Nitrogen-Fixing Plants

If you’ve read my posts from this spring, you’ll have endured me going on and on about peas and beans and how they fix nitrogen in the soil.  For those who nodded off during those episodes or who have just tuned in, I’ll go over it briefly.

Some plants have the ability to fix nitrogen in the soil.  Actually, a type of bacteria called a rhizobia invades the roots of plants in the Fabacea family and a few others, and fixes atmospheric nitrogen in nodules on the plant’s roots.  This is beneficial to both the plant and the bacteria, a process called mutulism.  It also benefits whatever grows around the plants because, when the plant dies, the nodules release their nitrogen into the surrounding soil.  In the case of long-lived shrubs and trees that fix nitrogen, as roots die off or are replaced, they release their nitrogen.

An edible forest garden is one where man mimicks the dynamics of an old-growth forest.  Why?  Because forests succeed without the aid of fertilizer, tilling, mulching, irrigation or any interference or ‘help’, as it were, from man.  How does it do this?  The plants that grow complement each other, providing what each other needs.  These relationships are called plant guilds.  You can create plant guilds, substituting plants that provide food for humans.  In a guild there is a taller tree which provides shade and leaf droppings (mulch), shrubs which provide more shade, mulch and habitat for animals and insects, plants that fix nitrogen in the soil, plants that have long tap roots called ‘miner’ plants, because they take up nutrients from deep in the soil and deposit them on the soil surface when their leaves die off, plants that attract pollinators, and plants that are ground covers to regulate heat and moisture.   Using permaculture practices for water harvesting and organic gardening, when the guild matures it should be almost completely self-sustaining.

Say you want to plant an apple tree.  That would be your tall canopy tree for the guild, which drops leaves as mulch.  Beneath it, you could plant a shrubby herb such as rosemary (another edible), daikon radishes (miners, leaving the cut leaves on the surface after harvesting the edible root), bush beans (legumes) and herbs such as dill, parsley and basil, some of which you allow to flower for pollinators.  As the tree grows, the plant guild can widen and others planted.

There are many plants, trees and shrubs that fix nitrogen in the soil.  All beans and peas including soybeans and fava beans do; when the plants are finished cut them above the soil so the roots stay put and decay where they are to release the nitrogen.  Cover crops such as clover and hairy vetch are grown and turned under to improve the nitrogen in the soil.  If you are from the Southern California area, perhaps you’d be interested in knowing what native plants are nitrogen fixers.

The native Southern California nitrogen fixers include: ceanothus, lupine, deerweed, California peashrub (endangered) (lotus),  and redbud.  Non-natives that are commonly used are alders, acacias, calliandra, sweet peas, guaja, and many more, as the Fabacea family is very large.  Use any of the natives in ornamental gardens and not only will you be improving the soil and the vigor of the surrounding plants, but providing much needed habitat for our native birds and insects.

Try building plant guilds; it is challenging and fun.  Many combinations of plants are suggested on permaculture