We have the same problem on our site which has been there for around a 100 years. Not only is the soil exhausted it is really bare in places and being on a slope it makes it even worse.
Many of the plot holders have adopted various strategies to bring the soil and the worms back
Green manures. several of us now sow that at various times of the year and dig it in.
Compost heaps: Produced on site using plot and home waste. spread on top, dug in, trenched and left to rot.
Bulk compost: Our local authority sells compost made from recycled green waste. Remarkable value at £14 tonne if you collect it yourself. We use it as a top dressing or make raised beds from it.
Wild flower beds / fallow. To encourage the wildlife back into the plot, several of us have "wild" beds which apart from pernicious weeds are left uncultivated with flowers to attract wildlife.
In the last 18 months we have seen a huge increase in not only the worms but also other elements of the food chain insects, slow worms, snails and slugs, birds, foxes. Although the increase in birds brings mixed blessings and the slugs and snails much, much less benefit