"PGPR" What is It?

[dice]

Here is a good summary on soil microbial life that mentions
nitrifying (converts ammonia to nitrates) and de-nitrifying
(converts various nitrogen compounds back to nitrogen gas)
bacteria and the conditions that are conducive to the
activity of de-nitrifying (evil) bacteria:
http://www.extension.umn.edu/distrib...ems/M1272.html

edit:
Looking at the rates at which ammonia and other nitrogen fertilizers
are converted to nitrates listed in the document above, winter legume
cover crops are not likely to leave enough ammonia behind to contibute
to BER in a summer tomato crop. By the time the legume is mowed in
late spring, before the tomatoes (or whatever) are planted, the only
ammonia still remaining in that form is likely inside the root nodules
or adjacent to them. Within a few weeks, before flowering and
fruit set in the subsequent row crop, other bacteria convert it to
nitrates (unless the soil is waterlogged, in which case conversion goes
the other way, back to nitrogen gas that escapes into the atmosphere).