Nitrogen-fixing trees in a hugelkultur bed

I've got access to quite a bit of a local, fast growing soft-wood leguminous tree Leucaena leucocephala (Lam.) de Wit (Lead Tree, to some people). I'll probably make a bed in the coming weeks, just in time for the local rainy season deluge (in South Thailand). Most or all of it will be fresh, so I'm wondering about the possibility of nitrogen loss. This is a nitrogen fixer, so will that not be an issue? Anyone have experience using this specific tree, or some other fresh nitrogen fixing tree in their hugels?

I wouldn't plant a tree in a hugelkulture because it will eventually collapse and breakdown. I would plant some perennial/annual legumes like pigeon pea or cow peas. Or any other legume.

To help with the nitrogen absorption by the high carbon wood you can layer in some manure for supplemental nitrogen.

Unless I've misread, the OP isn't talking about planting those types, simply using them in his hugelbed.

You are right my bad!

I say go for it! Use what you have eventually it will work its self out. Nitrogen loss is a bit of a misnomer. The nitrogen isn't really lost in this type of system, it is just tied up in the carbon for a time...eventually it is released back into the soil and adds to long term fertility. I would imagine that time frame would be pretty short in your climate, so like I said earlier...go for it, doing something is always better than nothing with this sort of thing. Observe, learn, and make it better next time!

Thanks everyone. Right, I wanna bury them. I'm hesitant to try out things that are for research, and for others to harvest from. So, maybe I'll mix some cow manure in there too, to make sure there's enough nitrogen present. Thanks Dave for correcting me. I shouldn't'a said nitrogen "loss", but I didn't realize it's tied up with carbon.

I have used mostly red alder, which is a nitrogen producer. It uses that nitrogen for growth and does not store large quantities within the wood. Therefore, as the material decays, it still uses up available nitrogen.

 I've incorporated thousands of pounds of coffee waste and the area is surrounded with alder trees, which provide good quality leaf litter.

Ahhhh...! I hadn't thought of it like this. Don't know why, but I had figured that nitrogen-fixers stored nitrogen. But, well, I don't know if I learned this somewhere, or if I assumed it myself. So, nitrogen-fixers don't store N, right? So, if I'm gonna use 'em in my hugel, they should be rotting, just like other trees, right? I've got access to a constant supply of used coffee grounds and will surely put them into any new bed.

Troy;
Let me see if I can give this a whirl. Beginning with the words "nitrogen-fixing". This means the plant, shrub or tree takes Nitrogen out of the air (N<sub>2<sub/>) and uses it or stores it while it is alive. This nitrogen can be used in the main trunk, roots, leaves or anywhere in the plant. For most plants, when you hear that they are nitrogen fixing, they have nodules in their roots made mostly of nitrogen (say peas or alfalfa). So, if that plant dies and falls over, the main plant will supply a little bit of nitrogen to the soil, however, the roots will supply a lot of very available nitrogen to that soil.

When you are considering using a "nitrogen fixing" tree as part of the wood core of a hugel bed, I would look at what part of the tree is nitrogen fixing. If it is the root, can you dig up the root of this tree to put into your bed? Or, better yet, can you locate your hugel bed on or around the stump of this tree?

I utilize the native mesquite trees for this purpose. I planted them just a few feet away from the base of the beds and their roots have grown upwards into the beds since. Mesquites are also nitrogen fixers. I just keep them pruned so that enough sun light filters through but still have some shade. In the summer down here, you have to have shade. A hugulkulture mound/bed never has to collapse; just keep on adding material as the years go buy ( they definitely get better as they get older, as you add material). I use wood that is already rotten. I then inoculate it over night in a slurry made of leaf humus and leaf fodder. Once the rotten wood reaches 100% absorption, I then burry it. Then I strip the layers of humus and leaf fodder from underneath the oak thickets on my place and mix it in with topping layers of the mounds/beds. Not having to worry about green or hard dry wood locking up nutrients, I can plant whatever I want without being concerned of having an unproductive season(s). I do let the area of which I stripped rest for 7 years. So, if you have the same available to you, I would do it. Plenty of beneficial bacteria and fungi in this naturally occurring layer that can provide all of the nitrogen and minerals that you need.

