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fertilizing elements required by plants in forms that insure plentiful crops and permanent fertility to the soil. It not only enriches the soil with the nitrogen, phosphoric acid and potash, which it contains, but it also renders the stored-up materials of the soil more available, improves the mechanical condition of the soil, makes it warmer, and enables it to retain more moisture or to draw it up from below."1 It has a forcing effect when fresh.

Barnyard manure rapidly undergoes change and deterioration. The latter results mainly from two causes: (1) Fermentation, and (2) weathering or leaching. Losses from leaching may be prevented by storage under cover or in pits, while proper absorbents and preservatives, such as gypsum, superphosphate and kainit, will almost entirely prevent destructive fermentation. The manure should be kept moist and compact. The loss is less in deep stalls than in covered heaps. The fertilizing constituents of well rotted manure are more quickly available to plants than those of fresh manure, and the former should be used when prompt action is desired. In the wheat lands of California manure is more or less visible for four or five years after its application to the land, and in the semi-arid region it must be used cautiously on unirrigated land. The light soils of the Pacific coast lack the moisture requisite for the absorption of wheat straw plowed under, and consequently it must be burned. This wastes the nitrogen element of the straw, but saves the ash ingredients for the land. Land treated with stable manure for 6 years gave an increase of 60 per cent in the yield of wheat. Ten tons has been given as a reasonable amount to apply to one acre of wheat land.

Guano.—The first shipload of Peruvian guano was imported by England in 1840. Two years later a company was organized to trade regularly in this substance. From 1.5 to 2 cwt. per acre of wheat was harrowed in with the seed. In the United States it quickly gained in popular favor. By 1876 the trade was regulated by national treaties, and millions of dollars were engaged in its transportation. Peruvian guano was used chiefly for its ammonia. The later guanos of the West Indies were rich in phosphates, and of greater advantage to some crops than the Peruvian. Guano was also one of the principal 1 Yearbook U. S. Dept. Agr., 1895, p. 570.

sources of nitrogen. Most of the guano beds are now completely exhausted.

Phosphoric Acid.—The four main sources of this are: (1) Bones; (2) phosphatic deposits like those of South Carolina, Florida, Tennessee, North Carolina and Virginia in the United States, or the keys of the Carribean sea; (3) accumulation of fossil and excrementitious material; and (4) Thomas slag, a by-product of the smelting of iron ores. Bones were used to a limited extent in England before 1810. They were ground until Liebig made the discovery of preparing superphosphate of lime by dissolving bones in sulphuric acid. One bushel of bone dust dissolved by one-third its weight of the acid is superior as a manure to four bushels of bone dust. The lime of the bones is converted into gypsum, and the phosphoric acid is reduced to a state more easily soluble and assimilable. Formerly bones were also often burned.

In 1817 superphosphates were first manufactured in England, and the first phosphate mined commercially in South Carolina was in 1867, six tons. The earliest form of mineral phosphate used for fertilizer was apatite. In Canada 10 years' experience has shown that finely ground, untreated mineral phosphate has no value as a fertilizer. The Bessemer process of manufacturing steel gives a by-product, rich in phosphoric acid, "produced by the union of the phosphorus of the iron with the lime of the flux employed." This is reduced to a fine powder and applied, without treatment, to the soil. It contains from 15 to 20 per cent of the acid. The yield of wheat seems to be little affected by the carrier or source of phosphoric acid if the material used is finely ground. Some 30 to 60 pounds of the acid should be applied to an acre of land.

Lime.—It has been claimed that this is one of the first mineral elements to show depletion. Sourness of soil often results. To correct this and supply lime, 20 to 40 bushels of lime per acre may be used. Lime has a tendency to work down, and should never be deeply plowed under. It can be applied to the soil most advantageously prior to the planting of maize in the rotation, for wheat does not seem to be directly benefited by it. If it is applied directly to wheat, it should be scattered over the plowed field a few days before seeding and at once harrowed in. The ground should be stirred again before seeding. The application of one ton per acre every 4 to 6 years is advised for Illinois uplands.

Marl.—This consists essentially of carbonate of lime. Often considerable amounts of organic matter, sand and clay are also present. It originated in the breaking down of fresh water shells. Its action is more slow and "mild" than that of lime. It has been "regarded rather as an amendment than a fertilizer." Its chief functions are in improving the tilth, neutralizing acidity, and promoting nitrification, besides supplying lime. The marls of New Jersey also contain potash and phosphoric acid.

