Topsoil depth for growing grass over a clay base

Question

I am in Tasmania, a southern state of Australia. My soil is clay (although not heavy clay) - grass will typically grow OK on it.

I have sprayed the area I want to grow new grass with glyphosate - as soon as everything dies off, I will be looking to sow new seed.

I have a rotary hoe that I can use if needed.

I will be bringing in new "lawn mix" topsoil which is just a topsoil with added conditioners, fertilisers and composts for growing grass.

What depth should I use over the existing soil base, and do I need to hoe the clay before I put the new topsoil down?

I have heard figures ranging from between 50-150mm, and I am working with a very large area, and don't want to purchase more topsoil than required. I'm also very open to any other advice you might have to share to ensure the success of the new grass.

Answer 1

Clay soil is often a nigtmare for lawns. They're dense, compact and have associated problems with root integrity for grass. Top soil must be pretty thick for a good lawn, but the real strategy is breaking up that clay. Unlike trees or woody plants, Grass doesn't have a deep taproot, grass produces a horizontal root called a "Rhizome" it's principal storage for nutrients and water and what it grows from if it's clipped or eaten (Grazed). A rhizome that can be several inches deep underground is less vulnerable to digging, drought, extreme heat. Clay soil must be broken up so tilling is a must and as you till mix it with some gritty sand for good drainage then add with it a top dressing.