I looked up Leucaena leucocephala (Katin). Wow, it looks like in Thailand this tree would be invasive but also a food for cattle and people. It looks like the seed pods are full of nitrogen, but have found nothing about the roots (you will just have to dig one up). If you dig up the roots and find nodules you will know you have found gold. The leaves also look like they have amounts of N,P and K among other things.





However, none of this, directly answers your question, "how well will this nitrogen-fixing tree do in a hugel bed?" When many people talk about hugel beds they do talk about nitrogen and other nutrients, however, the main point to having a wood cored raised bed is for water retention. If the wood is rotted, it will hold more water right away and it will already have fungal and bacterial activity that is great for wood cores of this type. What I would suggest to you is find some well rotted wood but also use the fresh stuff. If you need nitrogen and other kinds of amendments, throw roots, leaves, pods and whatever else from Katin (de Wit) you have. Try to have at least 1/4 of the volume the old rotted wood. Then the new wood, leaves (the leaves are considered green manure by some), coffee grounds, roots and so on. Both the rotted and the fresh wood might take in a little bit of the nitrogen the first year or two, but over the long run, you will have awesome water retention. You did not say how much water (rain) you get there.

Kenneth ... thanks for the reply. What I wanted to know about burying these leguminous trees as the base of the bed. But if there are any of these trees close to the bed, it'll be interesting to see if that part of the bed fares differently, and if that can be attributed somehow to the tree. Of course, if the tree is going to shade that part of the bed, then that's a factor that'll need to be considered.

Donald ... you got the right tree. Wikipedia cites a couple of reliable looking sources putting southern Mexico and northern Central America as its origin, and that it has invaded other tropical areas. It sure grows like mad around here. I haven't looked at roots of the trees to see if there are any nodules. My understanding about those nodules is, they have to have a pinkish color inside (you can tell by breaking them open) for the nodules to be fixing nitrogen. If they're white inside, then the bacteria necessary for taking the nitrogen in (from the air) is absent, though it's possible to innoculate the bacteria.

I've decided that if I use the trees, I'll strip off the leaves to mix with the soil, and put the branches and trunks further down in the pile.

Rainfall here, in far South Thailand is relatively high. About 60 inches (~1520 mm) per year. Oct-Dec are highest with on average roughly 8" (~200 mm) each of those three months. Starting in January though, there is little or some years no rainfall for several months, until May or so. So, I'd really like to get a bed started.

For whatever it's worth, I'll add that I'm doing this as part of my master's degree. Introducing several gardening methods that are appropriate to various living situations for suburban residents. For people who agree to try a bed, I'll help them build the beds, then determine their reasons for adopting or rejecting the various methods I'll introduce. I wanna encourage and contribute to the spread of urban agriculture. To this end, I wanna know what obstacles there are to its spread, and how to overcome those obstacles.

As far as hugelkultur goes, there are tons of rubber plantations here in the south, with many of them not far from where I live. Until last year the price of rubber was high, then it plummeted and rubber farmers who had been spending like there's no tomorrow started to suffer badly. (Market economies and mono-cropping disasters.) So I'm thinking that though rubber farmers aren't suddenly going to cut down their trees to grow vegetables in hugel berms, I see their dismay as an opportunity to introduce hugels. That said, I can't say for sure that any of the participants will even try out a hugel bed. But I need to know enough about these guys to explain things well, avoid some of the possible "problems", and what to do if these "problems" arise. So, I've made a couple myself. But since I don't have enough soil, and for other reasons ... it's complicated! Never mind!!

The people I work with for this research may or may not have rubber trees available, but we'll probably have access to Leucaena leucocephala (Katin). Since it grows like crazy, it's easy to find pretty much everywhere in the country. In addition to hugelkultur, I wanna introduce sheet mulch beds / lasagna beds, because both of these methods hold water well, which'll be important during the dry season. Plus, for folks who say they don't have a kitchen garden because they don't have the time for it ... what if they can make gardens that are really productive yet don't demand lots of their time and attention?

This is going to be qualitative research, so I'm not going to investigate the details of the beds themselves as a soil scientist might. But it would be interesting to have a soil scientist come and do tests. I'm especially interested in the microbial activity because I've become enamored with the lectures of Elaine Ingham and others. Fascinating stuff.

