On my own farm I use about one ton per acre of raw phosphate once every six years, thus adding at least 250 pounds of phosphorus at a cost of less than $8; whereas 200 pounds of the common "complete" fertilizer per acre yearly would cost $12 every six years, and would supply only 40 pounds of phosphorus. I do not use "complete" fertilizers, because there is plenty of nitrogen in the air and plenty of potassium in the soil; and because, by growing and plowing under plenty of clover, I not only secure nitrogen from the air and liberate potassium from the soil but also liberate the phosphorus from the raw rock phosphate applied to the soil. In beginning the use of raw phosphate where the supply of organic manures is limited, I apply one ton of phosphate and 600 pounds of kainit in intimate connection, turn them under, preferably with organic matter, then add ground limestone if needed, and thus prepare to grow clover. 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.