What will "fix" Nitrogen would be the legumes and they do not normally grow too well in the normal winters we have up here. Winter, or Cereal, Rye or winter wheat would be the most readily available cover crops for the time you are thinking of since both are seeded in the fall for harvest (in your case mowing) in the spring. Vetch might be a legume you could try but that is usually seeded with either the rye or wheat.

My tried and true solution is winter field peas. They'll get some growth in the fall and really take off in the spring. Even early when your soil is wet you just pull the plants up and have a very nice planting bed. Now if you don't turn in the top growth you won't get the benefits of the crop's nitrogen. Tom

Thanks for the advice folks.

A question about the winter field peas- Would I still get the nitrogen benefit if I shredded the top growth & worked it in as I cultivated the top few inches of soil, or does it need to be deeper? Or could I just put the top growth in the compost pile for later use?

Thanks

Any Nitrogen benefit you get from any legume will be in the nodules on the roots, not the top growth so what you do with the top growth will make little, actual difference in your soil other than as part of the total amount of organic matter added to that soil.

kimmsr, I'm not too sure about that. I think the nodules feed the host plant, in this case the peas. Then turning in the top growth puts the N back into the soil. Will look into this some more. Tom

I just Googled "legumes, nitrogen fixing bacteria". The top sites, from University of Edinburge, New Mexico State University and University of Florida all describe the relationship between the nitrogen fixing bacteria and the host legume as symbiotic. The bacteria infect the root, the plant supplies energy nutrients to the bacteria and the bacteria fix nitorgen from the air and supply it to the legume for its protein needs. Little to no free N is available to nearby plants and the N is returned to the soil with the death of the legume top growth. I think that by using legumes because the N fixing bacteria will supply N to other plants is probably one of the biggest "old wives tales" in gardening. Tom

I grew hairy vetch as my first cover crop. Supposedly you're supposed to add innoculant the first time, but I didn't know and my roots were covered with those things.

The stuff was not only gorgeous (I mean GORGEOUS...totally fell in love), it really brought in the ladybugs. I'll be growing it again this year.

skagit goat man...

I think the experience of organic
gardeners who have undersown legumes
with success tends to argue against
the idea that legumes only fix as
much N as they need.

I've been very pleased with a combo of cereal rye and austrian field peas.
If you are transplanting or planting in "hills", just mow it down in the spring and dig your holes.
If you are planting small seeds such as Lettuce or planting Corn by mechanical means, you will probably be more satisfied with a seedbed created by shallow tilling several times interspersed over a couple of weeks.
The cereal rye creates biomass (shoulder high plants) and the austrian field peas use the rye for support and they generate nitrogen.

Legumes "fix" Nitrogen from the air, provided the right bacteria are present to aid in that process. These plants "fix" that N in nodules on the plants roots and when the plants die that N is now there in the soil to be used by soil bacteria to feed other plants, just as as the bacteria die they become sources of nutrients for other soil bacteria that can be used to feed plants. Perhaps spending some time with the Soil Food Web Primer can help.

Here is a link that might be useful: Soil Food Web Primer

kimsr, First I want to be sure we're discussing the same topic, the symbiotic relationship between the rhizobium bacteria, the legumes which they infect and nitrogen fixation. It's not a discussion on the interactions of the entire "food web". I'm familiar with Dr. Ingham and her good work. It seems to me that when you wrote above that legumes fix N from the air and fix that N in the nodules on the plant roots you are contradicting Dr. Ingham. In the link you posted she talks of the organism fixing nitrogen from the atmosphere and making it available to the plants. She and I say that the bacteria fix the N and you say the legume does. Did you check out any of the university sites on the subject? Where on her site does she state that these bacteria in this symbiotic relationship provide free N into the soil? This shouldn't be a hot topic. It's been well studied and doucmented in the literature and is a stepping stone on which Dr. Ingham built her work.

decklap, on an annual legume as the seed pod fill out the bacteria generally lose their ability to fix nitrogen. This is because the plant feeds the developing seed and not the nodules. See Guide A-29, Cooperative Extension Service, New Mexico State University.

Tom

The top growth contains nitrogen. The nodules fix N, which gets incorporated into the plant. Therefore- if you turn in the top growth- you will add a source of N.

You can also pull it up, let it die on the soil surface, and mulch over it rather than the work of digging.

Saying that the legumes fix nitrogen is a short-hand way of saying that the bacteria in legume root-nodules fix nitrogen.

The plant provides an anaerobic environment and the food the bacteria need. The plant roots take up the fixed nitrogen to build amino acids etc. So the bacteria aren't pumping free N directly into the soil. Except that all biological systems are a little leaky.

Also plant root systems are in a constant state of turnover, with old sections dying and decaying, and new sections growing. So a permanent legume cover, like clover, will add significant N to the soil over time.

Legumes can afford to be a little less stingy with N than other plants, so all their tissues will tend to be higher in protein and other N-containing compounds. Both top growth and roots will return N to the soil when they rot. They tend to pump N-compounds into maturing seeds, so if you harvest them for food or seed you'll be removing significant N. That's also why it's recommended to turn under green manures as they begin to flower or just before. That and so they don't reseed themselves.

Hope this helps clarify things.

Jim

Interesting stuff guys. I don't mean at all to diminish the importance of the top growth vs. roots nitrogen discussion, but in this case, it's relatively moot since both the roots & the top growth will be going back into the soil. Just not sure whether it will be worked in as a shredded mulch or part of my compost pile. Either way, it's eventually going back where it grew.

One other consideration, I kind of alluded to it but didn't spell it out- Because I would rather not plow or rototill after this year, but do want to cultivate the top few inches, I'll want something that doesn't have such a thick, tough root system that working those few inches becomes too physically draining to do with hand tools.