If you add green softwood branches that are from growth within the last year, am I right to think that there will be enough nitrogen already in those branches that the problem you are worried about won't happen?

Plus, by trimming the tree, you will have temporarily made its root system larger than it needs to be to support the tree, so there will be some root dieback, opening up the nitrogen fixing benefits of the tree to its non leguminous neighbours.

Good luck with your studies, sounds like an interesting degree, and you will bring benefits and a better life to the people in the region.

OK, Troy, now we are getting somewhere. You want to help your community see the wisdom of some aspects of permiculture by introducing them to hugelkultur beds but you have some concerns as to how you will present it. You are also operating under the premise that you do not know enough to tell other people about it.

First of all, I would say that the people in Thailand are probably not much different than the people in the US. No one wants to be different. And thanks to some King of France several 100 years ago, everyone wants a grass yard (makes it look like they are wealthy). You will face people who do not want their property values to go down, people who do not want to spend the time, people who think they have bad soil and if no one else in the neighborhood is doing it, they do not want to be the first. They will not want to spend money, they will worry if wild life and other humans steal the produce from it (the fruits of their "labor") and they will look to you as the "expert" of the subject. I would say that is a bit daunting.

You have several big advantages here that I suggest you capitalize on.

You the expert. Here you have a great start by learning the incredible work of Dr. Ingham and the "soil food web". You are asking, you are looking, you are doing, keep doing that. Keep asking the questions, people (books and videos) will be there to give the answers. However, there is one thing you must do on your path to becoming an expert. You must do it yourself. I do not care if you live in an apartment or you do not have time yourself. Make the time, find the wood, find the ground and make a pile that is all yours!!! Take pictures (videos) of the construction, of the produce and keep doing this year after year. If you want, test the soil along the way, I am sure you have access to lab equipment.

You the advocate. Unless you start talking to someone, you do not know how they feel about hugelkultur. That is the first best step. The rest will follow. Let's get you started ... If a person says they have bad soil, you can tell them, hugelkultur makes good soil. You already know this from your reading of Dr. Ingham. Creating a pit of organic material with a dirt covering creates a soil food web over time. So, a hugel bed is a source of high quality soil creation. After a few years, if the person does not like having these beds for whatever reason, they can at least smooth them out and have very fertile soil to plant a yard or whatever. Hugel beds sequesters carbon. All of the carbon you put into the pit, pretty much stays in the pit and becomes part of the soil. A person can make it complicated or simple and it will still work. If all you have is wood, no amendments and very poor soil it may take more time, but it will become a rich source of soil capable of growing anything. I would say that this can also increase your property values. As far as being the first person in a community to try it, that person can try a low risk test in a place that no one sees. You should offer to support that persons efforts in making their pile a success and to help tell others in the community. Time/effort? Other than the initial effort, it can be as little or as much as you want to make it. Annuals can become perennials, fertilizer is grown and dropped (chop and drop). Remember, the community at large will not want to change overnight.

You the Community Leader. As time goes on and you become the expert I know you will, it will get easier. It will also get easier to bring the message into many other communities and perhaps have people from Laos, Cambodia or Malaysia come see your work. At this point, the message must be nice and simplified. Wood + dirt = good soil. At this point, you may also try to get restrictive laws lifted that go counter to putting these beds in (if you can do that in Thailand). However, the government (and local businesses) may help support you in getting your message out so do not discount the possibilities. By this time, you may have a few larger scale farmers (those rubber farmers) trying out "your" principles and having great results. What will help your cases at this level, is documenting everything that you are doing now. What you do today will be the evidence that can win the hearts of the people you will later try to convince. So, take good notes and a lot of pictures and/or videos.

You the regional expert and scientist.
You did not say what you are studying at the University, however, you are somewhat approaching the learning like a scientist, so I will make that assumption now. It is not enough to know that you get about 60 inches a year of rain and that you have some months that are dry. I am sure everyone who lives there has know that since they were kids. You should know what plants grow best locally (watered or not), what types of soil you have, where your streams and springs are, and a history of what locally has been tried and failed or succeeded. It is probably wise to get to know what kind of local animals you have to create habitat for or to create places for them to avoid (like your cash crop). Birds, pollinators, insects and other wild life (and domesticated) will teach you much about how to proceed. I would also say, you should prepare yourself to find the right lab tests and observational queues of the situations. It is also a good thing to brush up on your math skills (not much past calculus unless you become a hydrologist or geologist).

