THE CONTENTS OF THE SOIL 25 



ihoso winds an* kn<i\vn as "sand-storms." Most 

 •chores, i»articularly if sandy, arc luticli modified 

 l»y tlu' at'tion of wind. Hut tlu' wind lias an 

 infhionct^ u}»<^n soils even in tlio most pi\>toct('d 

 and t'ljuable regions. Tlic atiiiosplim^ contains 

 dust, much of wliicli is valuable itlant-food. 

 This dust is transported by winds, and it finally 

 settles or is earri«'d down by snow arid rain. 

 Although the amount ot' dust which is deposited 

 in any giv»>n tim«» may be slight, it is nevrTtlic- 

 less eontinuous, and has an important cllect 

 upon the soil. 



3. 7' In- Ursourccs of the Soil 



41. The soil affords a root -hold for phmts, — 

 a piac*' in whirh they can grow. It also supplies 

 the enviroiuncntal conditions which roots need, — 

 |)rotection, moisture, air, agreeable t<Mnp«Mature, 

 and other congenial surroundings. 



4'_*. The soil is also a store-house of plant- 

 food. Roberts calculates, fiom many analyses, 

 that in avrrage aLn-i<-ultural lands the suifai'c 

 eight inches of sod «>n each a<*re contains over 

 .'],Oi)n pcnmds of nitro^.'U, nearly 4,()()() jiounds 

 of phosphoric acid, and over 17,000 jxnmds of 

 potash. The.se three elements are the ones which 

 the farmer must chiefly consider in maintaining 

 or augmenting the pnxluctive power of the lun<l ; 



