Turnips are a good idea.. But I'm not real sure about what to plant without using fetilizer even though they will still grow in lower fertility soils. . I have used Delt-Ag in the past and I do think it helped new roots take hold as far as strength goes. It kinda gave them a "headstart" which will help in the long run. You could try rape, sugar beet, or brassicas, but you may have some issues with competetion for nutrients from other weeds/grasses if you dont use a herbicide either. And often times brassicas need a little more fertile soil depending on what you get.

These are just some thoughts, Im sure others will have better ideas/info