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other crops gave a poorer yield on land that had been continuously sown in wheat. "Land which produced three crops of wheat and one cultivated crop in a period of four years, gave almost as much wheat and more profitable returns than did the land which produced four crops of wheat in succession.'' * Experiments have been made in the continual culture of wheat on a certain piece of ground, there being no fertilizing of any kind, as, for example, the "experimental acre" in Kansas. This trial was begun in 1880, and by 1896 the yield was falling off. Permanent spots of diminished fertility had then appeared. Though they may have been due in part to surface-washing, partial exhausting was undoubtedly a factor.2

Historic experiments in growing wheat continuously without fertilizing have been carried on in England for over 50 years. "The yield has fallen to about 12 or 13 bushels to the acre, but for the past 20 years there has been little or no difference in the yield, except slight fluctuations due to seasonal conditions. So far as is known, the soil will produce 12 or 13 bushels to the acre annually for hundreds of years.''3

The Crop Rotations of the United States now generally practiced in some typical counties of states leading or prominent in their geographical divisions, are given below:

Pennsylvania.—Corn, wheat two years, grass two years (York, Franklin, etc.). Corn, oats, wheat, grass three years (Chester, Westmoreland).

Minnesota.—Wheat two years, oats, wheat, flax (Marshall). Corn, wheat two years, oats (Lac qui Parle). Corn, wheat two years, grass two years (Ottertail, Todd, etc.).

Washington.—Wheat, rest (Adams).

California.—Wheat, rest (Solano, San Joaquin, etc.).

Maryland.—Corn, wheat two years, grass two years (Montgomery, Frederick, Talbot, etc.). The rotation on dairy and stock farms includes wheat for only one year.

Oklahoma.—Wheat without rotation (Grant, Garfield, Kingfisher, etc.). Wheat, corn, (Dewey). Wheat three years, oats (Kay).

No crop, nor even any one class of crops, such as the cereals, should be continuously grown on a soil that will produce a variety of crops. On ordinary soils, cereal crops should be rotated every two to four years with a leguminous crop, such as clover or alfalfa. The North Dakota experiment station finds that wheat should have a good place in the rotation because it is a particular crop, and that the average yield of wheat is greatest when the crop follows either corn or potatoes. After these crops, placed in the order that they merit for preparing the soil for wheat, come summer fallow, millet, vetch, peas, wheat and oats. The more dry and unfavorable the season, the more important it was to introduce a cultivated crop into the rotation. The best rotations included a perennial grass, for which purpose brome grass is well adapted to North Dakota. The rotations vary greatly in different states, and soil, climate, and economic causes must determine which rotations are most advantageous for any locality. Summer fallowing is widely practiced on the Pacific coast, largely because there is practically no rotation feasible.

