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Welcome to the Bunnings Workshop community @justlearning123. It's wonderful to have you join us, and many thanks for your question about growing tomatoes.
I don’t love the sound of planting straight into that mix. Heavy clay on its own is already tough for tomatoes, and if the area stays wet, the clay will hold that moisture right around the roots. Even with compost added, a fifty–fifty mix is still going to behave much more like clay soil than a free draining garden blend, which puts the plants at real risk of root rot.
If there is that much clay in the ground, it’s unlikely you’ll be able to turn it into a suitable tomato bed just by mixing material through it. You’d be looking at something closer to ten percent clay and ninety percent compost or garden soil before it drains well enough, and even then, underlying clay tends to trap water.
I’d really encourage you to either build the planting area up above the clay layer, even just as a simple mound of good-quality soil, or set up a small raised bed. Large black plastic pots or grow bags also work brilliantly for tomatoes and give you complete control over drainage. Any of those options will give your plants a much better chance than trying to grow directly into heavy, wet clay.
Please let me know if you have any questions.
Mitchell
Our guide How to build a simple raised garden bed might be a good starting point for you @justlearning123.
Let us know if you need a hand sharing some photos so we can see what you have done to date.
Jason
