HOW ORGANIC FERTILIZER WORKS
Organic Fertilizer versus Chemical Fertilizer
To begin with, let us recall some basic facts about plant nutrition.
Green plants obtain raw materials for their biosynthetic processes in rather simple forms: carbon dioxide, water, nitrate, phosphate, and ionic forms of potassium, calcium, and other essential elements. Nitrogen, to choose a particularly contentious example, almost always enters the roots as nitrate, becoming assimilated by the plant’s biochemistry into organic compounds such as amino acids and nucleotides. There is no doubt, then, that nitrate is a “natural” plant nutrient. Nevertheless, a strict organic farmer does not wittingly fertilize his crops with nitrate – or with ammonium salts, which are quickly converted to nitrate by soil bacteria.
Why should a natural plant nutrient such as nitrate be regarded as unnatural when added to the soil as fertilizer?
To appreciate this argument, we need to go back into soil ecology beyond the immediate entry of nitrogen into the roots. 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 lowmolecular-weight products that support the growth of 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.
Now a farmer who wishes to add nitrogen fertilizer to the soil to support crop nutrition has 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. Alternatively, nitrogen can be added in an equally natural, but inorganic form, such as nitrate or ammonia. The first choice is the one made by the
organic farmer; the second is the conventional route of modern agriculture technology. The strict devotee of natural foods is likely to reject grain grown with inorganic fertilizer in favor of that grown “organically” with manure or compost, sometimes claiming that the nutritional value and keeping qualities are superior – a claim that at this point can neither be confirmed or denied.
Is there, then, any point in differentiating between the two ways of supplying fertilizer nitrogen? Indeed there is. Considering the soil as an integrated system, 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; it 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.
Thus, 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 upport the oxidative metabolism of solid bacteria. To accomplish that, carbon must be in the reduced state, combined with hydrogen, as it is in the nearly all more complex organic compounds. Although urea is an organic compound, by failing to support the rowth of soil bacteria, and therefore the formation of humus, it does not qualify 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 health 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) avoid 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
Used with permission from Hospital Practice magazine
Vol. 10, No. 4