Others can answer the more detailed parts.. But you could always just dig one up a bit to see if you see the nodules are formed. I tried hairy vetch for the first time late fall ... Not thinking they would do much. They seemed to barely hold on through winter since it was such a late start... But I just dug some up that are only a few inches tall and the all have nodules growing.

I will be interested what the experts have to say!

Divine: I'm sure tilling in fava beans at anytime is better than not growing them.. As far are decline in plant productivity.. I would apply compost, and trace elements(azomite,kelp, woodash) and you should have the productivity, if not better productivity than what your used to.. Your right on with growing a cover crop..

Nevermore: vetch survives your zone 6b.. What state you in? I was planning on planting vetch but people say it wouldn't survive and it has hard seeds and is a pain.. Did you inoculate the vetch with the bacteria or whatever? Do you think vetch would survive my zone 5 Ohio winter?

Thanks,
Joe

I'm not familiar with tilling, but I've always been told that it's best to cut legumes and leave the (hopefully nodulated) roots to decompose IN the soil.
I've cut favas, applied compost and other amendments, mulched and planted on the same day, although I'm not usually that organised!
I leave the fava stalks long enough to just protrude through the mulch, so I know where I can cut through the roots to plant.
From what I understand, 'fixed' nitrogen only becomes available to plants after being processed by soil bacteria, and exposing roots by tilling means a lot of nitrogen is lost.

Favas are a bit different than some/most green manures...the tops contain a decent amount of N and tilling is advisable for the tops.

If you're cutting then applying compost over it...it's about as good, though. The main thing is not leaving it around to volatilize off into the atmosphere during breakdown.

Tilling mature legumes of all types helps break up the nodules to make the N more immediately crop accessible, though (assuming the tilling is hard enough to scar/open/break nodules). That said, given that they're well protected in the soil, they'll all eventually be made available...it's just a question of how much of a "quick dose" of plant available N you're looking for in tilling vs. not tilling.

Fava, or Broad, beans have been used for eons to improve soils both as a green manure crop and because they can fix Nitrogen in their roots. But I doubt that they have even started that at 6 inch growth since most legumes do not start to fix N until about blossom stage.

Thanks for the replies, everyone. To clarify, by tilling I just meant chopping up the plants with a shovel and turning them under the soil. I was mistaken, the plants are actually about 18" tall and not 6" inches as I said. For anyone who's interested, I'm attaching a photo of one of my plant's roots to show the nodule development at this stage.

The thought of you tilling under favas just as they flower is breaking my heart. Forgoing the wonderful scent and the delicious beans for the sake of waiting a couple of weeks. At 18 inches you don't have long to go. I can't wait for my first plateful of the year. Why not eat the beans and then turn the plants under?

If you're doing it for green manure, that's the perfect time to till them in...well, before they set pods.

Once they start producing beans it starts "emptying" the plants of the nutrients (especially N and P...and some micros like Mg and Cu) you're trying to return to the soil.

Though it's not a huge N source compared to other legumes with root fixation, the tops are rather N-heavy, so tilling the whole plant in does a good bit of good as far as a green manure.