You the expert on water capturing earthworks. All of this is stemming from your desire to create easy ways to capture the water that falls. 60 inches of rain a year is a great place to start, whether it all drops in 3 months or it is dispersed across 12 months. Many other people make these beds (and I guess the lasagna) on much less rain. A great way to find out about that is the series of books from Brad Lancaster called "Rainwater Harvesting; for Drylands and Beyond". He does not talk about hugelkultur as much as he talks about creating ways to "plant" water in the land. The second volume of his book is about earthworks that you can do to slow water down and plant it. The subjects include, French Drains, Terraces, Berms and Basins, Mulching, Swales, Check dams and much more. Knowing how these work as well as hugel beds, you will be golden. Let me point out, with 60 inches of rain a year and some very simple earthworks you can create landscapes that will never need watering and produce mountains of food, even if you have a 10 month drought. Your weed pulling in the landscapes will be very easy and insects and disease will not effect your crop (soil food web) as long as you keep it a "no-till" environment. You can make annual plants into perennial plants. The list of advantages goes on.

Basically, what I am suggesting is that you dig this hole deep but also wide as well. You are the one concerned about looking like an expert (or an idiot). However, I think you will succeed. Why? Because you are continuing to ask. In life, that is what becoming an expert is all about (being unafraid to ask questions).

Thanks Donald. I'm busting my ass digging deep and wide, and actually overdoing it. I'm in agricultural development, a social science (I know, some people say these two words don't belong together). Although the technical aspects of farming interest me plenty, I know that because I'm in the social science field, so that's where I'm putting most of my efforts.

There are tons of theories out there explaining what I could come up against. The main theory I put to work in my research is called Diffusion of Innovations. Really hits the spot, but for sure, overlooks things that others have theorized. I'm not so sure it'll be so daunting to find some people to join my research. They might all reject the hugel, but they'll do something. Even if everyone rejects urban gardening, that's okay too. I want to know why people make the decisions they make.

At the moment, I don't need to go as widely into some of the various things you mentioned. Though they could very well help, I understand and agree with the 80-20 rule, so need to keep my focus sharp.

Though we're all human, and we all have some core things in common, many things that people of European descent share in common aren't so readily observable here. I have been here for quite a few years, consider myself a pretty keen observer (though certainly blind to some important things), and know the language quite well.

I appreciate your thoughtful response, really I do

We have this tree where I live in Hawaii. It sprouts VERY easily from cuttings or branches just left on the ground. You will probably want to experiment with how deep it needs to be buried in order to avoid this, before recommending it to others. Once established, it is also VERY difficult to remove; the roots are some of the toughest I have dealt with.

I have considered planting it here at my new place as goat feed and nitrogen fixation, but am very hesitant, due to just how aggressively it spreads and how hard it is to remove. But then I also wonder how much harm it would do even if it did spread, considering that it does, in fact, enrich the soil. For now, I have decided not to bring it onto my property and am using more manageable nitrogen fixers for my purposes, like pigeon-pea, sunn hemp, monkeypod, etc.

Anyway, just wanted to bring up the fact that it does root very easily. I tried mulching my banana tress with the cut branches at my last house, and luckily noticed they were sprouting before they really got a foothold (this was a rental in the suburbs and the owner definitely didn't want it taking over the yard). After that I always dried out the branches on cement before using them as mulch. In my suburban rental house, it was a constant battle to keep it from taking over. I gave up trying to dig it out, and instead settled for cutting it down frequently, but it still managed to multiply whenever I got slack.

Thanks Lila ... Seems I knew this but have forgotten. I buried some branches in a sheet mulch garden, and they surely stayed moist, but maybe they didn't have any contact with soil and that's why they haven't rooted. Funny. Anyway, I'll keep this in mind! Thanks a ton )

Thais eat the tender shoots raw, in case you're interested. They can be a bit astringent, so surely shouldn't eat a lot. Plus, Thais say they have high levels of Uric acid, which is in some way is bad for people with gout, or leads to a person getting gout.

