Food forests: nitrogen fixing plants and fruit trees

I've watched Geoff Lawton's food forest film and I have a question regarding fruit trees and nitrogen fixing plants. I was under the impression that fruit trees in the rosaceae family don't do well in nitrogen-rich soil. However, Geoff is going wild with nitrogen fixing plants and also planting apple and plum trees.

Is it correct that the rosaceae family does not do well in nitrogen-rich soil? If so, why does Geoff combine them?

It might be because Geoff is in the subtropics where it is more difficult to keep nutrients in the soil than in temperate areas.

Actually, in Gaia's Garden, Toby Hemenway mentioned that the Bullock brothers in western washington said they put in a caragana or a russian olive, both N-fixers, in the same hole with fruit trees. The n-fixers help the fruit trees get started, and then can be girdled or removed.

As I understand it, the natural fertility from plants and composts etc is released much more slowly, thus doesn't cause the same problems with overfertilization as chemical fertilizers, or even organic nitrogen additives like blood meal or cottonseed meal, which are released much more rapidly.

Using a lot of n-fixing and Dynamic accumulator species in the beginning can help build up rich soil that can support a mixed food forest of many layers. Once the trees and shrubs get larger, not as much sun gets through the canopy, so the nitrogen rich plants gradually play a lesser role. Most n-fixers seem to need lots of sun, so are most needed and most effective in the early stages of the forest garden.

Check out eric Toensmeier's web site.

http://www.perennialsolutions.org/all-nitrogen-fixers-are-not-created-equal

He quotes Martin Crawford suggesting that between 25 to 40 percent ( in a sunny situation) 50 to 80 percent (in shady situations) should be nitrogen fixing plants in a forest garden

best wishes

"between 25 to 40 percent ( in a sunny situation) 50 to 80 percent (in shady situations) should be nitrogen fixing plants"

Man... I'm gonna have to just plant a huge swath of eleagnus...

Hi David - I'm not sure what your situation is and what trees you are able to grow. What Martin Crawford does, is to plant a lot of Alnus Cordata (Italian alders) in his forest garden (they are really quick growing.) They grow, If I remember correctly to about 25 M tall) He begins to lop off the lower branches so that eventually the Alder canopy is way up above your edible trees so that they take up space up there rather than occupying the more valuable space at ground level and cast the least shade on your other trees. On our forest garden in Devon UK we have used Italian alders as our shelter belt so that we should get two yields from the the Alders. Shelter and nitrogen (from nitrogen fixing nodules plus leaf fall.) the only snag with this design is that the alders are quite late to leaf out, so perhaps Alders are not best used for shelter from your cold spring winds. However we are experimenting with planting ivy to grow up the alders to make them eventually more like deciduous trees.(Ivy is an excellent bee plant which would make for a third yield - Also the latest information suggests that Ivy does not harm or choke the host tree) I'd be interested to know what others think about this idea?

richard swier wrote:Hi David - I'm not sure what your situation is and what trees you are able to grow. What Martin Crawford does, is to plant a lot of Alnus Cordata (Italian alders) in his forest garden (they are really quick growing.)

I'm not sure if Italian alders will grow here. I'm in an interesting spot right between the tropics and the temperate regions. Unfortunately, this is one of the worst places in Florida to grow anything. It gets periodic freezes that reach down into the teens (usually for only an hour or less), and temperature in the summer that soars into the 100s. Chill hours are poor... and during the winter, we get weather in the 80s which wakes everything up, followed by cold that takes all the new growth off.

On the up side, we get rain in the summer and overall have nice mild winters. The eleagnus varieties I'm growing do quite well no matter what the weather... but the leucanas I've planted have frozen down to the ground. I like the idea of a rapidly growing species that makes plenty of biomass and feeds the soil - I'll have to look at alders more seriously. I've never seen them offered for sale in this area.

The only local nitrogen fixer I've seen is Albizia julibrissin, the "mimosa tree." It's not native, but it's tucked itself into many of our oak scrublands here. I've advised people with it in their yards to plant fruit trees next to them and chop the tops off now and again. The mimosas always look nice and green no matter how sad and sandy the soil.

Hi - I was curious about what nitrogen fixing trees might grow in your part of the world, so that you might be able to mimic what can be done in more temperate climates with Alnus Cordata. Ie a really tall nitrogen fixing tree that allows you to plant lots of things under it. This looks like might work really well: Paulowia Elongata (i.e. not the the invasive P tomentasa ) if it did it would be a fantastic candidate. Although perhaps your cold snaps might be a problem. Information seems to suggest it tolerates climate extremes. I found this on a Florida gardening website.

http://forums.gardenweb.com/forums/load/flgard/msg0518271715347.html


climatic info here:

http://www.worldpaulownia.com/html/zones.html


'If you want a long-living nitrogen-fixing plant, try a Paulownia, aka Royal Empress, Tree. The fastest growing one,, 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.'




On the strength of this I've decided to see if we can experiment with it here in the UK too.

