Just like humans, plants need an adequate supply of water and nutrients to survive. To thrive, they require living soil full of organic matter and microbial activity. In Kona, our soil contains a high percentage of volcanic rock and ash, which drains quickly and doesn’t contain or hold all of the nutrients plants need.
When we had a supply of free mulch readily available from the county green waste processing facility in Kaloko, we could easily access that to help enrich our soil. With the mulch now in faraway Waikoloa, we need to implement other more sustainable strategies for amending our soil. Growing and using nitrogen-fixing plants is one sure way to add nitrogen and nutritious mulch to our gardens.
Several nitrogen-fixing plants are available and do well here. Starting from the ground up, you can grow perennial peanut on fallow beds, between rows of vegetables or in a sunny orchard of fruit trees.Glircidia is a medium-sized tree often used to shade and nourish cacao orchards. Pigeon pea is a small tree that can provide some shade, soil nutrition and edible beans. Another edible plant that fixes nitrogen is the ice cream bean. All of these are available locally.
Each of these plants is unique in their ability to collect nitrogen from the air, through a relationship with bacteria that lives inside special root structures called nodules. The nitrogen fixed by the bacteria is transferred to the plant in exchange for the food the plant provides the bacteria. The young stems and leaves of these plants are also nitrogen-rich. When pruned, the branches are used as mulch in garden beds or orchards.
When the nitrogen fixers are cut down, the nitrogen in the nodules is slowly released into the soil and made available to surrounding plants.
Many of the plants that fix nitrogen are in the legume family but not all legumes are capable of fixing nitrogen. Most of the legumes grown in Hawaii fix nitrogen. Several species are particularly well-suited to offer nitrogen fixation for Hawaiian gardens.
Though the recommended species are excellent nitrogen-fixing plants they can serve other purposes in your landscape. The trees and shrubs can provide shelter from wind and from midday sun and heat. Several also offer fodder for animals or food for your table. Some make excellent timber or fuel wood and some help stabilize the soil and prevent erosion. All offer some measure of wildlife habitat and some have medicinal and cultural uses.
Perennial peanut, Arachis glabrata, is a low-growing ground cover with small round leaves and attractive yellow flowers. It is an excellent plant for hillsides, where it can prevent erosion. Its resistance to drought, nematodes and pathogens and its minimal fertilizer needs make it an excellent choice for ornamental landscapes and farms. This plant’s ability to fix atmospheric nitrogen for its own fertility adds to the desirable qualities that make it a xeriscape plant, worthy of consideration for any landscape. By covering long cuttings of the plant with soil, you can create new mats of perennial peanut to use in your landscape.
Gliricidia sepium, sometimes known as Madre de Cacao, is often used in cacao orchards to provide a light overstory and nutritional source. It can be grown as a hedging plant, to form a barrier or trimmed as a specimen tree. It quickly develops long, thin branches that make excellent material for mulch. Its cut or living trunks also make excellent fence posts. Its lavender flowers, as well as its rapid growth rate, make it a desirable plant. Gliricidia is easily propagated from cuttings.
Pigeon pea, Cajanus cajan, can be grown as a perennial shrub or small tree. Like other nitrogen-fixing plants, pigeon peas have several uses. They are often used to offer light shade for greens or other vegetable plantings that need protection from the sun’s heat. Prunings from pigeon peas offer nitrogen-rich mulch and its seeds are edible. The beans can be eaten green or harvested when the pods are dry, yielding beans known in Spanish as gandules. Though pigeon pea is a perennial, it is somewhat short-lived. New trees can be grown easily, however, from some of the tree’s prolific crop.
The ice cream bean, Inga edulis, is another example of a nitrogen-fixing plant with multiple uses. This tree is well-suited to upper elevation sites since it prefers to grow in acid soil and a humid environment. Its major distinguishing characteristic is the beans it produces. Ice cream beans can be up to 3 feet in length and are filled with a sweet cotton candy-like pulp that provides an enjoyable treat. The sweet pulp hides a row of seeds useful for propagating new plants.
Two native Hawaiian plants can also provide nitrogen fixation. Both the native koa and the smaller koaia can make wonderful additions to a landscape while offering an additional nitrogen source. Koa is better suited to upper elevation sites, while koaia can thrive in smaller landscapes at lower elevations.
All of these plants are locally available and planting a few can get you started on a path to sustainability as you begin to produce your own fertility on site. They are all easy to propagate and can be kept small by regular coppicing that enables full use of their nitrogen-rich cuttings.
Watch for classes or go to agroforestry.net/pubs/nftguide.pdf for more information on growing and using nitrogen-fixing plants.
Diana Duff is an organic farmer, plant adviser and consultant.