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PLANT
NUTRIENTS -
PLANT FOOD FOR HEALTHIER PLANTS & IMPROVED YIELDS
Plants obtain nutrients for their biosynthetic
processes in the form of carbon dioxide, water, nitrate,
phosphate, and ionic forms of potassium, calcium, and
other essential elements. Nitrogen generally enters the
roots as nitrate and becomes assimilated by the
plant’s bio-chemistry into organic compounds.
Accordingly, nitrate can be classified as a
"natural" plant nutrient or can it?.
In a natural system, nitrate in the soil is derived
from the gradual breakdown of humus, the dark, complex,
polymeric material that gives the soil its
"tilth." Nitrogen is integrally bound to the
carbon atoms that make up the organic structure of humus,
which is itself the end product of a complex chain of
events that carries nitrogen into the soil. The main path
of entry begins with the deposition of organic
nitrogenous compounds on the soil in the form of animal
feces and urine and the dead remains of animals and
plants. These largely organic materials are subjected to
hydrolytic and oxidative degradation by decay
microorganisms, yielding organic low-molecular-weight
products that support the growth of soil microbial flora.
These processes finally yield a mass of microbial cells,
which on their death, together with some other remains,
become humus. The other source of soil nitrogen is
nitrogen-fixation, which also delivers the element to the
soil system in organic form. Thus, in a natural soil
system, untouched by human technology, nitrogen enters
into the system in organic combination with carbon,
largely as the nutrient for microorganisms that
eventually produce humus.
Farmers who wish to add nitrogen fertilizer to the
soil to support crop nutrition have two main
alternatives. Nitrogen can be added in a natural, organic
form - as plant residues, manure, sewage, food wastes, or
for that matter, in the form of any nitrogenous organic
compound that can be metabolized by the soil’s
microbial flora and thereby yield humus. In the
alternative, nitrogen can be added in an inorganic form,
such as nitrate or ammonia.
Soil is an integrated system and there is a vast
difference in the outcomes of the two methods. Because
nutrient uptake is a working-requiring process, it must
be driven by the root’s oxygen-dependent energetic
metabolism. Humus is much more than a store of nutrients;
is also the chief source of the soil’s porosity,
hence of its oxygen content, and therefore of the
efficiency with which nutrients, such as nitrate, are
taken up by the crop.
The critical difference between the alternative means
of supplying nitrogen fertilizer is that the organic form
leads to the production of humus, while the inorganic
form does not. The use of synthetic urea as a fertilizer
provides an informative test of this distinction. Urea
is, of course, an authentic organic compound and is, in
fact, an ordinary constituent of a clearly natural source
of nitrogen-urine. The scientific agronomist may often
cite the organic farmer’s objection to pure urea as
a fertilizer - it is a fairly common one in modern
agriculture - as evidence of the irrational basis of
organic farming. But is it?
While urea is, indeed, an organic compound, it will
not support the bacterial growth that is essential for
the formation of humus. When urea is metabolized, the
products are ammonia and carbon dioxide. Thus, urea
yields carbon in a form that will not support the
oxidative metabolism of soil bacteria. To accomplish
that, carbon must be in the reduced state, combined with
hydrogen as it is failing to support the growth of soil
bacteria, and therefore the formation of humus, it does
not quality as an "organic fertilizer."
The intensive use of inorganic nitrogen fertilizer (or
urea) may so overload a humus-depleted soil with nitrate
as to cause it to leach into surface waters when nitrate
levels may readily exceed public heath standards. Leached
nitrate also wastes expensive fertilizer synthesized from
an increasingly diminished supply of natural gas. Apart
from any other possible and yet to be established
virtues, the use of organic fertilizer (as defined above)
avoids these difficulties and holds the promise of
restoring the natural source of soil fertility - humus.
While it remains to be seen whether food grown in such
naturally fertile soil contributes distinctively to the
health of people, the practice can, it seems to me,
contribute significantly to the health of the soil and
the economy.
Dr. Barry Commoner
Director, Center for the Biology of Natural Systems.
Vol. 10, No. 4
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