_Slow penetration of Water._—Soils of this class are wetted with extreme slowness by irrigation water; so that when first taken under cultivation it sometimes takes twenty-four hours to soak the land for twelve inches in each direction. Irrigation furrows must be placed very close together and in large numbers, in order to ensure the wetting of the soil so that the crop shall not suffer from lack of moisture at a distance of two or not more than three feet. Where the irrigation furrows are drawn farther apart a fine stand of grain may be seen within eighteen inches of the same, while farther away the crops may be dying from lack of moisture. This difficulty is by no means infrequent in the arid region, and is difficult to overcome except by frequent and thorough tillage, which gradually increases the rapidity of water-penetration; as has been shown in the soils of the alluvial prairies of the Yakima country in the State of Washington. It is necessary, however, to take care that they shall always contain an adequate amount of humus or vegetable matter, in order to prevent re-consolidation by the burning-out of the humus during the warm, rainless season. There is an unmistakable resemblance between these dust soils of the Northwest and the “putty” soils mentioned above; both showing a very low percentage of clay with a relatively large amount of the finest sediments, with a sudden downward break of the curve before the coarser grain-sizes are reached. It would seem as though the absence of these intermediate grains favors the close packing of the fine sediments in the interstices of the coarse ones, thus bringing about the imperviousness, which is the chief obstacle to their cultivation. _Effects of coarse Sand._—Coarse sand intermingled with heavy clay soils has but little effect in improving the tilling qualities, unless carried to such excess as renders it financiallyimpracticable. In actual practice it is frequently possible to improve such soils by properly distributing upon them the washings of the adjacent hills, which will always carry sands of many grades; and when it is intended to improve garden land by hauling sand it is important to choose the latter so as to complement the deficient grain-sizes of the soil. The sand of wind drifts or dunes is generally well adapted to such improvement, being, as Udden[28] has shown, of a fairly definite composition of sufficiently wide range of grain-sizes for the purpose. The effects of humus in modifying soil texture are discussed farther on. [28] The Mechanical Composition of Wind Deposits, Bull. No. 1, Augustana Library Publications; 1898. CHAPTER VII. THE DENSITY AND VOLUME-WEIGHT OF SOILS.