The short answer is to buy a load of good garden loam and to improve it with manures etc but that is not necessarily all that is involved because of drainage issues. If you have heavy clay soil one thing to avoid is turning the hole into a pond again. If the surrounding soil doesn't drain well and you fill the hole with good soil then during wet periods the hole will fill with water turning the fill into a bog which will kill your veges.
What is the surrounding soil like down to the depth of the pond? Is it stratified, if so in what way?
Another way to investigate it is to put water into the hole and time how long it takes to run out and also check if it runs out but only down to a certain level. While this direct test will give good information it has some potential difficulties due to the size of your hole. It might be better to dig a much smaller proxy hole the same depth nearby to test in this way.
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