Nitrites come from a variety of sources of nitrate within the agricultural
environment, including high-nitrate fertilizers. Nitrates are taken into plant
tissues and used in the synthesis of proteins, but only at a relatively-slow
rate compared to what is often present. For instance, Ag Extension
recommendations often specify the application of an entire season's
(highly-soluble) nitrogen/nitrate needs at one time. This virtually ensures not
only uptake of more nitrate than can be metabolized by a plant at once, but also
leaching of further quantities of highly-soluble nitrates into ground water, a
common problem in agricultural areas, as well as under dairies and feedlots due
to leaching of stored manures. Regulatory agencies in most localities now
require covering of manure piles to prevent leaching, but urines are still
ignored, as well as nitrate leaching from agricultural applications.

Nitrates and nitrites are metabolized in the human stomach into nitrosamines,
which are carcinogenic and corrosive to the system. Soule and Piper (Farming in
Natures Image, 1992) report that "At concentrations of forty-five parts per
million, [nitrates from fertilizers] can cause fatal methemoglobinemia disease,
or "blue baby syndrome" in infants receiving formula made from contaminated
UNITED STATES IS NOT USED BY CROPS" [my caps]. Nitrates are only
extremely-slowly degraded, and so very persistent, once they have reached
groundwater, due to the absence of oxygen and bacterial action. Further problems
with surface waters are produced concurrent with the groundwater problems, of
course, including suffocation of fish caused by depletion of dissolved oxygen by
nitrate-jazzed bacterial and algal metabolisms gorged on nitrates washed into
creeks, rivers and lakes. 

The safest way to apply nitrates/nitrogen/manures/ammonia/etc is by
incorporation into a mulch with active bacterial and fungal populations, a
little at a time, or by using leguminous green manures, which release their
nitrogen slowly over a season (40% if mowed, to 60% if tilled in, during the
first year, the total over about 3 years). Greenhouse operators, not known for
savvy in avoiding chemical use, have nonetheless acknowledged, for reasons
having purely to do with productivity and costs, that nitrogen should be applied
gradually, in tiny doses, evenly throughout the growing season for maximum

Nutritionists recommend holding back nitrogenous fertilizers for several weeks
prior to harvesting leafy greens for human or animal consumption, to give the
plants time to metabolize nitrogen taken into their tissues in excess of what
can be quickly used. This is especially important for young children.