David, you might want to look into native legumes: http://www.nsis.org/garden/family/legume.html

Looks like Pawlonia would work here - I didn't realize they were N-fixers. And 15' a year? Heck yeah. My kind of tree!

Good link, Tyler - thank you. Still not many (if any) options for tree legumes on that list, though. That's been my problem. The really great stuff grows a few hours south of me... dang it.

Richard, thanks for the link to Eric T's site. I found a nice list of n plants for Colorado. Now I'm working on assembling a list of plants that are drought-tolerant and might actually be able to survive and thrive here.

It is like a big puzzle to find and pull together data from lots of sources, to eventually design guilds that might work together and create a bounteous garden in many layers.

I don't know how to do it, but would it be possible for us permies to work together to create guilds of plants we find that might work in our areas? Maybe have 1 thread for each different region/biome? And anyone in that region could share what they have learned? I know somewhere I saw a discussion about a data base, but don't know where I saw it, and a big data base would be too general, and too confusing, I think. It seems more meaningful to me if each region had its own section.

Done a little more research on the Pawlonia it seems like there is a bit of conflicting information about it. Some suggest that it is not a root nitrogen fixer after all. Some websites claim that its leaves fix nitrogen - something I have not come across before? This seems to be a characteristic of some tropical nitrogen fixing plants. Perhaps there is someone out there who can enlighten us? Anyhow, it seems like it is used extensively in China in Agroforestry situations to increase the yields of companion crops. That suggests potential suitability for forest garden type situations.

Dj, yes I agree I suppose we are all pioneers at the beginning. I guess it means we all potentially could make significant contributions/ and mistakes!!! in working out how to do it in our own particular set of circumstances. We are really lucky here in Devon UK as Martin Crawford has blazed a trail and shown how it can be done in the conditions we have in the UK. So it is easy for us here to see how well his designs works in practice and therefore feel confident to follow his recommendations. his two acre demonstration garden is about fifteen years old and is very impressive. Lots there on you tube if you want to get a feel for his garden. Furthermore I haver found that his various books are excellent resources in helping you to design a garden - highly recommended.

http://www.amazon.co.uk/Creating-a-Forest-Garden-ebook/dp/B003Z9JMYI/ref=sr_1_10?ie=UTF8&qid=1363083139&sr=8-10

There is a chapter in this book devoted to designing the nitrogen fixing plant aspect of a forest garden. The principles that he uses are described very clearly although they are expressed in the context of a cool temperate climate I imagine some of those principles will be universal. However for the detailed application of those principles in different circumstances adaptation will no doubt be necessary.

The main issues to consider with the designing nitrogen fixing in a forest garden situation, as I understand it are:
1 That most nitrogen fixers are quite light demanding - (elaeagnus is a rare exception)
2 if the cover of nitrogen fixing plants need to be 25 to 40 percent ( in a sunny situation) 50 to 80 percent (in shady situations). To get this kind of cover you need to fill much or most of you under-story/shrub layer with nitrogen fixers - you have to start sacrificing a lot of this very valuable space to nitrogen fixing plants when you might prefer to use that space for edible or other kinds of yields . Therefore it makes sense to get your nitrogen fixing done way up in the over story.
3 This conclusion is even more convincing when you also consider that: If you use nitrogen fixers in a sunny situation you need only half the area to be covered by nitrogen fixing plants compared to those in a shady situation.








Anyone wanting more detailed information on nitrogen fixing plants might benefit from this:

http://www.agroforestry.co.uk/publorders.html which is Martin Crawfords website where you could get a booklet called:

Nitrogen-fixing Plants for Temperate Climates
by Martin Crawford
2nd rev Edition, 1998. ISBN 1-874275-38-6. A4, 89 pages.
Plants which fix nitrogen (ie utilise nitrogen from the air) are some of the most useful plants in agriculture, horticulture, forestry and agroforestry. Their use can lead to much reduced fertiliser usage, lower losses of soil nitrogen through leaching, and improved soil fertility through increased soil organic matter.
Most people only know of the legumes as nitrogen-fixers; however, there are several other plants groups which do so, notably the so-called actinorhizal plants (including alders, Elaeagnus, sea buckthorn) which are mostly of temperate origin and better-suited to cool temperate climates.
This directory describes the different groups of plants which fix nitrogen, including the legumes, the actinorhizal plants, liverworts, and lichens. Most fixation occurs via a symbiotic relationship with bacteria in root nodules, and the nodulation status of temperate genera are listed, with notes on cross-compatibility between different strains of bacteria and different genera.
The nitrogen contributions which N-fixing plants can make are discussed, and an overview of the different uses given, including their use in forest and fruiting gardens as well as in forestry and agriculture. The main parts of the directory describe over 450 different species and their in-depth uses. This includes tables, organised by plant type, showing conditions required and uses of all species.


Not sure if this would be of any help to you in Colorado dj?

I won't be around for a while as I'm off to work on the forest garden - no internet access there. So keep up the good work