Disadvantages of Nitrogen fertilizers
Farmers use manure as a source of nitrogen for their crops and
as a way to dispose of it. Many also apply commercial nitrogen
fertilizer, since there's no way to know exactly how much
nitrogen is in the manure. This practice ensures that the plants
get enough of the nutrient to produce profitable yields.
An unintended result can often be too much nitrogen, more than
the plants can use before it washes away in rain, snowmelt
runoff, or subsurface drainage to surface waters. Most of the
nitrogen eventually ends up in rivers and eventually in seas,
where it can create periodic dead zones. Marine animals either
move out of these oxygen-starved waters or they perish.
According to scientist Michael P. Russelle, using manure to
fertilize legumes may help keep excess crop nitrogen out of
bodies of water.
Legumes can capture nitrogen from the air with assistance from
bacteria that live in nodules on their roots. The bacteria
convert the nitrogen into amino acids that plants use to make
proteins. The bacteria in turn get nutrients and energy from the
plants. For this reason, farmers usually don't apply manure or
commercial fertilizers to these crops. In this case, the damage
that the excess of nitrogen causes to the environment is
diminished and If the legumes don't have enough nitrogen, they
will start making their own.
Russelle suggests that fertilization should be done in strictly
controlled conditions, in order to reduce the risks of
contaminating the soil and the water. By carefully planning
fertilization, scientists are able to control pollution of the
environment, discouraging the improper use of natural and
commercial fertilizers.
By using research results, scientists estimate how much of the
crop nitrogen is supplied by nitrogen fixation and how much of
it comes from other sources.
This means that while water-quality experts and watershed
managers will be able to use basin-scale maps to help determine
good candidate areas for manure application to legume crops,
farmers will need to use more detailed information about their
fields to best use the manure to derive both production and
water-quality benefits.