Neem has been grown for years in much of South and Sotheast Asia
as well as Africa.  It is known to have a wide variety of uses, most
importantly insecticidal and medicinal.  Your mention of it as a
fixer of Nitrogen is new to me, and quite frankly I would view it
with some skepticism.

One example of it's insecticidal properties was demonstrated in the
following reference:

Olaifa, J.I. & A.O. Adenuga 1988. Neem products for protecting
field cassava from grasshopper damage. Insect Science and its
Application 9(2):267-270.

The authors had significant results using both Neem oil extract
(from the seeds) and Neem kernel water extract for the control of
grasshoppers in cassava.  Both neem products outperformed the
chemical control products.

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