I don't believe it. AFAIK, only the pea family can do this. Grevilleas can extract more phosphorous from the soil, which you can then use by composting the grevillea. Wattles can 'fix' nitrogen.

Casuarinas can fix nitrogen, too - but Nathan is right. For a good quick job at it the pea family (legumes) are tops. You'd only grow Casuarinas if you wanted the trees, which I understand people do in parts of Africa, for their qualities as windbreaks, soil erosion preventers, and timber providers - both for making things, and for firewood.
I have a suspicion that there is some new research somewhere to say that Grevilleas do fix nitrogen in a minor way, but I think that, like Casuarinas, you wouldn't really be growing them to improve the soil for another crop the way you'd use a legume for green manure.
Peas of various kinds, on the other hand, can be grown for food in soil of poor fertility - but I expect you already know this.

Trish

Thank you. I normally spend 8-10 hours a week researching topics on the Internet and that was the only mention I saw, so I had my suspicions.

Trish, you mentioned 'casuarinas', this sounds like another plant (used for soil improvement, firewood, livestock feed)that we are trying to introduce - Calliandra calothyrsus. Is 'casuarinas' possibly another name for the same plant, or have I possibly stumbled on a second option?

Trish,

I immediately went to the Internet to check on this tree. It is definitely different from the calliandra, and I was very excited about Casuarina equisetifolia until I found this note on a website from Purdue University -
"WARNING: Casuarina can exhaust the soil moisture, lower the water table, and restrict understory growth, leaving the soil exposed. Some species are agressive weed species."

Do you have first hand knowledge of this species? The area where I work is subject to prolonged drought and I would not want to introduce a plant that would exacerbate the problem, no matter what the offsetting benefits.

Thank you,
Cheryll

There are lots of (allo)casuarinas. There are numerous species, some grow in deserts, some grow along river banks. Any plant which is well adapted for a climate has weed potential.

I would suggest you only import male allocasuarinas and grow them from cuttings, but there is a significant chance that some dimwit will import a female.

Casuarina and Calliandra are two very different plants. Calliandra is a group of New world legumes including some species from the US Southwest and Mexico and some from the tropics. Casuarina is pretty much Australian/Indonesian I think.

I would be very cautious about introducing Casuarina to a new area. In some parts of the world they have naturalized and become very invasive.

Some plants in the genus Alnus can also fix nitrogen.

I am a semi-retired agronomist who is working in other Third World countries on increasing the productivity of small farmers.
I advise you to go through the Internet and check out the research being done in places like Kenya. Sites like FAO, CIMMYT, ICRISAT and others have good stuff. Have a look at 'Conservation farming for the tropics' in the search engiune and see what comes up.
I am sure that there are adapted legumes which fit in with the farming system in Kenya.

It seems that Grevillea are able to fix nitrogen.
http://cat.inist.fr/?aModele=afficheN&cpsidt=17038770

Abstract
A field experiment was performed in eastern Kenya to estimate N2 fixation by Sesbania sesban over an 18-month period using the 15N dilution method. The influence of three reference species, Senna spectabilis, Eucalyptus saligna and Grevillea robusta, on the estimates of N2 fixation was also assessed. Percentage Ndfa (nitrogen derived from the atmosphere) was calculated based on foliar atom excess (FAE), above-ground atom excess (AAE) or whole tree atom excess (WAE) data. The differences in atom% 15N excess values between species and plant parts are presented and discussed. We recommend the use of several reference species for estimating %Ndfa and that the different results obtained should be carefully considered in relation to the issues being addressed. In this study, Senna was the most suitable of the three reference species because its N uptake pattern and phenology were very similar to those of Sesbania. When well established, the amount of N fixed by Sesbania accounts for more than 80% of its total N content, according to FAE-based estimates. We estimated the Ndfa by Sesbania after 18 months to between 500 and 600 kg ha-1, depending on whether FAE, AAE or WAE data were used and on the choice of reference species. The substantial accumulation of N in planted Sesbania highlighted its potential to increase the sustainability of crop production on N-limited soils. We consider the 15N dilution method to be appropriate for quantifying N2 fixation in improved fallows in studies, similar to this one, of young trees with high N2-fixing ability.

If someone buys this document I'll very happy to know if this tree is able to fix nitrogen and if this tree is able to fix a lot of nitrogen. Thanks.
PS : I'll not buy it because I'll not be able to understand a lot of things. I am not english speaker.