. Proceedings of the American Society of Agronomy . ^ airtight in order to prevent evaporation, as thesoil usually requires aeration for normal growth. Over the soil onthe surface, about two or three inches of fine gravel are placed afterthe plants have been well started. All is then covered with a gal- Plate Fig. I.—Battery of 24 cans used at the Nebraska Experiment Station (1910)in determining water requirements of crops in relation to weather factors.(Courtesy Nebraska Exp. Sta.)

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