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Denitrifying organisms thrive best in a soil at least slightly organic, and so packed as to exclude the oxygen of the air. The nitrifying bacteria are unable to develop in organic matter, but its presence to some extent is not fatal to them. The presence of nitrogenous substances has a deleterious effect upon the cultures of nitrifying bacteria, which seem to fix atmospheric nitrogen only in the absence of plenty of nitrogen in the soil, consequently little benefit is to be obtained from inoculating soils containing a good supply of nitrogen. Most of the nitrifying germs seem to exist in the first foot of soil, while few, if any, exist at a greater depth than 18 inches. Other bacteria, such as those which change the sulphur and the iron compounds, also exist in the soil.
Need, Time and Application of Fertilizers.—Nearly all wheat land that is under continual cultivation, even if crop rotation is practiced, yields larger returns when fertilizers are properly used. In each individual case local conditions and the economic position of the wheat grower must determine to what extent it is advantageous to fertilize. Each farmer must, in a large measure, learn by experience whether the application of a certain fertilizer is profitable under his circumstances. It is now well known that yield does not increase in proportion to the amount of plant food applied, and that the increase in straw is greater than that in grain. In determining the value of such application, it must, of course, be remembered that more than one crop is benefited. The wheat crop may be increased either by direct fertilizing or by the residual effect of fertilizers applied to other crops in the rotation. The composition and condition of the soil determine the relative importance of different fertilizing constituents. Phosphoric acid used alone generally increases the yield of wheat grown anywhere on the glacial drift area of the United States.1 Either nitrogen or potash applied alone does not seem to increase the yield greatly, while the application of both with phosphoric acid gives the greatest gain. A fertilizer that can usually be found on the market is one containing 4 per cent each of ammonia and potash 1 Hunt, Cereals in Amer. (1904), p. 75
and 12 per cent of available phosphoric acid. By applying from 250 to 500 pounds of this commercial fertilizer per acre the best general results are obtained. If land has been quite exhausted by continuous wheat growing, the proportion of nitrogen and potash should be greater. Commercial fertilizers are best applied by means of an apparatus made for this purpose and attached to the wheat drill. They may also be broadcast just in front of the drill. In the case of winter wheat, most of the nitrogen is often applied early in the spring so as to prevent loss through drainage during the winter.
Kinds of Fertilizers.—These naturally fall into two classes, barnyard manure and commercial fertilizers. It is only on farms where the supply of manure is not adequate to preserve a high state of soil fertility that commercial fertilizers are economical. In general farming the former has the greatest relative value on account of individual or combined physical, chemical and bacterial influences not yet fully understood. Considering equal weights, however, most commercial fertilizers contain more plant food than manure does. It is also claimed that when applied to wheat they will produce larger returns. Nevertheless, the lower cost of farm manure always makes its use more profitable than that of other fertilizers. Where both are
used, it is most profitable to apply the manure to crops grown in the rotation, such as maize, and to apply the commercial fertilizers directly to the wheat crop.
Manure.—It is claimed that the soils of China have been in continuous cultivation for more than 4,000 years without falling off in productiveness, and that the continued soil fertility is due to the utilization of all animal manures and of sewage. During the eleventh century in France, stable manure was unknown as a fertilizer, though flocks of sheep were used for this purpose. Stable manures were utilized in the medieval husbandry of England, and they have been used to great advantage in France and Germany for over a century.
In America manure has always been utilized as a fertilizer by progressive farmers, but it has also been looked upon as a farm nuisance. It has been charged with producing dog fennel and various other weeds, and with "poisoning" the soil. In parts of Oregon and South Dakota it has been burned, sometimes for fuel. It has been hauled into ravines in California, into the creek in Oklahoma, into a hole in the ground or to the side of the field in Kansas, to the roadside in Missouri, to great piles in North Dakota and Idaho, and to the river in the Mohawk valley.1 It is estimated that the farmers of the United States annually lose over $7,000,000 by permitting barnyard manure to go to waste. As the fertilizing value of the manure annually produced by the farm animals of the United States is calculated at over two billion dollars, it must, however, be very generally utilized, a fact which does not excuse the foolish and useless waste. The fertilizing value of the average amount produced yearly is estimated for each horse at $27, for each head of cattle $19, for each hog $12, and for each sheep $2. The amounts of fertilizing constituents in the manure stand in direct relation to those in the food of the animal, and have a ratio to them varying in value from one-half to unity.
Experiments have shown that equal weights of fresh and of rotted manure have equal crop-producing powers. As 60 per cent of the weight is lost in the rotting process, manure should be used in fresh condition. "Barnyard manure contains all the
1 Yearbook U. S. Dept. Agr., 1902, p. 529; Industrial Commission lOxlxxxvill. (
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