It seems to me that any crop that is eaten would not be very soil building as you are removing nutrients from the soil. An uneaten cover crop can be soil building. I like the daikon radish cover crop which may be called forage radishes or "tillage radishes' under PVP registration. If these radishes are planted about 5 inches apart in a carpet fashion, they can grow to nice root size.

When you say that your soil is 'fine', do you mean fine in particle size? Clay is the finest size and sand the largest unless you have rocks.

He possibly means something that encourages good micro-flora, that is, mycorrhizae. I am unsure of the mic. contributions of many of the plants I grow. I do know that allium develops the flora and all brassica and beta suppress it. Generally, fruiting plants develop the flora, including potatoes (part of the fruiting family solanaceae) and all legumes. On the other hand, right now all the best candidates for a crop before the end of season are brassica: daikon, pak choi, arugula, kale, broccoli raab, etc.
Unsure of the mic. association of lettuce, carrot, celery, chicory.

If one wants just nitrogen, any bean will do. I suggest favas (the greens are as good as pea greens, and favas fix three times the nitrogen). For biomass, it is hard to beat daikon, if you only eat 50% of it. Fast types of chicory also leave large roots in the soil, but unlike daikon here in Michigan, they often don't die in winter.

There are many cool season legumes,including beans that fix N as well as improve tilt.

Fava beans are awesome...but they can grow rather tall.

As far as mycorrhizae-helping plants...most TRUE natives to an area help with this. Additional research is needed to determine the best plant for your situation, but true natives are generally the starting point for looking into this. Ericoid and ectomycorrhiza fungal-friendly natives are what you're looking for. Ectomycorrhiza plants tend to form vast root systems, though...so that may or may not be a concern. Either way, pick carefully the plants you choose for size and spread.

Plants do not, as they are growing, encourage the fungi that form the mycorrhizal (or symbiotic) relationships but a good healthy soil will.
Legumes, Fava Beans or any other beans, Peas, Lentils, Peanuts, etc. are all plants you can grow and eat the produce of as well as benefit the soil they grow in. Aside from those almost any plant growing is better then leaving the soil bare and exposed to the ravages of Ma Nature, besides she does not like bare soil and so plants things we do not want growing there (we call them "weeds") to keep the soil covered. Even corn could be grown and turned in to benefit the soil.

There are so-called mineral "accumulators" like comfrey and nettles, and many other medicinals.

All good points, but the op has an immediate time frame to work with on his or her plot.

Sure plants engage or not engage in mic. relationships. Woodland derived vegetables generally do, and seashore derived vegetables generally do not. Those who do start a relation make better use of organic matter. Brassicas, in my experience, just want fertilizer, and not too acid soil.

From a Bonnie Plants web site, "Never stop learning about how to improve the soil. It pays off in healthy, more productive plants."
http://bonnieplants.com/library/articles/soil-soil-building/
What ffreidl is asking about is known as succession planting.

Here is a link that might be useful: succession planting

there are specific ones like the nitrogen fixers, and like comfry take nutrients from deep down. but my experience would say especially in vege' gardens everything you grow converts what it need in the soil, so we recycle our spent vege's right back where they grew.

that is followed by lots of green mulch with kitchen scraps tucked under, too easy really.

len

Here is a link that might be useful: lens garden page

Well, all I can say is: Wow!! I love this forum!

Thank you everybody. You all have hit on exactly the things I was wondering about: Nitrogen-adding plants, mineral accumulators, Myccorhizzae (sp??).

I never thought about Daikon radish or fava beans, so that's interesting to consider.

Regarding favas, does anybody know if the greens have any of the chemical in them that the beans have which are the reason you're not supposed to eat them raw?

Thanks again!

The fava greens are very nice, if a bit stringy (then again pea shoots are stringy too). Eat raw or cooked. The greens do not induce favism, it is a substance that is specifically in the seeds. I use cattle fava (cover crop favas), which is much cheaper, seeds are as big as a soybean, the plants get five feet tall if they can overwinter (here they die, Zone 8 and higher they overwinter). The taste and appearance of the greens, cattle vs established varieties like Windsor, is indistinguishable. Speaking of the huge plants, they provide you not only with the most nitrogen, but also the most biomass over winter. There is a reason they have been heavily used for millennia.

Thanks Glib. That's great info. Appreciate it!