Clover hits a lot of the things on my wish list, but the clover I've seen (to be fair, I have no idea what strain) is almost impossible to kill. Have seen deer mow it down time after time, yet it still comes up if they leave it alone for a short time.

Look up annual alfalfa, or hairy vetch. Either might fit the bill.

The annual alfalfa (I read several sources calling it by the name "nitro alfalfa") just might be the ticket.

Anyone have experience with it? In this area, can I expect good results by planting it late in the harvest season or after harvest (late Sept. through Oct.), mowing in early spring (late March through mid-April), and lightly incorporating through cultivation up until planting time (~ mid-May)?

My apologies for the volume of questions... much to learn.

Tom,

Often when I've undersown the sead pod never fully
develops but I get much better production from
the *adjacent* plants than I have otherwise and
in those cases I don't add any additional fertilizer
once the legume is undersown. Im harvesting well
before the legume dies and is turned under as
are most growers who use the technique. Same observation
would hold true for farmers who are adding legumes
to hay fields to nurse grasses. Years and years of
practice by literally thousands of growers the world
over pretty well put to rest the issue of whether or
not legumes make N available to other nearby plants.

Greg, Steve's suggestion of cereal rye and field peas is a good mix but if the rye gets too much spring growth it can be a bear to till in. It has a great root system. Something else to consider is planting oats in the later part of the season and let it winter kill.

Decklap, I agree with you 100% about cover crops being beneficial to the garden. But I don't see how you make the jump to attributing the benefit to free N from the rhizobacteria. There are all kinds of benefits from cover crops. A few years back our Extension Office sponsored trials of growing an annual rye between rows of sweet corn and the results showed it to be of great benefti (I don't remember what) and there's no N fixing with annual rye grass. Years ago people saw ships sail over the horizen and dissapear and concluded the world was flat. Early astronomers saw the movement of the stars and planets and determined everything rotated around the earth. Their observations were correct but their conclusions wrong. Tom

Tom,

You're asserting with great certainty that living legumes don't release N to the soil, but do you know this for a fact?

It's true that living roots and nodules wouldn't release much N, but what about root turnover? When legume roots die their decay would release N. The decaying nodules would release N. The rhizobacteria would go dormant until they have another living legume root to infect.

Root turnover can account for many of the benefits of cover crops through aeration and the addition of organic matter and nutrients. The N-fixing and high N content of legumes suggests that N release could be an important benefit.

If you have data that supports your assertion that living legumes have no affect on soil N, please outline it here. It seems too simplistic to me.

Hi Jim, That's the info I got from NM State, UF and Edinburgh. Found them through Google. No sense of me outlining it because the sites are from the people that did the research. One point from NM State is that the rhizobacteira nodules continually turn over during the season and that when the seeds start to mature the nodules stop fixing atmospheric N because the plant no longer supplies them nutrients. If I read it correctly the bacteria in the nodules die. Anyhow read the artilces that are available. I'm only talking about these bacteria, I realize there are all types of other interactions going on. I'll be gone from the computer for a week. Tom

I'm confused as to why there would be a problem with a low, non-woody cover crop like clover reseeding in the garden. Wouldn't it then be a living mulch AND a useful cover crop at once? Couldn't you just grow among it/between it? Is it so aggressive that other plants would have trouble around it?

Thanks for your thoughts.

I wonder the same thing, arugula.

A basic question: is the air providing the N for the legume nodules the air surrounding the plant above ground, or the little pockets of air between the soil particles underground?

I've grown a few different cover crops and I don't till.
Here is what I learned.

Low growing white clover, grew beautifuly with irrigation and in a hot/dry climate, can be killed by drying it out. Not easy to cultivate through the thick growth though. Nitrogen fixing.

Buckwheat, can be killed off by mowing when it goes to flower or by frost. Easy to cultivate through, great for beneficial bees and wasps. Not Nitrogen fixing.

Oats, frost seemed to kill it but some if it did manage to re-seed itself. not too bad to cultivate through. Not Nitrogen fixing.

Winter Wheat. We waited till it flowered and then weed whacked it down. Very little grew back. Kinda like cultivating through grass roots but not too bad. Not nitrogen fixing.

Spring wheat, we planted it at the wrong time of year and it didn't really flower so was much harder to kill by weed whacking. Finally pulled up the clumps and flipped them roots to the sky. Still kinda weedy and difficult to cultivate through the clumps. Not Nitrogen Fixing.

All these trials took place in Central Florida. All beds were built as lasagna beds over cardboard with lots of mulch, mushroom compost, and compost over them. Each bed was fitted with micro sprinklers. Each bed was 100 SF. Cover crops were planted in the fall. We did get a couple hard freezes that winter, it is now the following summer.

This past spring I left the spring wheat bed fallow.

The winter wheat bed grew wonderful sunflowers, soy beans, and we are still harvesting squash and zucchini from it.

The buckwheat bed was planted with pumpkins and underplanted with the white clover as living mulch. The pumpkins didn't do that great but I've not yet had much luck with pumpkins down here.

The Oat bed was planted with sunflowers that did great. Had been underplanted with peanuts but a garden helper pulled most of them out thinking they were weeds.

The clover bed I tried planting corn in but I think an irrigation malfunction may have doomed that, the corn failed miserably. I left that bed to dry out over late spring early summer and hopefully kill off most of the clover. The bed is now planted with winter squash, seedlings just coming up.

Good Luck in your search and I hope to hear what you try and how it works for you, how you kill it and how the hand cultivation goes through the stubble.

Here is a link that might be useful: TCLynx