The following is from the 1983 Handbook of Energy Crops by James A. Duke, this is an unpublished work.

Leucaena leucocephala or Leadtree is valued as an excellent protein source for cattle fodder, consumed browsed or harvested, mature or immature, green or dry. The nutritive value is equal to or superior to alfalfa. Leadtree has gained a favorable reputation in land reclamation, erosion control, water conservation, reforestation and soil improvement programs, and is a good cover and green manure crop. The leaves, used as a mulch around other crops, are said to significantly increase their yields. It is said to possess the power of extracting selenium from the soil and concentrating selenium in the seed. This could be used to ameliorate seleniferous soils if the feed were discarded or used for some purpose other than feed. Seeds yield about 25 percent gum worthy of commercial investigation. Seeds after softening are strung as beans into various items of jewelry for tourists in Puerto Rico and the Virgin Islands. In the Philippine Islands, young pods are cooked as a vegetable and seeds are used as a substitute for coffee. Ripe seeds are sometimes eaten parched like popcorn. Wood is hard and heavy (sp. gr. 0.7), the sapwood light yellow, the heartwood yellow-brown to dark brown, used for fuel or charcoal. Plants are used in some countries for shade for black pepper, coffee, cocoa, quinine, and vanilla and for hedges. In many places, however, renegade seedlings have created a noxious weed situation. The dipilatory chemical mimosine has been used, experimentally at least, to shear sheep.
Folk Medicine
Medicinally, the bark is eaten for internal pain. A decoction of the root and bark is taken as a contraceptive, ecbolic, depilatory, or emmenagogue in Latin America. However, in experiments with cattle, leucaena had no effect on conception.

Chemistry
Seeds and young leaves yield 4 percent of mimosine, which causes loss of hair in non-ruminant animals, especially in horses, mules, donkeys and hogs. Leaves also contain 0.08 percent of the glucoside quercetrin. Per g of N, there are 294 mg of arginine, 88 cystine, 125 histidine, 563 isoleucine, 469 leucine, 313 lycine, 100 methionine, 188 methionine + cystine, 294 phenylalanine, 231 threonine, 263 tryosine and 338 mg valine. Leucine protein makes a better animal feed than copra in several amino acids, and is equivalent to alfalfa in most of them. If leucaena makes up half the animals diet, problems result due to mimosine (3–5 percent, on a dry-weight basis, of the protein). Heating the leaves or adding ferrous sulfate reduces the mimosine or its toxicity. Raw young leaves are reported to contain per 100 g edible portion: 68 calories, 79.5 percent moisture, 2.9 g protein, 0.8 g fat, 1S.3 g total carbohydrate, 1.8 g fiber, 1.5 g ash, 553 mg Ca, and 51 mg P. Raw, tender tops and pods contain per 100 g edible portion: 59 calories, 80.7 percent moisture, 8.4 g protein, 0.9 g fat, 8.8 g total carbohydrate, 3.8 g fiber, 1.2 g ash, 137 mg Ca, 11 mg P. 9.2 mg Fe, 4,730 mg b-carotene equivalent, 0.09 mg riboflavin, 5.4 mg niacin, and 8 mg ascorbic acid. The genus Leucaena is also reported to contain hydrocyanic acid, leucaenine, quercitrin and tannic acid.

Description
Arborescent deciduous small tree or shrub, to 20 m tall, fast-growing; trunk 10–25 cm in diam., forming dense stands; where crowded, slender trunks are formed with short bushy tuft at crown, spreading if singly grown; leaves evergreen, alternate, 10–25 cm long, malodorous when crushed, bipinnate with 3–10 pairs of pinnae, these each with 10–20 pairs of sessile narrowly oblong to lanceolate, gray-green leaflets 1–2 cm long, less than 0.3 cm wide; flowers numerous, axillary on long stalks, white, in dense global heads 1–2 cm across; fruit pod with raised border, flat, thin, becoming dark brown and hard, 10–15 cm long, 1.6–2.5 cm wide, dehiscent at both sutures; seeds copiously produced, 15–30 per pod, oval, flattish, shining brown, 18,000–24,000 per kg; taproot long, strong, well-developed. Tree grown as an annual when harvested for forage. Fl. and fr. nearly throughout the year.

