Lately I've been planting a lot of fruit and nut trees. For several years, my understanding has been that a sustainable orchard has involved nitrogen fixing plants. So far I use only goumi berries and siberian pea shrubs. I love goumi berries and would grow them for their fruit alone, and siberian pea shrubs are said to have many uses, so they make sense for me as well.
But I've always wondered at the efficacy of these plants, and others, at actually aiding other plants in growing. I'd like to think they'll play a role in helping my orchard to be more resilient in the future. Like I said, the nitrogen fixers I use have other roles to play, so it wouldn't be a total wash if they turn out to not help my trees. Lately I've met some people and seen some posts on here that say that perhaps these plants don't work as advertised. I'd like to know people's opinions on this, their experiences, and anything else relevant to the discussion. Thanks!
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I think it really matters on what timescale you are looking at and like you said annual versus perennial plants.
If you plant perennial nitrogen fixing plants my understanding is that you won't get much benefit during the first year. But leaf litter, any chop and drop material, etc. will steadily build soil and increase the nitrogen level. In my area the classic example has been large amounts of red alder growth in previously cleared areas increasing nitrogen levels in the surrounding waters. So for the long term health and fertility of your site I think perennial's could have a great benefit but I don't think you will get that sudden growth that you might get from just applying nitrogen fertilizers. Of course that method is not sustainable but it does provide very short term benefits that people can be drawn to.
I do think nitrogen fixing annuals that are left on the field to decompose and not harvested and removed can provide a great benefit. I think this benefit would be seen earlier than with perennial nitrogen fixing plants. Though I do think the benefit would still likely be less in the very short term than using nitrogen fertilizers but you will get lots of negative impacts overtime.
But nitrogen fertilizers don't have to be the toxic icky stuff. It could be animal manure which could give you a nice short term boost and also help in the long run.
I think ideally we would use animals for the short term boost (plus some long term benefits), use annual nitrogen fixing plants for the mid-term boost, and in the long run use perennials nitrogen fixing plants (through chop and drop or just wait for the plants to die on their own). Just keep this cycle going and I think you would see great results.. It seems to me that combining these three elements would create great soil, build a healthy level of nitrogen, and provide lots of other nutrients that your plants need. Plus my understanding is that this would reduce disease by building a healthy and diverse community of life in the soil and increase the water holding capacity of the soil.
Nitrogen fixing plants do in fact take nitrogen from the air to build their cells vs taking it from the soil like other plants.
Nitrogen fixing plants WILL NOT however just squirt it out everywhere for other plants to use it. Other plants only get it after the nitrogren fixing plant (roots/leaves/fruits/seed/stem) has been killed and it decomposes.
Now if a cow or bird or bugs comes along and eat the nitrogen rich legume and then dies or poop/pee, the nitrogen will be released from the plant and enter the soil.
All that said, just letting nature take it's course (bugs/birds/winter/etc), without you doing anything special alot of legume will release 150lbs of nitrogen per acre. I think dutch clover can release 150lbs per acre. Not too sure how much Goumi releases. But every single nitrogen fixing plant species "fixes/release" different amount of nitrogen so you would have to plant the best one for you.
But the real take away if to keep your soil life very active, with all these worms and microbes pooping+peeing and dieing and decaying you will have alot of nitrogen available. so encourage soil life.
I don't have much experience with perennial NFP's, but I do annual NF cover crops twice a year. I can pull these plants up and clearly see the pink nitrogen nodules on the roots. The key is to terminate the plant before all that N is pulled up into the plant to create seeds. So it's a matter of timing. If you wait too long, you don't keep that N in the soil profile.
Just chopping and dropping the leaves and branches of a nitrogen fixing perennial does nothing more than mulching with any other plant. The nitrogen levels of the leaves of a NFP are not more rich in N than any other plant. Dumping a bag of grass clippings would be just as beneficial—perhaps moreso. The problem is that unless you are composting that N rich with a carbon source (the old greens vs. browns ratio that composters are trying to get right), all that N just gasses off.
So I'm all for chop and drop mulching. But that doesn't fix N in the soil profile.
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