174 . SCIENTIFIC AGRICULTURE. 



tence of the thick crusts of nitrate, carbonate and chloride of 

 soda, which are met with in Peru and other parts of South 

 America, India and Egypt These salts are brought to the 

 surface by capillary action, in a state of solution, and deposited 

 as the water evaporates. 



Contractibility on drying. Some soils contract or shrink 

 on becoming dried after rains, much more than others; and 

 this appears to be in proportion to their power of retaining 

 water. Thus clay and peat diminish in bulk one fifth on being 

 perfectly dried after saturation, while sand maintains the same 

 bulk in either state. [Johnston.] This contraction in clay 

 soils has a tendency to tear and injure the small and tender 

 roots of plants. 



Power of absorbing gaseous matters. rThe necessity of free 

 access of air to the soil has already been noticed; and in 

 proportion to the amount of air which is admitted into the soil, 

 will be the oxygen and other gases absorbed and made avail- 

 able to the roots of vegetation. Clays, peat and humus absorb 

 more oxygen than sandy soils ; this is due partly to difference 

 in porosity, and partly to the chemical character of each. Be- 

 sides oxygen, soils absorb carbonic acid, ammonia, nitric acid, 

 and other vapors which contribute to fertility. All soils absorb 

 gaseous substances the most readily when in a moist state ; so 

 that dews and showers are of great benefit, in bringing the soil 

 into a condition to extract from the air fresh supplies of the 

 gases. 



Power of absorbing heat. The earth is capable of absorbing 

 lieat during sunshine, so as to attain a temperature above the 

 surrounding air. Dark colored, brown and reddish soils absorb 

 heat most rapidly, and become warm the soonest They also 

 become from three to eight degrees warmer than other colored 

 soils, and by this means they promote the growth of vegetation 

 better than those of other colors. This property gives an 

 additional value to dark soils over light ones, in countries 



