PHYSICAL COMPOSITION OF SOILS. 



PHYSICAL ANALYSIS OF DUST SOILS. 



105 



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 num- 

 bers, 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 vege- 

 table matter, in order to prevent re-consolidation by the burn- 

 ing-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 imper- 

 viousness, 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 financially 



