Friday, March 05, 2010

Chop 'n' Drop: Growing mulch with legumes and other NFTs

Nitrogen-fixing trees, or NFTs as they're sometimes abbreviated, are mother nature's best kept secret. Especially if one's goal is to create a forest garden.

Where to begin? Well, how about here: Nitrogen Fixing Tree Start-Up Guide. It pretty much tells you everything you should know, but I'll summarize.

Nitrogen fixing plants are plants that fix nitrogen. What does this mean? Well, nitrogen is a major component when fertilizing one's garden, and it can be a very costly component, and the synthetic nitrogen in fertilizers can actually kill off microorganisms in the soil. The ironic thing is that the air around us is about 80% nitrogen, but most plants can't use it. This is where nitrogen fixing plants come in. In a way, "nitrogen fixing plant" is a bit of a misnomer, since the plants themselves aren't fixing the atmospheric nitrogen. Instead, they live in a symbiotic relationship with the bacteria housed in their roots, and these bacteria capture that nitrogen and trade it with the plant in exchange for starch. When the leaves fall off the tree, or when the tree is pruned, these little bits decompose and make all of that nitrogen available to the plants around them without the need for additional nitrogen fertilizers.

This isn't a new concept. In fact, farmers have been growing crops of clover and other ground covers for years for this very reason. The advantage of growing trees instead of ground covers is that the trees are perennial--you never have to plant them again. Also, the sheer amount of biomass being produced is overwhelming (in the best possible way). Some part of these trees are also often edible (although, many are also poisonous, so do your research). Ice cream bean sounds like an amazing species, if only I lived in a subtropical climate...

Another benefit is that many of these species can take a lot of abuse, both in terms of the types of soils and conditions they're willing to put up with, but also the extensive pruning they can survive. Pollarding is the common practice, as in the picture to the right. All but a few lower branches or buds are cut off of the tree, and the tree will regrow from what is left with full vigor. Everything that is removed from the trees can then be used as mulch.

This is particularly useful in a forest garden or orchard type situation where you can have NFTs planted in among your fruit and nut trees. You don't have to worry about them overshadowing your fruit trees because you're going to severely cut them back each year. And you don't have to worry about transporting mulch because you can throw it down on the ground right where you cut it to release all of the nitrogen they've collected. You'll also be promoting a great diversity of soil microorganisms and fungi which help break down the woody material.

But hey, some NFTs are actually edible by humans, and you don't want to prune them back. Not a problem. They're still going to lose their leaves in the fall, and even that will add a good bit of nitrogen back into the soil near the trees.

Unfortunately there aren't many trees in the legume family, the most well-known nitrogen fixers, that do well here in the temperate pacific northwest. This family includes trees commonly referred to as acacia, wattle, and locust. Black locust (Robinia pseudoacacia) and rose acacia (Robinia hispida) are the only two that I've found that are reported to do well here. The seed pods pictured at the top of this post are from a neighbor's black locust, and I intend to experiment with those. Fritters are made from the flowers dipped in pancake batter, and that's certainly another thing I'm looking forward to.

But, there are plenty of nitrogen fixing trees outside of the legume family. Wikipedia has a list, though not a complete list, if my research has treated me well. Also, not all of them are trees. However, it should be a good list to get you started in conjunction with Dave's Garden and Plants for a Future, both of which I've mentioned previously, and both of which have amazing databases of plants.

Ask around and do your own research to find out what works best in your area. But in any case, start growing some of these nitrogen fixing trees. They really are one of the best things you can do for your garden. What I did? I used Dave's Garden's local plant search to find all plants that grow well in my area, and then used my browser's search tool to look for plants from the genera listed on Wikipedia, as well as others I've found throughout my research. The Dave's Garden database is maintained by users, so there's not guarantee that ever plant that does well in your area will come up in the search, but it's a good starting place.

Nitrogen-fixation is also covered in Gaia's Garden, which I mentioned previously, and which is a book everyone should own. The Biology of Soil: A Community and Ecosystem Approach also seems like a good--although more scientifically intense--look at what's going on in our soil. What I skimmed through seemed relatively approachable, but quite in depth. It will be going on my list of books to buy.

Good luck, my fellow gardeners.

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