Germplasm
Over 100 cvs and botanical varieties, and several closely related or synonymous species contribute to the leucaena genepool. The commoner 'Hawaiian' type, native to coastal Mexico, is now a widespread tropical weed. It is versatile in adaptation and has become a serious weed in cultivated areas and wastelands. The 'Salvador' type, less aggressive and more tall and tree like, producing twice the biomass as the Hawaiian types. The 'Peru' type, treelike also, branches low down on the trunk, it contains several cvs highly productive of forage. There is quite a variability in mimosine content and cleaner cvs are in order. Colombian cvs and Leucaena pulverulenta have much less mimosine. Assigned to the Middle America Center of Diversity, leadtree or cultivars thereof is reported to exhibit tolerance to aluminum, disease, drought, high pH, heavy soil, laterite, light frost, limestone, low pH, salt, slopes, weeds, and wind. (2n = 104, 36).

Distribution
Native throughout the West Indies from Bahamas and Cuba to Trinidad and Tobago, and from southern Mexico to northern South America. Naturalized northward to southern Texas, California and southern Florida, and southward to Brazil and Chile: also naturalized in Hawaii and the Old World tropics.

Ecology
Requires long, warm, wet growing seasons, doing best under full sun. In Indonesia it is grown to 1,350 m, in Java to 1,080 m. Natural stands are found mostly below 500 m in areas of 6–17 dm rainfall. Its growth rate is slower, at higher altitudes. About one dm per month is required for good growth. The plant is known for its drought tolerance. The leadtree thrives on a wide range of soils, but the most rapid growth is on deep clay soils which are fertile, moist and alkaline. It tolerates aluminum and soils low in iron and phosphorus. It grows best on neutral or alkaline soils, but does poorly on acidic latosols unless Mo, Ca, S and P are added. Its deep root system permits it to tolerate many soil types, from heavy soils to porous coral. Ranging from Warm Temperate Dry to Moist through Tropical Very Dry to Wet Forest Life Zone, leadtree is reported to tolerate annual precipitation of 1.8 to 41.0 dm (mean of 30 cases = 14.9), annual mean temperature of 14.7 to 27.4°C (mean of 30 cases = 24.0°C), and pH of 4.3 to 8.7 (mean of 21 cases = 6.1).

Cultivation
Trees, propagated by seed or cuttings coppice well. Some seedlings less than one-year old will produce viable seed. Seeds remain viable from several months to several years. The hard waxy seedcoat makes scarification necessary before planting. For forage, seed should be sown 2.5–7.5 cm deep, planting at onset of wet season. Leadtree responds favorably to fertilizer and lime. Irrigation and cultivation may be necessary. The crop soon produces a dense stand.

Harvesting
The crop can be cut at any stage for silage or fodder.

Yields and Economics
Leadtree produced 56 MT/ha/year green forage in Hawaii at 24 m altitude. With adequate moisture yields of 80 MT/ha have been obtained. Two year old trees have yielded 4.5–7 kg pods per tree. Duke (1981a) reported annual DM yields are ca 2–20 MT/ha, equivalent to up to 4,300 kg protein per hectare, nearly double the yields of alfalfa. In his pphytomass files, Duke (1981b) reports DM yields of 2–13 MT/ha/yr in Australia, 14–16 in Brazil, 15–20 in Cuba, 35 in Mauritius, 13 in New Guinea, 15–19 in Taiwan and 3–21 in the Virgin Islands.

