Subject: Re: what plants "fix" nitrogen?

From: Tristan Hatton-Ellis <[email-masked]>
Date: Wed, 28 Feb 1996 19:28:26 GMT

[email-masked] (Gad Zukes!) wrote:
>In <[email-masked]> [email-masked] (Bryan Cort)
>writes: 
>>What plants can i grow in my garden that will "fix" nitrogen?  I would
>>be interested in both food crops and flowers.  The only one I can
>>remember is peanuts, and I may well be wrong.
>>
>>>Bryan Cort
>>[email-masked]
>
>Hi Bryan - All legumes fix nitrogen - sweet peas, garden peas, snow
>peas, sugar snap peas, crowder peas, string beans, shell beans, dry
>beans, fava beans, scarlet runner beans, cleome (flower), and PEANUTS!!
>In addition, ground covers include clover and alfalfa. 

Actually, not quite all legumes fix nitrogen - some of the more shrubby 
species and trees have lost the habit. In addition, because nitrogen 
fixation depends on a partnership with a symbiotic bacterium, it is 
necessary to have the bacteria present in the soil to induce root nodules 
to form. It is in these nodules that nitrogen fixation takes place. 
Particular strains of the bacterium are specific to particular legumes, so 
if you are growing an exotic legume (especially an unusual one) the right 
strain of bacteria may not be present in the soil. However, in the garden, 
where nitrogen is not usually too limiting, this is unlikely to affect the 
growth of the plant much.

Alders (Alnus spp.) also have nitrogen-fixing bacteria; there are also 
various nitrogen fixing organisms living free in the soil.

Finally, blue-green algae (cyanobacteria) fix nitrogen. These are common in 
ponds and lakes in summer, but are not exactly most peoples first choice to 
encourage in gardens! Some are toxic, and may cause fish deaths when they 
occur in large numbers. The little floating fern Azolla filiculoides (a US 
native I think) harbours cyanobacteria on its surface, and since it grows 
very fast in suitable conditions might be quite useful for fixing extra 
nitrogen.

Don't forget that plants are limited by other things besides nitrate 
availability. Phosphate and Potassium are particularly important, and trace 
elements help too. Application of too much nitrate causes lots of leaf 
growth but little flower, which isn't necessarily desirable. However, using 
clovers (for example) as a green manure is a good idea, and will certainly 
improve overall soil quality by upping the humus content.

Tristan