Key points:
- 3 subfamilies of the legume family can fix nitrogen
- Symbiotic bacteria (Rhyzobium) convert nitrogen in the air to a form plants can use
- Repair damaged land in tropical and arid regions with initial plantings of 90% nitrogen fixers
As mentioned in the previous article in this series, beneficial partnerships are the way of nature. In particular, some microbes (Frankia and Rhizobium) form associations with certain plants allowing them to fix atmospheric nitrogen into a form that plants can use. These symbiotic partners can help us to rehabilitate damaged landscapes, preparing the soil for a succession of more long-term plants.

This piece focuses on woody plants that associate with the bacteria of the genus Rhizobium. We can see from the diagram below that there are 3 subfamilies of the family Fabaceae (AKA Leguminosae). These families are Faboideae (AKA Papilionoideae), Mimosoideae, and Caesalpinoideae. Note that not all the trees in these subfamilies are nitrogen-fixers. Among the Caesalpinioideae, 23% are nitrogen fixers. For Mimosoideae, the figure is 90%, and for Faboideae, 97% are nitrogen-fixers.

As the diagram shows, Mimosoideae contains the nitrogen-fixers Acacia, Albizia, Calliandra, Enterolobium, Leucaena, Mimosa, Paraserianthes, and Pithecellobium. Caesalpinoideae‘s nitrogen-fixers are Chamaecrista, Cordeauxia; and Faboideae has Cajanus, Dalbergia, Erythrina, Flemingia, Gliricidia, Pterocarpus, Robinia, Sesbania, and Tephrosia.
To rapidly revegetate a damaged landscape, be sure to include plenty of these species to help quickly build up the soils. In areas of very problematic soil, such as arid, tropical and subtropical regions, make 90% of your initial planting of trees nitrogen fixing, pioneer species (associating with either Frankia or Rhizobium), and 10% of species your long-term canopy overstory species. When the system reaches maturity, the proportions will be reversed with 10% nitrogen-fixing, support species and 90% canopy species. The same formula could be followed for humid temperate regions, but the soils in these area are not so fragile and can stand a lower percentage of nitrogen fixers. A 70/30 or even lower may suffice in these areas, as the seasonal cycles of death and regrowth feed these soils well.
As the diagram below shows, the nitrogen-fixing support trees can be pruned (coppiced, pollarded, shredded or sacrificed) to provide mulch, fodder, fuel or fibre. As this is done, the roots of the tree self-prune, releasing nitrogen into the soil.

The highest concentrations of nitrogen are to be found in descending order in the seeds, the seed pods, the flowers, the leaves and then the woody parts of the tree. Inter-planting with fruit or nut trees naturally provides more soil nitrogen. But interplanting also makes the job of chop-and-drop mulching that much easier.








In the interests of saving people’s crops from raiding elephants, and to save the elephants themselves, one popular technique, widely taught and promoted in permaculture circles is the “Ha-Ha!” fence. The “fence” is actually a trench about 1.5 to 2 metres deep with steep edges to create a barrier that elephants cannot cross (so people with the fence can watch the elephants and say “Ha-ha!” as their crops are safe). In this way, farmers’ crops can be protected without having to harm or kill elephants.
The drawback to this method, however, is that people have to put in considerable effort to dig these trenches out to protect their crops. Now, thanks to the ingenuity of a Thai villager, adopted by the Elephant Conservation Network (ECN) and the Zoological Society of London, there appears to be a simpler, less energy intensive method to deter elephants. The villager had strung old CDs along his fence in an effort to scare off the elephants. It was observed that, particularly during a full moon, “the CDs twisted and shone, mimicking a person with a torch.” It would be a simple matter to rig up a few throwie-type, battery-powered LED lights so that nights without the full moon would also have protection.
Industrial activity and industrial agriculture consume large amounts of the water that is available. And predictions for continental inlands forecast that global warming will bring drier conditions than those that currently exist. Of the total land mass of the Earth, 47.2% is arid, much of that converted to desert by human activity with a further 25% of the land at risk of desertification. And this problem is not limited to poor nations. While 66% of African lands suffer from desertification, 40% of the pastureland in Texas is now too arid for use. And pivot irrigation is turning patches of land on the Great Plains of North America into salted desert in 3 to 4 years. Even former patches of the Amazon rainforest are very close to becoming desert. Global water consumption is rising so fast that by 2025 demand will surpass availability by 56%. We need to shift to sustainable water usage, or we risk creating a planet largely comprised of desert.
Many building codes have a lot of catching up to do when it comes to greywater. A system can be built with a diverter to the currently approved (i.e. unsustainable) system. The diverter can then be used to flow water into the greywater system once the code changes. This system can also be used to deal with freezing conditions mentioned above.
An effective, safe greywater system slowly filters water through the soil so that microbes can devour the organic material in the water. The system should be designed such that people never come in contact with the greywater. The systems can be used to assist food production, provided that the plants are not root crops. Don’t spray grey water on plants (or anything else). The water could contaminate the plant and contaminated water droplets could be inhaled.
The elbow will effectively halve the flow, but you will need to ensure that the pipe leading up to the elbow has at least 50 cm (about 20”) of straight section. Also, the slope of the pipes should be kept at a fall of 1:48 or steeper to keep the water flowing.















In North American culture, bats are portrayed as scary creatures. Fortunately for me, I learned at a very young age that bats eat mosquitoes; and since learning this, I have always viewed bats as friends. Most of my permaculture designs call for bat houses, but this element of the design is usually ignored by those implementing the design or is met with strange looks when I suggest the idea. I hope this piece will explain my desire to incorporate bats into designs.
About 70% of the more than 1,000 species of bats in the world are insectivorous.1 They assist us in controlling biting insects, but their use in insect control is much broader than this.
While some may indeed have bats in their belfry, it is perhaps more useful to build homes for them in locations that are beneficial to your aims. Bats seem to have an easier time locating bat roosts located on poles or on the sides of buildings, and tend to occupy them more readily. They like the interior temperature to be from 26 to 38ºC.7 [See below for links to roost designs.]