Energy
With its rhizobium, leucaena can fix more than 500 kg N/ha. On 3 to 8 year trees, annual wood increments vary from 24 to over 100 m3/ha averaging 30 to 40. Dry leucaena wood has 39% the calorific value of fuel oil (10,000 cal/kg), lecaena charcoal 72.5%. In , Molokai, Hawaii (Brewbaker 1980) a 400-ha farm of Leucaena leucocephala on a four year rotation is expected to fuel a two megawatt facility producing 11.6 million KWh/yr. This will replace about 22,000 barrels of diesel. "Wood yields of the giant Leucaena equal or exceed those of other tropical trees and can be the equivalent annually of 30 barrels of oil per acre." Conservatively, rounding their 22,000 barrels per year down to 20,000 barrels, or about half of Panama's daily import, two 400-ha farms may satisfy one day's requirement for Panama, and 730 400-ha farms may satisfy annual requirements or 292,000 ha. Conservatively again, 300,000 ha of Leucaena could satisfy Panama's current energy requirements, providing in the process more than 450,000 metric tons of nitrogen-rich dry macter using the assumptions adopted by Brewbaker (1980). (Curtis and Duke, 1982)
According to NAS (1977b) the fertilizer value of the "fines" is as follows:
N 2.2–4.3%
P 0.2–0.4
K 1.3–4.0
Ca 0.8–2.0
Mg 0.4–1.0
Even under favorable conditions, continual browsing, or cutting and removing the wood or foliage will deplete a Leucaena plant of some vital nutrients; fertilization is then required, particularly P, S, Ca, Mo, and Zn. "It adapts badly to acid soils. Lime pelleting addition of a special Rhizobium strain as well as fertilizer containing Mo, P, S, and Ca are needed to get it started. Leucaena also grows poorly in high-alumina soil-5 and requires careful fertilization with phosphate and calcium. With fertilization, good yields are possible in aluminous soils." Small lateral roots near the soil surface carry Nitrogen-fixing Rhizobium nodules which are usually 2.5–15 mm in diameter, frequently multiobed. Functioning nodules are bright pink inside. The Leucaena-Rhizobium partnership is capable of annually fixing more than 500 kg/i (200 metric tons/400 ha). In 1974 Panama consumed 14,400 MT Nitrogen, probably imported. Apparently IRHE and RENARE have discussed using Eucalyptus for their fuel farms, with seed from Australia. Presumably the seeds will be from species or ecotypes adapted to Tropical Moist Forest of Darien. Many species of Eucalyptus may not adapt to such tropical heat and humidity conditions. Leucaena may be more adapted to the climate if not the soil. Many ecologists and foresters would recommend first an examination of the native species already adapted to an area, then consider the existing introduced species before introducing new aliens. Too often the alien species, introduced without its natural enemies, may propagate and invade to become a weed. Almost all species recommended to us as a biomass candidate including many of the botanochemical species have great weed potential.

Biotic Factors
Leucaena is relatively resistant to the pests and diseases prevalent in Hawaii, but extensive plantation culture may invite the breakdown of this apparent resistance. Twig borers, seed weevils and termites, as well as damping off may hinder the plant. Herbivorous mammals may be fond of the seedlings. Bananas may do better in the shade of leucaena than in full sunlight due to reduced damage by Sigatoka disease.

Now, what this study tells us is that the roots do indeed have nodes of nitrogen fixing bacteria (Rhizobium), That using the leaves and pods as soil amendments is very beneficial.
It also mentions that it is a prime fodder tree for livestock.
If you are going to use the wood in a growing mound, it will work just as any other wood, unless you are planting the near surface roots that contain the Rhizobium nodules, you will benefit greatly by using high nitrogen amendments as the between log fill (leaves, pods, spent coffee grounds, etc.).
Once you have built a layer, water it and continue upward.
Good luck with your growing mound, it should perform quite nicely.

Wow. Thanks for this. Looks like this is indeed a good candidate for a hugel bed, as long as the branches are kept down low. One thing Duke didn't mention here is what Lila mentioned, that the branches can root easily. So, it seems best to keep the branches low in the pile.

That's really interesting info on leucaena, Bryant. Makes me think I might reconsider planting it in the future. I am always looking for good, tropical alternatives to alfalfa for my milking goats. Luckily there are a lot of tree legumes here that seem very comparable to alfalfa, and grow like weeds. We have one here called Falcataria moluccana that has become the most hated tree in my area, due to it's habit of spreading like wildfire, then growing extremely tall, fast, and flimsy, and falling down on power lines in big storms. The government is actually now trying to eradicate it (good luck!) My goats love the leaves more than anything, so I am actually allowing some to grow on my land and just keeping it small and not letting it flower, by continually chopping the branches. It's lovely because the wood is light and weak and very easy to cut and carry to the goats.

It seems like the best plants are often double-edges swords

Troy, I didn't know that about the leucaena shoots. Maybe I'll give it a try someday. I reckon I'll still prefer kale, but good to know in case it's ever needed