Nitrogen Fixing Trees (NFTs) at Rancho Mastatal
By Nic Donati, Core Team Member
What are Nitrogen Fixing Plants?
A nitrogen fixing plant is a common term used to describe plants that have a symbiotic relationship with microorganisms on their root nodules that allow them to fix nitrogen. The vast majority of these plants are in the Fabaceae family (the legume family), but there are other families of plants that have relationships with microorganisms that allow them to fix nitrogen. There are many trees and soft stemmed plants that have symbiotic relationships with microorganisms on their root nodules that allow them to fix nitrogen. We often in permaculture design refer to all of these as NFTs (Nitrogen Fixing Trees) for simplicity although some of the plants we are referring to are not actually trees.
Root nodules on a Poro sapling
What is Nitrogen fixation?
Nitrogen is essential to all life forms as it is used in the biosynthesis of all nitrogen containing organic compounds such as amino acids and proteins. Nitrogen makes up approximately 78% of the earth’s atmosphere, so it is in abundance, however in its gaseous form (N2) it is not readily available to plants as it cannot be easily metabolized by them. Nitrogen fixation is the process of converting N2 into a compound that can be readily metabolized by most organisms such as Ammonia (NH3).
Nitrogen fixation takes place naturally in the soil by microorganisms. Some species of plants are able to form symbiotic relationships with these microorganisms and we are interested in those plants in permaculture because of the role these species can play at increasing the fertility of our soil in a natural way. Permaculture adheres to the philosophy of working with nature rather than against it and capitalizing on these naturally occurring phenomena is a great practical application of this. We use them a lot at Rancho Mastatal in our food forests, alley cropping systems and our annual vegetable garden system.
Getting Nitrogen into the ground
Nitrogen is one of the most important nutrients for the growth and survival of plants and deficiencies in this key element will lead to problems in plant development. In recent decades this has been dealt with in conventional farming by applying chemical fertilizers. The application of these fertilizers, however, has led to huge environmental degradation. Organic growers may use naturally occurring nitrogen rich amendments like manure, blood and bone etc…. Either way it requires the addition of amendments to our soil. At Rancho Mastatal we absolutely use natural soil amendments, typically we apply dried cow manure, compost, vermicompost and other amendments that we have locally available or make ourselves, this requires a huge amount of work, collecting, hauling and applying them. We ideally want to generate all of the nutrition that our trees need right near our trees so that we don’t need a lot of resources to look after them. NFT’s present a wonderful opportunity to use the natural processes that are taking place in the soil to benefit the trees we are interested in being productive.
Agroforestry
An agroforestry system at the Rancho
Our warm, humid, tropical environment lends itself to the development of agroforestry systems. Our natural ecosystem is the rain forest and our goals as a tropical agro-forester is to create a forest environment utilizing tree species that have yields for human consumption. We typically think of these yields as food, but they could also be fodder for feeding animals, timber for building and branches for firewood amongst others, from now on we will refer to these as tree crops. We also need support species for our tree crops, which we may or may not directly use the yields of. NFTs are good examples of these support species as their roots and leaves are richer in nitrogen than other species. It means when we prune our nitrogen fixing tree and arrange its biomass around our tree crops; we are feeding the soil in multiple ways:
1. The leaves and branches around the tree break down over time and add organic material, and nutrients to the soil
2. Whilst they are breaking down, they are also protecting the soil from the harsh sun and rain.
3. The root die back that is created by pruning our NFT releases nutrients and helps aerate the local soil.
4. Even when not pruned the roots stabilize the local soil and the leaves provide protection and shade from sun and rain.
In our environment we use many different species of NFTs and we would like to share the 9 main species we use, what we use them for and why. Not all NFTs are created equal and some provide different functions than others. As I said earlier we also use the term NFT rather liberally as some of these species are actually not trees!
1. Black Wattle (Acacia mangium)
This is number one on the list as it is the most abundantly used NFT on our site. In recent years we have moved away from using it, as it can quickly get large and unwieldy, though there remains a significant number still on our site. It grows fast like most NFTs and handles pruning relatively well, but not great with a fair few dying each pruning season in comparison to some others on this list. It needs full sun and will naturally form a tall straight trunk. It is commonly used in other countries as a valued timber species. The interesting thing about this species is how it really doesn’t look too much like an NFT. Most species in the legume family have a very characteristic leaf pattern. Acacia mangium though does not have this characteristic leaf pattern and you would not think it was an NFT by looking at it. However its first emergent leaves do possess the familiar NFT leaf pattern and give away its family! It is not super easy to propagate from seed like some other NFTs and we have had no success with cuttings. We mainly used this species planted along contour lines when establishing new orchard spaces and since then have moved away from using it so much. We plant it at tight spacing and prune at the end of each dry season to provide biomass for our tree crops.