Nitrate of Soda.—Trade in this as a fertilizer began between 1830 and 1840. The supply is limited. In 1860 all estimates showed that it would last nearly 1,500 years. By 1900 these estimates had fallen to less than 50 years, and the world's markets were annually consuming nearly 1,500,000 tons, the United States requiring about 15 per cent of this amount. It is by far the most expensive fertilizer in use, and it is the best carrier of nitrogen.

Potash.—The main sources are wood ashes, and, since 1860, the products furnished by the potash industry at Stassfurt, Germany. Several forms are imported from Germany, each containing a different but correctly warranted quantity of this fertilizer. Nearly 500,000 tons are imported annually by the United States. This is over half the product. Since 1860, the Stassfurt salts have been almost the only source of concentrated potash. Good wood ashes contain perhaps 10 per cent of potash. They were long used without the real reason of their value being known. Besides potash, ashes often contain considerable lime. Hartlib gave 23 fertilizers and means of fertilizing the ground, and among them were included lime, marl, ashes and chalk.

Gypsum or Land Plaster.—Those who are experimenting with this material report varied results. It has long been used, however, and the most reliable conclusions seem to be: (1) That gypsum has undoubted fertilizing value on many soils; (2) that its chief value depends on three processes: (a) Preservation of ammonia and perhaps other nitrogenous forms; (b) decomposing potash and phosphorus-bearing silicates, liberating these two elements for plant-food; (c) affecting soils physically, making them granulated, or loose and mellow; (3) and that it decomposes sodium carbonate and thus breaks up the so-called "black-alkali."

Common Salt.—This has also been used as a fertilizer for at least several decades. In the eighties it was a common practice in England to sow salt in the early spring on wheat land that was too rich, the idea being that a larger deposit of silica in the stalk would result, enabling the wheat to stand better. While it has been found a valuable agent for increasing the yield of barley, it is of less importance in raising wheat.

Miscellaneous Fertilizers.—A great many other materials have been used to a greater or less extent as fertilizers. Among them are: Animal products, as wool waste and the refuse of modern slaughterhouses, blood, bone, hair, horn, hoof, etc., which with fish, manure and sulphate of ammonia from the gas works, are still the main sources of nitrogen applied to crops; swamp muck, marsh mud, sea-weed, sludge, poudrette, potassium, cottonseed meal, rape-cake, burnt clay, charred peat, soot and green manuring crops. The latter are simply plowed under, a practice widely followed in the United States, especially with alfalfa and other legumes. Where stock can be raised, green crops and cottonseed meal have nearly as great a value for fertilizer after feeding as before, and yield the additional intermediate product of milk or beef.

It is interesting to note that the aborigines taught the early settlers of New England the value of fish as a fertilizer. Fish or fish waste should be composted. Quicklime is used in France. Fish compost readily yields its elements to growing crops, consequently it should be applied in the spring, and not deeply covered. Sludge is the precipitant of sewage, and poudrette is the same reduced to a dry powder. A part of their value lies in the germs of nitric ferment which they contain. Some 40 tons of wheat straw leached and burned on the soil contribute to it 8 pounds of phosphorus and 680 pounds of potassium, besides the nitrogen leached into the soil before the straw was burned. This immensely increases the yield of wheat. Mulching with straw does not seem to be of any benefit to wheat, whether applied for fertilizing or for winter protection.

Fertilizing by Irrigation.—To show the fertilizing value of irrigation waters, some analyses are given below.

COMPOSITION OF IRRIGATION WATER.

Results expressed in parts per 1,000,000

[table]

The waters of the Nile seem to have the largest amount of nitrogen, 1.7 per cent, all the others having merely a trace. Some 24 acre-inches of Rio Grande water add to the soil about 1,075 pounds of potash, 116 pounds of phosphoric acid, and 107 pounds of nitrogen. The same amount of Delaware river water contains 741.08 pounds of materials, while the Mississippi, St. Lawrence, Amazon and La Plata rivers average 655.6 pounds of solid matter for every 24 acre-inches. As a rule sewage waters from the cities have the highest value for irrigation, and muddy river waters stand next. Waters containing sulphate of iron are positively injurious when applied to land. They usually issue from peaty or boggy swamps. While the fertilizing value of sewage waters is unquestioned, and while their use has been almost universally favored, objections have been made 1o them on other grounds. To say the least, they undoubtedly contain a hidden danger, and if used at all, it should be with the greatest of care. It has been claimed that "the use of sewage for fertilizing purposes is not to be commended because of the danger of contaminating the soil with

1 Rept. Mont. Exp. Sta., 1902, p. 62.

2 Exp. Sta. Record, V. 14, No. 11, p. 1057.

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