386 MISCELLANEOUS FA&M SUBJECTS 



is only a dry soil, or one which can be dried by the wind, that will 

 blow seriously. The exposure of a soil is even more important than 

 its cohesion. If the wind is unable to get at the soil there can be 

 no blowing, no matter how dry or loose the soil or how strong the 

 wind. In nature the degree of exposure depends almost entirely 

 upon vegetation. If a soil is well covered, the low r er layers of mov- 

 ing air are entangled in the plants and retarded, and the air next 

 the soil has only a very low velocity. The wind does not reach the 

 soil at all. For this reason, and because of the actual holding of 

 the soil by plant roots, a cover of vegetation is a nearly perfect pro- 

 tection against blowing. 



It is obvious that both of the soil factors affecting blowing, 

 namely, cohesion and exposure, are mainly controlled by the water 

 supply. A moist soil is always coherent, and a dry one is only sel- 

 dom so when loose. Similarly the covering of vegetation (native 

 or artificially planted) which a soil will bear, and hence the degree 

 of protection with which it can provide itself, sustains a direct and 

 closely corresponding relation to the water supply. It is to be 

 expected, therefore, that soil blowing will be an especially serious 

 problem in regions of low rainfall. In the arid region, soil blowing 

 is the rule, while in the humid regions it occurs only in exceptional 

 cases, mainly on sandy soils of very low cohesion and situated in 

 windy regions. 



Damage by soil blowing may be and usually is twofold: Di- 

 rectly, to the soil, and indirectly, to the plant or crop covering. The 

 injury to the soil itself is usually through removal, and there are 

 many cases in which the blowing away of the fertile surface layers 

 exposes subsoils insufficiently weathered and filled with humus to be 

 ready for crops. More generally important, however, are the in- 

 juries to the crop itself by blowing out or burial, or the direct cut- 

 ting effect of flying sand grains. The danger is especially great to 

 new seedings or plantings, since a great proportion of the ground is 

 bare and unprotected. It is no uncommon occurrence for farmers 

 in the arid and semiarid sections to lose an entire crop through the 

 blowing out of the seed or the uprooting, burial, or cutting off of the 

 young plants. After the plants are larger they become not only 

 better able to survive burial or exposure of roots, but also more 

 efficient in covering and holding the soil and preventing its blowing. 



It is apparent from the facts just stated that the means by 

 which the damage may be prevented or decreased must be, in prin- 

 ciple, two: (1) Increasing the cohesion of the soil, and (2) de- 

 creasing its exposure to the wind. The cohesion may be increased 

 practically (1) by increasing the water content of the soil, (2) by 

 increasing the amount of humus which it contains, and (3) by 

 modifying its texture, as, for instance, by adding clay or by leaving 

 it in small clods instead of in a finely pulverized condition. The 

 exposure may be decreased (1) by providing a cover of growing 

 vegetation, (2) by leaving the stubble of the last crop still standing 

 on the land, (3) by providing an artificial cover of straw, brush 

 lines, etc., and (4) by planting windbreaks to protect the fields. 



