If you want a long-living nitrogen-fixing plant, try a Paulownia, aka Royal Empress, Tree. The fastest growing one, Paulowia Elongata, is the fastest growing tree in the world and has leaves that make great organic matter to add to your soil, as they have much nitrogen in them.
You can find the trees online pretty easily and they aren't too expensive, only around $15 for a tree. If the tree is only about a foot tall, don't worry. The tree will outgrow that size in no time. I should know, I have one! : D

I would recommend Paulownia Elongata, as it is virtually identical to the most popular one, Paulownia Tomentosa, but has a much greater disease resistance and grows a bit faster.

You probably CAN'T find this tree at your local Home Depot or Lowes. Your best bet would be online.

PS: Paulownia trees start incredibly easily from cutting, without any added hormones! (Sorry, my tree isn't big enough to take cuttings from yet) To start by cutting, wait until spring or pull leaves off in a specific area, and wait until you see new leaf buds forming; cut a few centimeters before you hit the bud(s)(there should be two buds, as leaves tend to grow 180 degrees from each other) and place in moist potting soil. A new tree should come up within a few weeks.

TClynx,

When I go to my local farmers' market, I usually buy my produce from Worden Farm in Punta Gorda. It is an organic farm and the interns and farmers that work there man the stand as well. I take the opportunity to ask them a little about their farming techniques on occassion. Farmer Chris told me they use sudex cover crop. I believe he said it is a tall grass, and I think he spoke of nematode suppression. You might look up Worden FArms and either email him or call him. He's a really nice youngish guy who is very much into what he does. I'm sure he'd be happy to share any info with you.

Anna

Look up perennial peanut . It fixes nitrogen in the soil . Makes a fabulous ground cover 10 months out of the year in zone 9 .
from the link below :
"
Perennial peanut has recently shown promise as an ornamental groundcover due to its high resistance to drought, nematodes, and pathogens and its minimal fertilizer needs. This translates into savings in water, energy, dollars, and reduced impacts to the environment. It is not only beneficial to the environment since it requires no supplemental nitrogen or phosphorus fertilization or pest control, but it also is aesthetically pleasing, can be walked on, and has edible, peanut flavored flowers."

Here is a link that might be useful: Guide to Using Rhizomal Perennial Peanut

tc--bear in mind that nitrogen-fixing plants do not generally make the nitrogen they create available to other plants until the nitrogen-fixing plant dies. in fact, the little reddish nodes on the roots don't _store_ much N--whatever is produced is used by the plant. so, you have to return the WHOLE plant to the soil (the green growth contains nitrogen) to make available the N that the plant has produced. does that make sense?

in other words, it's hard to use N-fixing shrubs/trees as a source for N, because typically you can't return (easily) the plant material to the soil.

the best plant for doing what you want to do is the velvet bean, though sun-hemp might work, too. if i recall correctly, ifas has determined that bahia is the best cover crop--yes, it requires some N input, but it produces more (in green growth) than it uses.

in the past, i've grown lots of crowders and then, at the end of the season, mowed them and used the plant residue as a mulch. worked great.

Actually there are ways to return a fair bit of the nitrogen from nitrogen fixing plants to the soil, it requires plants that are big enough to allow you to chop and drop (during an appropriate time of year when there is enough moisture for the plants to re-grow.) Understory size trees and shrubs are the most appropriate size plants for this. You simply walk around and prune the plants off at the desired height (often just above head high) and drop the clipping around a nearby fruit tree or other needy plant as mulch. When you prune the top of a plant, the root mass will generally self prune and the excess nitrogen from the excess root mass will be in the soil and the nitrogen from the pruned top is now mulch. This is how to utilize nitrogen fixing plants if you don't own a tiller.

I plan to use the crowder peas as initial annual nitrogen fixing ground covers but they only give an initial kick start, I need to start growing some seedlings of some more long term support species.

Right now on my list (for shrub or small tree size plants) I have Southern bayberry or wax myrtle, Pigeon Pea, and Gumi (if I can find it.) I know there have to be many more options out there.

This thread is very interesting and I hope to use some of the wisdom.
The opposite of a nitrogen fixing plant is sunflowers I just found out. They deplete the soil, NOW I find out :-(
I think cotton is another soil negative plant.

Denise

Sunflowers will only deplete the soil if you don't return the residues of the plants to the soil. Granted, the season after growing a bed packed with sunflowers, you will probably need to add more nitrogen rich substances in order to grow any greedy veggies, but it is good to change things up a bit anyway.

Here is where much of our current agriculture methods are running into problems. Using chemical fertilizers which only provide a few nutrients to the plants being grown. Then harvesting and taking the nutrients in the plants away. Finally doing it all over again and again. This depletes the soil, constantly taking away and only adding the minimum to manage to grow the crop for the least amount of money. And we wonder why our food is no longer as nutritious as it used to be, we depleted all the trace and micro nutrients from the soil decades ago and are only replacing the bare minimum to produce crops that will look good enough to sell in the stores.

by the way, the nutrients we ate with those veggies have been excreted into bowls of nice clean drinking water and sent back to the drinking water supplies in a hope that dilution will take care of any pollution problems (but this isn't really effective anymore) or perhaps some of the solids were sent to a landfill or maybe they have been spread on fields to raise fodder for livestock. The nutrient cycle is broken, it is not sustainable to keep taking from the soil and never returning anything to it. Sure you can go elsewhere to get stuff to help build up the soil but then you are simply moving the depletion to another place.

The trick is to keep as much organic mater on your property as possible so then you only need to replace what does leave. It takes time to build up really good soil and a functional system that doesn't loose too much but I can see that my soil is already much better after only a couple years in the house.

PS:T The Paulownia tree I was telling you about sheds its leaves every Fall, but they disintegrate into the soil within days, unlike Maple trees which its leaves stay on the ground until someone picks them up.

Try to only grow sterile and non aggressive types of Paulownia. They can spread fast.

Florida coontie fixes it's own nitrogen, but it doesn't give it back into the soil.

Senna mexicana var. chapmanii

senna ligustrina

both S Florida natives, fast from seed, and prolific reseeders

When you annually cut back nitrogen fixing plants, they shed some of their roots in order to keep a certain stem to root ratio. This adds nitrogen to the soil as do the prunning which you allow to break done on the ground. But if the nitrogen fixing plants are simply allowed to grow bigger and bigger each year then they don't add much in the way of nitrogen or biomass to the soil until they outright die.