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.
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“The whole modern conception of the world is founded on the illusion that the so-called laws of nature are the explanations of natural phenomena.”
“… even when all possible scientific questions had been answered the problems of life would remain completely untouched” (Wittgenstein)