By far the most important agencies under the farmer’s control for the liberation of plant food are the decomposition products of fermenting or decaying organic matter, such as green manures, crop residues and ordinary farm manures. In the decomposition of these organic materials sour or acid products are formed. Thus vinegar, containing acetic acid, is formed from the fermentation of apple juice, hard cider being an intermediate product. Sweet, chopped, immature field corn becomes sour silage in the silo, lactic, acetic, carbonic and other acids being formed. By a similar process cabbage is turned into sauerkraut. Likewise sweet milk becomes sour, with the formation of lactic acid. Oxalic, citric, tartaric, succinic, malic, gallic and tannic are other well-known organic acids. Some of these are contained in the sap or juice of certain plants, and these or others are formed when crop residues are decomposed in the soil.
In the ultimate decomposition of organic matter the carbon appears in the form of carbon dioxid which when combined with water forms carbonic acid. Though this is a very weak acid, its solvent action is very important.
But, in addition to the various organic acids and carbonic acid, we have also to consider the formation of nitric acid in connection with the decomposition of organic manures. Nitric acid is one of the strongest known, and in solvent power it is excelled by no single acid. The nitrogen contained in crop residues and other organic manures is chiefly in chemical combination with carbon, oxygen and hydrogen, much of it in insoluble protein compounds. Normally this organic nitrogen is transformed in the soil, first into ammonia nitrogen, next into nitrite nitrogen, and lastly into nitrate nitrogen, these three transformations being effected by biochemical action produced by different kinds of living microscopic organisms called bacteria. Though detectable amounts of free nitric acid do not accumulate during this process of nitrification, the soluble nitrate or final product is formed by the action of nitric acid upon a mineral base, such as calcium, magnesium, or potassium, which may have been in the soil in insoluble form, so that the nitrogen must pass through the form of nitric acid in the transformation into nitrates.
While the organic matter applied to the soil contains about twenty times as much carbon as nitrogen, and while corresponding amounts of carbonic acid and important amounts of intermediate organic acids must be formed, it is of much interest to know that even the nitric acid formed in the transformation of organic nitrogen to nitrate nitrogen in sufficient quantity for a given crop is seven times as much acid as would be required to convert raw rock phosphate into soluble phosphate to furnish the phosphorus required for the same crop. A knowledge of this definite quantitative relationship should help us to appreciate the possibilities of decaying organic manures in the important matter of making plant food available, including potassium, calcium and magnesium as well as phosphorus and nitrogen.