Leah Sattler wrote:I have also read that nitrogen fixing plants such as alfalfa or beans must be tilled back into the soil to release the nitrogen and make it available to other plants because the nitrogenis stored in little nodules in the roots. Maybe there something I missing or maybe each year some of the roots die back and introduce the nitrogen to the surrounding soil. either way from my research I have concluded that nitrogen fixing plants are a much more complicated scenario than they are often presented as. I personally will stick to them as cover crops in a cultivated garden or utilize them if they have other redeeming qualities as companion plants but don't paln on wasting much time or space on them just for the sake of thier abilitity to fix nitrogen.
Following this debate with interest as this is precisely what I researched.
My research showed that they do release some nitrogen into the rhizosphere, but there is a time lag. I studied grass and clover, in the absence of grazing animals, significant amounts of fixed nitrogen did start to appear in the grass around September/october. I suspect that a big part of that was leaves dying back because of frost etc. Over winter probably a lot more fixed nitrogen was released because over the course of 3-4 years, nitrogen was building up in the system.
, growing legumes purely for their rhizodeposition is not a good way to use them. Their are many ways you could use them without tillage though. You could use a cut and drop method or feed the cuttings to animals. I would suggest cutting and dropping the mulch somewhere else, rather than back onto the legumes (legumes prefer to grow in low nitrogen). MOST OF THE NITROGEN IS IN THE LEAVES, not in the root nodules.
The way I think of it is that legumes put a lot of nitrogen into THE SYSTEM. "The system" to organic growers, means soil, plants, animals, manure and people. If you are organic, these things should
all be connected. If you are only thinking "how much nitrogen will this legume provide for my plants RIGHT NOW RIGHT HERE", you are not thinking organically/holistically. If someone grows a monoculture of beans, harvests them and sells them, and at the end of the season rips up the plants and bins them before cultivating the soil and leaving it bare over winter, they cannot expect to have an extra 400kg of nitrogen ready for their next crop. Likewise with intercropping. Some nitrogen will find its way to neighbouring plants, but because of the time lag, if you are growing summer annuals, think hard about what will happen after the crops are harvested. A winter cover crop (best a non legume) would be a very good idea, as there will be a lot of nitrogen in the form of organic compounds and decaying bits of root and leaf and exudate which will not sit around in the winter rain.
If, on the other hand, you are growing a perennial polyculture including legumes, the nitrogen from the legumes will eventually find its way into the other plants, it might take a few years but basically, with legumes, the system will be self sustaining for nitrogen at least.
I once worked on a farm where the farmer was attempting to improve his soil with an alfalfa monoculture. He planned to grow it for a few years and then plough it in. It was an OK plan, but I suggested as an improvement, I cut some of the alfalfa and mulched gardens with it. He was very surprised by the results of this which were extremely good vegetable yields. He thought that it would rob his poorest soil, but the alfalfa grew back fine. Ploughing up legumes does produce good results but a lot of nitrogen gets wasted, especially if there is a long delay between cultivation and planting. Actually I suspected that the real problem he had wasn't lack of nitrogen, but compaction from too much heavy machinery and cultivation.
If you intercrop right you get two crops where you would otherwise get one, and the yield of each crop will be roughly similar to what you would get in a monoculture. Legumes and non legumes can share soil. Caveat: Patrick Whitefield has noted many cases where intercropping doesn't work so good, and emphasises that companion plants should share space as well as soil, so tall plants and an understorey for instance. He also noted that the three sisters planting never seems to work outside Mexico.
Legumes do have to be used right. But basically sustainable
agriculture could not work without them. Not using them is not an option, however, for some of us, with small gardens there may be more practical ways to quickly build soil fertility than planting legumes and waiting for rhizodeposition.