Nitrogen fixers are an integral part of any garden food forest or perennial system as a lack of nitrogen is often a limiting factor in ecosystems. Nitrogen fixers have the unique ability to take nitrogen out of the atmosphere and convert it into a form which is available to other plants. They do this with the aid of symbiotic bacteria which in make nitrogen available to plants in return for sugars and nutrients from the plant. These bacteria are visible as white nodules which form on the roots of Nitrogen fixers.
White Nodules contain N fixing bacteria
Most plants which can fix Nitrogen are members of the Legume family. This family includes peas, beans, clovers as well as a wide range of shrubs and trees.
In the garden we incorporate Nitrogen fixers by regularly planting peas, beans and clover, either amongst other plants or as part of a crop rotation.
In perennial areas we include a range of fast growing pioneer species such as Tagasaste to fix Nitrogen and improve the soil before or as we plant fruit trees. As the fruit trees mature and increase in size many of these Nitrogen fixers can be chopped down and used as mulch. If this is done in a well thought out manner fast growing trees can be used to block light and suppress the growth of weeds without sacrificing the fast growth of fruiting species. Groundcovers or intercrops of clover or beans can also be incorporated into design.
Tagasaste is a fast growing tree of 3 - 5 m which is useful as a stock feed and which is used in native restoration projects for its prolofic flowers which feed many types of native birds. It can spread but is not highly invasive. Standing beneath a tagasate tree in flower is an amazing experience, the air above you is constantly moving with bee's and birds moving through the tree.
I recently got some seed from Unitec which after boiling for 30 seconds I planted in my hot house. I now have about fourty seedlings which I will plant around my property particularly along the grass verge which runs down the side of the section as well as a few food forests I am working on around the place. After 3 - 6 years the trees will be cut out and the branches used for mulch and the trunks for firewood.
Mature tree in flower