Shelley,
First of all, there is a negative effect on the soil ecosystem (responsible
for cycling minerals in the soil and making nutrients available to plants)
when you add any salt. And NPK fertilizer is a salt. This has the effect of
requiring more fertilizer for every subsequent crop. So, to me, the argument
that "a molecule is a molecule" is moot. The natural nitrogen cycle in the
soil can provide all crops with enough nitrogen to eliminate the use of
nitrogen fertilizers.
There's no shortage or lack of availability of nitrogen- the atmosphere is
70+% N and this can be tapped through the use of legumes as green manure
crops. And the N2 carried into the soil through rainfall can be introduced
into the soil nitrogen cycle by naturally occurring nitrogen fixers. This
assumes that the soil has been managed properly for the existence and thriving
of a balanced ecosystem.
Those using chemical fertilizers, for the most part, also use pesticides.
These have an even more dramatic negative effect on the soil ecosystem. They
kill off the plant nutrient cyclers, disease suppressers and symbionts. This
leaves the niches open for a population explosion of plant pathogens.
Insecticides have been found to be better at killing bacteria than insects and
certain classes of bacteria are responsible for producing natural nitrate in
the soil. So, the use of these means you HAVE to use manufactured N
fertilizer.
Also, when you suppress the natural nitrogen cycle in the soil by disrupting
the soil ecosystem and add nitrate fertilizer, you slow down root growth.
Ammonium (NH4+) is the start of this natural cycle as a waste product from
predators in the soil. This is oxidized by certain bacteria and converted to
nitrite. This is further oxidized by other bacteria into nitrate (NO3-).
Killing off these bacteria and adding nitrate as the only N in soil results,
as Arnold J. Bloom, Professor Dept. of Vegetable Crops, UC Davis puts it:
"In the field and the laboratory, tomato roots grew more rapidly under NH4+
nutrition than under NO3–. Changes in rhizosphere pH may mediate the
differential effects of NH4+ and NO3– upon root growth. Absorption of NH4+ and
its subsequent assimilation acidify the rhizosphere, whereas absorption of
NO3– alkalinizes the rhizosphere. Acidity may displace Ca2+ from the root cell
wall and increase its extensibility; alkalinity may diminish cell wall
extensibility. Thus, exposure to moderate levels of NH4+ should stimulate,
whereas exposure to high levels of NO3– should inhibit, root growth. This was
observed in both soil and solution culture."
And, only 20 to 30% of the nitrate fertilizer added to the soil is absorbed by
plants. The rest winds up in our ground and surface water leading to poisoned
wells and massive blooms of water plants and algae in streams.
So, arguing about molecules seems, at least to me, the wrong place to look.
Our attention should be focused on what promotes soil fertility and the
growing of high nutrition food. I believe the soil life can lead us to the
answers.
This leads me to a question- Has anyone out there heard of research being done
at Duke about plant nitrate preferences? Is this just a rural (as opposed to
urban) rumor?
Alex McGregor
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