Down to Earth – Episode 1: Treating clay soil (1990)

[A close up shot of a plant growing on a trellis pans to Peter Cundall walking to camera with a spade over his shoulder]

Peter Cundall: In the final analysis everything wonderful comes from healthy soil. That’s why it’s probably our most precious resource and Kevin Handreck, look he’s been an inspiration to gardeners everywhere because of his deep knowledge and understanding of soil. That’s why we at Gardening Australia are so delighted to be associated with this project.

Here is a video cassette that’s crammed with information. It’ll be used again and again and every time you see it you’ll learn something new.

[Music plays, CSIRO logo appears and a verandah with flowers appears on screen]

[Text appears: Down to Earth with Kevin Handreck, CSIRO Division of Soils]

[New text appears: in conjunction with Gardening Australia, 1 Treating Clay Soil]

[Image changes to Kevin Handreck standing outside a new home prior to the establishment of a garden]

Kevin Handreck: We spend a very large amount of effort in very carefully selecting plants for our new garden but most of us don’t take much notice of the soil that they’re going to be growing in. If that soil is lousy then we’re going to have a great deal of difficulty in getting a good garden established. On this site here we’ve got a quagmire down the bottom here and up the slope here we’ve got massive runoff of water that’s taken the soil and the clay subsoil down to that lower area.

[The camera is panning over the ground as Kevin describes the different areas]

This is about the worst possible start to a garden you can have. The builder has just bulldozed up the subsoil to the side. He’s taken away the surface soil and put it somewhere else. Perhaps he’s going to even sell it back to us. But what they’re actually trying to do here now is to put a little bit of well, I suppose it’s topsoil. Yeah, it’s not too bad but it’s still a bit clayey. They’ve put a thin veneer of that over this crud underneath and now they’re putting some instant lawn over the top of that. Now that’s a recipe for disaster. I’ll show you just over here what’s happened.

[Camera zooms in on Kevin walking across the instant lawn]

This lawn doesn’t look too bad. It’s fairly lush but you can see that I’m really staggering around in it. It feels like quicksand underneath me here or perhaps it’s quick clay because that’s really what it is. It’s just soft clay under here. You can see that there’s water lying all over this site here.

[Camera zooms in on the water collected in the bark chips next to the grass]

There’s no drainage system here at all. The water isn’t getting away from this site and we might as well just have a lake here in fact. But if we do want to have a garden here what we need to do is put a drainage system in to take the water away but more importantly, the soil itself has to be improved.

[Camera pans across houses in a new estate]

All of the houses in this area are built on much the same sort of heavy clay subsoil. This site here you can see that there hasn’t been any landscaping done yet so I can show you just what should be done to improve this soil, get it ready for planting. The subsoil surface here has all compacted. The machinery’s run over it and the rain has beaten down on it. It’s very hard for water to get through that sort of compacted surface and of course if water can’t get through plant roots have a great deal of difficulty in getting through too. The first thing to do in this sort of site is actually to relieve that compaction, to start digging into it like this and you can see it’s pretty hard.

[Kevin starts to chip the ground with a spade, but little impact is made as the ground is so hard]

I’m going to be here all day and probably all weekend before I can do anything with that so I’m really going to have to get some machinery in.

[Images changes to show Kevin crouching by small piles of sand, compost and soil]

OK well, look once we’ve relieved the compaction we can add several different things to this soil, one or several of them anyway. We could add sand. Now there’s some caution here. If you want to add sand just be very careful. Just adding a little bit is not going to make much difference at all to the properties of this soil down here. If you really want to add sand use two or three times the volume of soil that you’re incorporating it into. That will then make a sort of a loam out of the clay. The other things that you can do are to put on some gypsum, something like one to two kilos of gypsum per square metre of soil. That will allow the clay particles to aggregate together and so allow water to infiltrate between them and of course the roots can then go down and get the water down there. The other thing that you can do is to incorporate some organic matter, some compost. You can buy it in. There’s plenty available these days. Just a caution about mushroom compost, it’s very alkaline and you may increase the pH of your soil too much if you use a lot of mushroom compost so just check that. Check the pH.

Now the final thing that you need to do after incorporating all those things into the subsoil is to put some topsoil over the whole lot. It may have been stockpiled on your block and then you can just bring it back over or you might have to buy it in. We’ve got some here that’s rather a nice sort of mixture of some soil and some organic matter. That’s good so you spread that over and then you’re ready to do your planting.