Nearly six tenths of the earth's land surface receive an annual rainfall of less than twenty inches, and can be reclaimed for agricultural purposes only by irrigation and dry-farming. A perfected world-system of irrigation will convert about one tenth of this vast area into an incomparably fruitful garden, leaving about one half of the earth's land surface to be reclaimed, if at all, by the methods of dry-farming. The noble system of modern agriculture has been constructed almost wholly in countries of abundant rainfall, and its applications are those demanded for the agricultural development of humid regions. Until recently irrigation was given scant attention, and dry-farming, with its world problem of conquering one half of the earth, was not considered. These facts furnish the apology for the writing of this book.

The book now offered is the first attempt to assemble and organize the known facts of science in their relation to the production of plants, without irrigation, in regions of limited rainfall. The needs of the actual farmer, who must understand the principles before his practices can be wholly satisfactory, have been kept in view primarily; but it is hoped that the enlarging group of dry-farm investigators will also be helped by this presentation of the principles of dry-farming. The subject is now growing so rapidly that there will soon be room for two classes of treatment: one for the farmer, and one for the technical student.

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