2. Cratylia argentea
This is the one species on the list that isn’t technically a tree, it is more of a climbing vine, and boy does it climb fast!!! It is an abundant source of nitrogen rich biomass, it handles pruning excellently and pretty much always bounces back. It grows very easily from seed and produces really beautiful, abundant purple flowers (great for pollinators) at the beginning of the dry season, which really facilitates the collection and storage of seeds. You have to be careful with this species as it loves to climb and it will quickly swamp tree crops if left to its own devices and can strangle and kill them. Its greatest characteristic is its speed of growth and abundant biomass, but this is also the feature which gives us a love hate relationship with this species! It loves full sun but will tolerate some shade. We are using this a lot to fill in gaps in our contour plantings of Acacia mangium where a tree has died.
3. Mountain immortelle (Erythrina poeppigiana)
Known locally as Poro, but be careful as there are several species that are known as Poro in Costa Rica. It does really well from cuttings which makes it an excellent choice as a living post for fences or a living post for trellises for climbing crops like passionfruit or dragonfruit. It has an attractive flower which is great for native pollinators and grows easily from seed. The one drawback of this species is it is typically spikey which can make it a little unfriendly to work with and to use as biomass.
4. Brazilian Fire Tree (Schizolobium parahyba)
Is by far the fastest growing NFT on the list along with Cratylia argentea, the main difference being them is it is a tree rather than a vine and it grows straight up…. Very quickly! It can grow up to 30m in 5 years, as you can imagine the wood is very light and spongy. It doesn’t produce a huge amount of foliage and creates a very light shade, it is supposed to keep its leaves during the dry season too as one of its major advantages, however in recent years it has dropped all it leaves so I will leave that one up to you. We typically grow this from seed and use it as a light shade for tree crops.
5. Quickstick (Gliricida sepium)
This species is known locally as Madero Negro, grows well from seed and has about a 50% chance of survival when planted from cutting. The mature tree is a low growing, typically not straight, that produces excellent high quality heartwood timber. The heartwood timber is known for its durability, rot resistance and is used a lot for foundations. It is also used a lot as a living fence post and the leaves can be used to deter fleas and kill rats. At the Ranch we typically use it as a living post for vanilla and black pepper. It tolerates shade, as in it will survive in shade, however don’t expect a lot of growth.
6. Ice Cream Bean (Inga Edulis)
The fruit of the ice cream bean
This is one of the few NFTs that also produces an edible fruit in our environment. Locally the fruit is known as Guava or in English it is called Ice Cream Bean. There are often many wild forest varieties of this tree, some of which produce beautiful heartwood timber. Inga Edulis however produces useable timber, but it is prone to termite damage and should only be used for furniture and non structural elements. It produces huge and abundant leaves and reacts quite well to pruning, making it an excellent choice as an NFT. If you are continually pruning it though for biomass don’t expect to get any edible harvest from the tree. This will grow into a very large tree if left unmanaged.
7. Venezuelan Rose (Brownea macrophylla)
This is primarily planted as an ornamental, that produces a very beautiful and striking flower at the end of the rainy season. It also tolerates shade quite well and will still produce some flowers. It can be frequently pruned but doing this will effect flowering. We struggled for many years to source this rare beauty!
8. Peanut Grass (Arachis pintoi)
Again not technically a tree but provides a formidable and fast growing ground cover. One of the important permaculture principles that we apply is covering all of our soil. Peanut grass is able to do this quickly and effectively even in very poor soils and can survive our harsh dry season and bounce back quickly once it starts raining again. It produces pretty yellow flowers and is very resistant to mowing or weed whacking. We try to use this as much as possible as a groundcover however it prefers full sun and whilst it will survive in shady areas does not really grow or spread quickly. It is most easily produced by cutting and has a very, very high success rate. Seeds can be purchased but really why would you bother? It is easier to plant via cutting.
9. Wild Hops (Flemingia macrophylla)
A bushy shrub that doesn’t get taller than a couple of meters on our site which is one of its biggest benefits! It is easy to propagate from seed and cuttings making it a great choice for our site. It seems to tolerate shade relatively well, but does prefer full sun. Its low bushy growth habit means it doesn’t swamp other plants like the Cratylia can, but it doesn’t produce nearly as much biomass. Although relatively small amount of biomass is produced in comparison to some of the other NFTs the leaves do take comparatively long to break down and provide excellent mulch. In comparison to the Schizlobium’s feathery leaves that break down incredibly quickly. We mainly start these in our bamboo planters either from seed or cuttings and use them to fill holes where other NFTs have died or not taken well.
Want to learn More?
These are the main NFTs we use at the Ranch. If you are interested in learning more about how to use Nitrogen Fixing Trees in your landscape why not join us for a Permaculture Design Course or Agroforestry workshop.
We also have some of these trees available seasonally in our nursery, so get in touch if you would like to buy some.
Happy planting!
Other blogs related to soil fertility
Improving Soils in the Humid Tropics
Making Microbes: Fungal vs Bacterial Soil Life