28 SOILS. 



Summing up the facts concerning the water of the sea and 

 of landlocked lakes, with reference to the ingredients of soils 

 needful for the nutrition of plants, it appears that the rock-in- 

 gredients leached out in the largest amounts (lime alone ex- 

 cepted) are those of which the smallest quantities only are re- 

 quired by most plants; while of those specially needful for 

 plant nutrition, only potash is removed in practically apprecia- 

 ble amounts by the stream drainage. 



Result of insufficient Rainfall; Alkali Soils. When the rain- 

 fall is either in total quantity, or in consequence of its distribu- 

 tion in time, insufficient to effect this leaching, the substances 

 that otherwise would have passed into the drainage and the 

 sea are wholly or partially retained in the soil ; and when the 

 rainfall deficiency exceeds a certain point, the salts thus re- 

 tained may become apparent on the surface in the form of 

 saline efflorescences, or as it is usually termed in North Amer- 

 ica, " alkali." l Their continued presence modifies in various 

 ways the process of soil formation and the nature of the soils 

 as compared with those of regions of abundant rainfall (" hu- 

 mid climates ") ; one of the most prominent and important re- 

 sults being that, besides the easily soluble salts mentioned 

 above, the carbonate of lime formed in the process of decom- 

 position is also retained, and imparts to the soils of regions of 

 deficient rainfall ("arid climates") the almost invariable 

 character of calcareous lands. There is thus in the United 

 States a marked and practically very important contrast be- 

 tween the soils of the arid region west of the Rocky Moun- 

 tains and those of the " humid " region between the immediate 

 valley of the Mississippi and the Atlantic coast. These differ- 

 ences and their practical bearings can be best discussed after 

 first considering more in detail the chemical decomposition of 

 the several soil-forming minerals. 



1 In some cases the soluble salts originate in rocks impregnated with salts from 

 marine lagoons or landlocked lakes, or directly from their evaporation residues. 

 But this is the exception rather than the rule. 



