The Importance of Guilds and Nitrogen Fixers

How is it that the natural world provides excessive abundance while not relying on any external sources of nutrients? Nature produces her own fertility needs, firstly through accumulating organic matter on the soil surface which protects the soil, adds to the layer of humus, and stimulates the biological activity of the soil. The natural world, however, also takes advantage of the abundance of nitrogen in the air to supply plants with one of the most important nutrients they need. Our air is made up of almost 70% nitrogen, and almost all plants need major amounts of nitrogen for healthy growth. Nature, then, was left with the question of how to take the nitrogen from the air and deposit it in the soil where plants could use it.

Nitrogen fixing plants have the ability to absorb the nitrogen in the air through their leaves and “fix” the nitrogen in the soil through nodules that grow on their roots. Leguminous plants such as beans and peas do this as well as many other different types of trees, bushes, and shrubs. If you have ever pulled up a bean plant by accident when weeding your garden, you may have noticed many small white nodules sticking to the roots of that plant. Those nodules are pure nitrogen and are contributing to the growth of the plant and to the overall soil health. When that bean plant dies, the nitrogen in the nodules stays in the soil. With nitrogen-fixing trees and bushes, pruning the branches causes the tree to “shed” some of its root systems. The nitrogen nodules “fixed” onto those roots are then released into the surrounding soil for other plants to take advantage of.

For that reason, almost all agrarian cultures developed a traditional planting system that planted a carbohydrate crop with heavy nutrient needs (like corn or wheat) together with a leguminous crop that added needed nitrogen to the soil (such as beans or peas). The three sisters planting system adapted by the Mayans of Central America combined corn, beans, and squash to make sure that each plant contributed to the overall health of the soil with the beans adding needed nutrients for the other two plants.

More recent agrarian cultures have relied on nitrogen-fixing cover crops such as alfalfa, vetch, or winter peas that were plowed into the soil to add fertility and organic matter for the next season´s crop. In perennial tree-based agricultural systems, there are a number of nitrogen-fixing trees and shrubs that can be grown together with your fruit and nut orchard. These trees and shrubs add needed fertility to the land for the other producing trees, and also respond very well to aggressive pruning so that they don´t outcompete your productive trees through causing excessive shade.

This forestry system that relies on heavy pruning is called coppicing or pollarding. Certain trees respond well to pruning. Even when cutting the tree all the way to the stump (coppicing), these trees will quickly send up new shoots and continue to grow. Other trees won´t allow you to cut them down to the stump, but you can completely prune them to about your waist height (pollarding) and they will grow back.

These forestry systems offer you large amounts of organic material through leaves and branches that can either be made into wood chips for mulch or left to rot through a chop and drop system. Either way, you are adding important organic material to the soil. Since fungi are the only creatures able to decompose the lignin in tree wood, branches that are left as mulch also invite a fungal-dominated soil which is important for any tree-based perennial agricultural system.

Furthermore, if the species that you choose for your pollard or coppice system or nitrogen-fixing trees or bushes, every time you prune them they will also release an important source of nitrogen in the soil. Miracle Farms is a diversified orchard in Quebec, Canada. When the owner of this orchard established his trees, he planted one nitrogen-fixing tree or shrub for every two fruit trees. Once established, these nitrogen-fixing trees provided all the nutrients for his orchard helping him escape the expense and hard work of having to fertilize thousands of trees several times per year.

Nitrogen fixing trees and bushes don´t only provide nutrients for the surrounding soil, but many of them also offer important sources of food and fuel. Below we offer a short list of some of the best nitrogen-fixing trees and shrubs and what they offer.

Alder: This genus of trees fixes nitrogen, adds abundant organic material to the soil through its leaves, and also provides fantastic firewood. It responds well to pruning and pollarding.

Black Locust: Another nitrogen-fixing tree that offers a great source of firewood. It grows fast, can be coppiced, and also provides fragrant flowers that attract bees to help with the pollination needs of your orchard. It is some of the highest BTU firewood you can find.

Goumi: This nitrogen-fixing bush produces tasty red berries that are great for jams or jellies.
Seaberry or Sea Buckthorn: This nitrogen-fixing bush offers extremely nutrient dense yellow fruits that are high in Vitamin C and anti-oxidants.

Chipilin: Another nitrogen-fixing bush that provides a protein dense green leaf that can be eaten in soups or salads.

By adding nitrogen-fixing trees or bushes to any agricultural system, you will be helping to improve the soil through adding needed nutrients and a source of organic material. At the same time, many nitrogen-fixing species offer useful fruits, firewood, and other benefits to the homesteader.

Tobias Roberts

After working in the development industry for over a decade, Tobias decided it was time to stop advising Central American farmers how to do things if he didn´t have a piece of land to live coherently with what he taught. Together with his family he runs a small agro-forestry farm, tourism cooperative, and natural building collective in the mountains of El Salvador.


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