CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 73 



It is immediately apparent that the arid soil is poorer 

 in silica than the humid soil, but richer in iron and alumin- 

 ium, indicating a less weathered condition of the feldspars. 

 Due to a greater amount of leaching, the humid soil is 

 much lower in phosphoric acid, lime, magnesium, sodium, 

 and potassium. The humus in arid soils is somewhat 

 lower than in the soils under better conditions of rainfall, 

 as one would naturally expect. The amount of easily 

 soluble material is higher in arid regions, due to the lack 

 of heavy rain and the tendency for soluble salts to accumu- 

 late. A comparison of the analyses above with Clarke's 

 estimate regarding the composition of the earth shows 

 that the humid-region soil has moved farther away from 

 the average soil-forming rock than the soil produced 

 under conditions of aridity. 



Biologically, organisms are found active at greater 

 depths l in arid regions than in humid regions, because 

 of the loose structure of arid soils and because of their 

 good aeration. Such soils are seldom water-logged. In 

 humid regions bacterial action is limited very largely 

 to the surface foot of soil, since only there are the aeration 

 and the food conditions adequate. The intensity of 

 biological activity in arid soils is very largely governed 

 by moisture, and when moisture conditions are satisfied 

 bacterial changes may be expected to take place rapidly. 

 Cases are on record in which the soluble salts due to 

 bacterial action have become of such concentration as 

 to be toxic to plants. 



56. Soil color. — Another characteristic of soil is its 

 color, which has originated during the processes of soil 



1 Lipman, C. B. The Distribution and Activities of Bacteria 

 in Soils of the Arid Region. Univ. of Calif., Pub. in Agr. Sci., 

 Vol. I, No. 1, pp. 1-20. 1912. 



