268 E. W. Hilgard — Peculiarities of Rook - Weathering. 



rise to "alkali soils," which characterize the arid region to a 

 greater or less extent all over the world. Their chief soluble 

 salts are usually the chloride, sulphate and carbonate of so- 

 dium, varying in relative proportion from place to place at 

 short distances. These differences are largely due to cross- 

 reactions between the salts of the alkalies and alkaline earths 

 under the influence of carbonic acid formed within the soil 

 by the usual processes, resulting, e. g\, in the formation of sodic 

 carbonate and calcic sulphate from sodic sulphate and calcic 

 carbonate. Magnesic sulphate is similarly formed. But on 

 exposure of the soil to free diffusion of air, the process may 

 go backward so far as to re-form the original compounds. 



The peculiar effects of sodic carbonate upon clay, causing it 

 to diffuse from the flocculent condition into single-grain struc- 

 ture, frequently results in the formation of " alkali hard pan " 

 at the depth to which the annual rainfall usually reaches. 

 This characteristic of " black alkali lands " is illustrated in the 

 diagram before you, showing that at the depth of three feet 

 there is a maximum of accumulation of carbonate, cementing 

 the soil into a hard, tough mass which is quite inaccessible to 

 ordinary means of disintegration, and yields reluctantly even 

 to dynamite ; but is quickly and completely resolved into a pile 

 of crumbs when subjected to the chemical action of gypsum 

 and water. This affords a ready means of reclaiming " black 

 alkali " lands, so far as the present price of land plaster will 

 permit ; since the sodic sulphate thus formed is at least four 

 times less injurious to vegetation than the carbonate. 



Unfortunately there is no practically available chemical 

 reaction by which the highly injurious common salt could be 

 changed into sulphate. The same is true of the calcic and mag- 

 nesic chlorids, which are occasionally, though fortunately not 

 very frequently, found in the soluble salts of alkali lands. 



The difficult solubility of calcic phosphate explains why the 

 leaching process which causes such large differences in the 

 other important plant-food ingredients, is not apparent in the 

 average phosphoric acid-content of the two regions. 



It goes without saying that so far as plant-food ingredients 

 are concerned, alkali lands are from their very origin very 

 rich in them, and when reclaimed by leaching or otherwise are 

 profusely and lastingly productive. Aside from a high 

 " reserve," they always contain water-soluble potash, and very 

 commonly nitrates and phosphates also. 



What action these salts exercise in the weathering process is 

 not well established except as regards the sodic carbonate, 

 which undoubtedly is active both in dissolving free silica and 

 in decomposing silicates."" That common salt in presence of 



* On the Geologic Efficacy of Alkali Carbonates. This Journal, August, 

 1897. 



