The Rhode Island Experiment Station began a series of experiments with different forms of phosphorus in 1894. If we add together all the hay and grain crops grown during the decade following the first year of these experiments, we find that the increases per acre were 14,580 pounds for raw phosphate and 14,550 pounds for acid phosphate, on unlimed land; while lime and raw phosphate produced 27,030 pounds, and lime and acid phosphate 29,690 pounds, of increase; and the acid phosphate cost three times as much as the raw phosphate.
In commenting upon these investigations the director of the Rhode Island Experiment Station states that the raw phosphate gave very good results with such farm crops as oats, peas, crimson clover, millet, soy beans, and so forth, but very poor results with such garden crops as turnips, rutabagas, cabbage, beets, lettuce, squash, and so forth, and its use for these garden crops is not advised.
In 1890 the Massachusetts Experiment Station began investigations with different phosphates applied in equal money value, and in his report for 1900 the director states that the raw rock phosphate ranks above the acid phosphate both as an average for the entire period and as an average between 1895 and 1900, during which time the land to which no phosphorus was applied produced only 55 per cent as much as where raw phosphate was used—a result worth every farmer's consideration.
More Bushels and Tons
The Ohio Agricultural Experiment Station has reported investigations covering sixteen years in which raw phosphate was compared with acid phosphate costing twice as much per acre. As an average of all results secured, 320 pounds of raw phosphate applied with manure on clover sod produced 8.4 bushels more corn, 4.7 bushels more wheat, and 0.49 ton more hay per acre than where manure alone was used, and 320 pounds of acid phosphate, costing twice as much money but containing only half as much phosphorus, applied with the same amount of manure, produced 7.5 bushels more corn, 5.1 bushels more wheat, and 0.46 ton more hay than where the manure alone was used.
Now I have presented the averages or summaries of all investigations that have been reported covering ten years or more where equal money values of raw phosphate and acid phosphate have been used, or where any apparent provision was made to supply some organic manure, whether as farm manure, green manure or merely as clover grown in the rotation; and I invite the reader to mix his own brains with these data and not to expect me to state whether he should use the relatively cheap ground natural phosphate rock or the more costly manufactured acidulated phosphate in the improvement of his own soil in systems of permanent profitable agriculture.
Making Phosphate Available
If the natural rock is used it should be ground so that at least 90 per cent will pass through a sieve with 10,000 meshes to the square inch, and of course its content of phosphorus (from 12 to 15 per cent) or of so-called "phosphoric acid" (from 27 to 34 per cent) should also be guaranteed. Moreover it should be used liberally and in connection with plenty of decaying organic matter. People sometimes ask, "How much of the phosphorus in raw phosphate is available?" The best answer to this question is, "None of it; and, if you are not going to make it available, don't use it."
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.