If your ground has too much clay you're stuck with a big problem.
if there is some drainage Calcium Sulphate also called Gypsum will improve drainage even when yr plants are quite advanced. they might turn yellowish for a day or so but the Gypsum won't hurt.
next time maybe make raised beds. I 've grown plants in six inches of soil over pure clay.
"Life is ... a Continuum ... with no Beginning and no End. It is not going Somewhere ... it is always Here." Osho
"Human consciousness, at the same time as being a form of Awareness, Sensitivity and Understanding, is also a form of Ignorance.
The ordinary everyday consciousness that we have ... leaves out ... more than it takes in. And because of this it leaves out things that are terribly important. It leaves out things that if we did know them would allay our anxieties, fears and horrors. We would have a deep, interior Peace.
Now ... the question that is absolutely basic for all Human Beings is: What have we left out?