1 N. D. Bui. 48, p. 735; Bui. 39, p. 458.

2 Kan. Bui. 59 (1896), p. 90. * Indus. Com., 10:clxxxviii.

Crop Rotations in Foreign Countries.—In Canada, summer fallowing is rapidly becoming general throughout the territories, where the profitable corn crops of the United States cannot be grown on account of the latitude. The system of agriculture most prevalent in Russia is the three-field system, which is universally practiced in the center of the Russian wheat belt. The usual sequence of crops is winter rye, spring wheat and fallow. The arable land is divided into three corresponding parts. At a given time each part is in a different stage of the system. Other crops are being introduced, and this is lessening the area of fallow land. Among the private land owners this signifies progress in agricultural methods. Among the peasants it frequently signifies a harmful overworking of the land, the penalty of which is the drastic retribution of greatly reduced yields. Another system, still more primitive than the three-field one, is also found in Russia, especially in the steppes of the southeast, where the greatest extension of the wheat area is taking place. By this system the land is tilled until it becomes exhausted. It is then allowed to lie fallow in order to recover its fertility. This may require 10, 15, or even 30 years. In Archangel, Olonetz, Vologda, Viatka and Perm, the forest must be cleared to prepare the new land for cultivation, but in the southeastern provinces of Orenburg and Astrakhan, in New Russia, Kherson and northern Caucasia, all that is required is to plow the land. As population grows, this wasteful method of farming is being replaced by the three-field system. Impoverishment of the land by continuous wheat cropping has been the custom in Argentina. Sixty per cent of the wheat is raised under the renting system. The colonist owns nothing which grim necessity does not compel him to own, and he practices his ruinous methods of farming until the land is completely exhausted. Then he fastens the bullocks and horses to the carts, packed with his many children and his few miserable pots, boxes, beds and implements, and travels until he finds new fields. Mixed farming as known in the United States is little understood or practiced in Argentina, and the farmer is generally either a wheat grower or a maize grower. There is complaint of the methods of farming in all parts of the Republic, however, and a practice of rotating crops is already beginning, by alternating wheat and maize, or by planting the land with alfalfa after three or four years of wheat cropping.1 For the best crops of wheat in Egypt, it is sown every fifth year, the rotation being (1) cotton; (2) "birsen" (clover) or "full" (beans); (3) wheat; (4) dura (maize); (5) "birsen." A commercial success has been made of growing wheat and alfalfa together on the dry uplands of North Africa. In Algeria two rows of wheat are sown 4 inches apart. A space of 40 inches is left between the double rows, and in this space the alfalfa is sown. Wheat is sown only every other year. This is of interest, as alfalfa is now the greatest American fodder crop, especially in the arid southwest where durum wheat is being more extensively grown.

Experiments with Mixed Crops have been made, chiefly in Canada and North Dakota. Results seem to be in favor of unmixed grain, although wheat and flax have an advantage under certain conditions, as when wheat is apt to lodge, or when there is a superabundance of moisture. In the latter case flax has increased the yield of wheat as much as 6.5 bushels per acre, in addition to giving 1.2 bushels of flax per acre.

IRRIGATION.

Historical.—Irrigation is of prehistoric origin. Water, as was shown in a former chapter, is one of the greatest essentials of all plant growth, and it is also one of the most variable quantities involved. Since the effects of these variations upon vegetation appear quickly, they must have been noticed at an 1 U. S. Dept. Agr., Bu. of Statistics, Bui. 27 (1904), pp. 41-42.

f early date, and then it was only another step to supply artificially the needed water. Irrigation was a condition that was indispensable to the settlement of large portions of western America, Australia and South Africa, In meeting these problems during the nineteenth century, the Anglo-Saxon race had its first experience with extensive irrigation. Throughout all the centuries of previous history, the art of irrigation was quite exclusively the possession of Indian, Latin and Mongolian races. It was used extensively by the ancient Chinese, Egyptians, Persians and by the people of India. The Homeric Greeks used small canals in irrigating. In Italy, it was probably as old as the Etruscans. The Romans borrowed the system from the east, and brought it to their country and southern Trance. The ancient Peruvians also practiced it, and in Spain it dates back to the Iberian life existing under the Roman conquerors.

Modern Irrigation in Foreign Countries.—Irrigation is more or less extensively practiced by all of the great nations of the globe, even in subhumid and humid regions. As a rule, however, the wheat crop is not extensively irrigated, for irrigation is more profitable with other crops. The total area watered runs into millions of acres in most of the European nations. Wheat is frequently irrigated in the Po valley. In Mexico, Argentina and Australia, wheat is irrigated to some extent. Both streams and wells furnish the water. Extensive systems have been planned for Australia, and over 1,000,000 acres could be irrigated in New South Wales alone. Argentina contains large areas which are irreclaimable except by irrigation. The lower valley of the Nile with its delta comprises another great irrigation system, 6,000,000 acres being under cultivation. Egypt is so arid that dry farming is impossible. In 1902 British enterprise completed a dam across the Nile at Assuan. It is built of granite, and is 70 feet high, 23 feet wide at the top, 82 feet wide at the bottom, and 1% miles long. It is the largest irrigation dam in existence, and the reservoir has a storage capacity of over thirty billion cubic feet. The largest increase in irrigated area in recent years has been made in British India, where about 30,000,000 acres have been reclaimed or made secure for cultivation by constructing new supply works. It has been estimated that 80,000,000 acres more can be reclaimed in India. In 1892 over $150,000,000 had been

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