbourne said knowing as much as possible about the soil was a key part of creating a successful garden. He was speaking to GGP members during a live-streaming event in February. Written by GGP member and volunteer Megan Hallowes.


Landscapers need to have good information before deciding how to provide the right soil environment for plants in their projects Dr Peter May, one of Melbourne’s leading soil scientists, told GGP members earlier this year.

Dr May said professional soil testing measured things that influence plant growth such as drainage, pH and salt and nutrient content.

“I encourage you to do as much soil testing as you can afford or can convince your clients to pay for,” he said.

He added many landscapers make decisions based on just looking, or on past experience, and that while sometimes this would lead to acceptable outcomes, there were times when it will not.

Knowing the soil conditions at the start can prevent costly mistakes.

As an aid to understanding soil drainage in a garden, Dr May recommends “the percolation test” which is simple and provides valuable information.

“Dig a hole as deep as you need for planting. Fill the hole several times, allow it to drain between each filling, then make a final fill and leave it to drain and measure how long it takes to empty. What this is going to give you is an indication of how well the site drains overall which you can use to make decisions about whether you need to install artificial drainage, or whether you need to change your planting palette…….Ideally you want it to drain in less than 24 hours.”

Once you’ve got the particular information on soil together, then you can decide: if it’s ok as it is; if the site requires minor tweaking (eg adding nutrients); if the site requires significant inputs; or the site requires an entire new profile.

The cheapest and greenest option is to make as little disturbance as possible.

“Remember that plant selection is cheaper than soil modification, especially where soil is poorly drained.”

Dr May said a useful, hands-on guide on how to use soils in landscapes was “Soils for Landscape Development” by Simon Leake and Elke Haege.

“In my view, every professional gardener should have a copy and use it.”

This book and others show that there is a finite number of different soil “designs” required in most landscapes.

“When we’re building soils from scratch or renovating seriously degraded soils, ultimately what we’re trying to do is fit them in to a relatively small number of patterns,” he said.

For example, it could be turf, or turf with wear, or flowering plants, or shrubs. There’s a standard profile design for each of these options.

Once the design has been classified, one can classify the site soil resource.

Highly disturbed, poorly drained soils are what a lot of garden contractors have to deal with.

A soil improvement method trialled in the United States, known as “Scoop and Dump”, has had success rebuilding compacted, degraded soils.

You spread compost over the degraded soil, go over with a backhoe, mulch and plant.

“It is a low tech, relatively cheap method and the published research information seems to suggest it provides remarkable soil improvement and enhanced plant growth,” he said.

Melbourne research recorded better drainage and better water holding capacity using this method.

“It means you can work with pretty nasty stuff and avoids having to dig it all out and replace it which at the end of the day is much more complex,” he said.

If you do have to build a soil profile from scratch, Dr May recommends having specifications based on things you can measure. Normally, the specification includes ranges for pH, salt, nutrient content, toxicity and a measure of drainage rate.

“Why I like Leake and Haege is that it provides cut and paste generic specifications you can take to the supplier. For example, if you are planting trees in a garden bed you would have a specification which is slightly different from trees planted in a lawn.”

It is important when importing soil to build new profiles to keep compost close to the surface (no more than 200 mm deep) to avoid anaerobic decomposition.  Most guidelines also suggest that maximum added compost levels of around 20% by volume will reduce the risk of soil volume loss as the organics decompose.

During the presentation, Dr May showed a photograph of the Forest Gallery at the Melbourne Museum, which is an example of a completely artificial soil profile in action. It is a recreation of cool temperate rainforest in the Eastern Highlands and has been operating since 2001.

References: “Soil for Landscape Development by Simon Leake and Elke Haege, “Soil Design Protocols” by Timothy Craul and Philip Craul.

 For more information on the “Scoop and Dump” method: Susan Day at Virginia Tech: and Nina Bassuk (Cornell University):

GGP members can access the hour-long video of Dr May’s presentation via the GGP Industry library.