82 NATURE AND PROPERTIES OF SOILS 



conditions of rainfall, as one would naturally expect. The 

 amount of easily soluble material is higher in arid regions, 

 due to the lack of rain and the tendency for soluble salts to 

 accumulate. Biologically, organisms are active at greater 

 depths^ in arid than in humid regions, because of the loose 

 structure of arid soils and because of their good aeration. 

 Such soils are seldom water-logged except from improper ir- 

 rigation. 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. 



22. Special cases of soil formation. — Having compared 

 the weathering of granite and limestone under different cli- 

 matic conditions, it is interesting to note the quantitative chem- 

 ical changes of these rocks as they are reduced residually to 

 soil under humid conditions. The following analyses^ indicate 

 the elements that are likely to be lost to the greatest extent 

 during the process. (See Tables III and IV, page 33.) 



The soil resulting from the decay of the granite was a deep 

 red clay, with numerous quartz grains present. The soil 

 from the limestone was very plastic and high in silicate silica. 

 Leaching has probably gone on to a very great extent in both 

 soils. It is noticeable in both cases that the bases, such as 

 calcium, magnesium, sodium, and potassium, have suffered 

 severe losses. The carbonate has almost wholly disappeared 

 from the limestone clay, indicating that a residual soil from 

 such a rock will probably need an application of lime. (See 

 Figs 6 and 7, pages 34 and 35.) 



^Lipman, C. B., The Disirihution and ActwUies of Bacteria i/th Soils 

 of the And Begion; Univ. Calif., Pub. in Agr. Sci., Vol. I, No. 1, pp. 

 1-20. 1912. 



^Merrill, G. P., Weathering of Micaceous Gneiss; Bui. Geol. Soe. 

 Amer., Vol. 8, p. 160. 1879. 



