188 



THE SOIL: INORGANIC MATTER 



can be supplied and maintained. Fig. 41 shows the method 

 of supplying water to such soils. The lack of organic matter 

 is a serious drawback in arid soils, for it is easily "burned 

 out." Decomposition is very rapid, for the pulverulent soils 

 are naturally well aerated. 



Fig. 41. — Arid soil under irrigation. Sugar beets. 



(6) 1. Sand Soils. — The term sand is based on size of 

 soil particles and not on chemical composition, although 

 from the nature of its formation there is usually a distinct 

 chemical composition. Technically a sand soil consists of 

 any coarse material whether composed of pieces of lava, 

 coral, shell, or pure quartz. Since in humid regions, however, 

 sands are derived by the action of water and carbon dioxide 

 on rocks containing quartz, such soils are composed largely 

 of silica, Si0 2 , since this is the least soluble of any of the 

 ordinary minerals, and breaks up slowly largely by physical 

 agencies. There are present in addition pieces of feldspar, 

 mica, and hornblende. As a rule sandy soils do not contain 

 much organic matter for the reason given above — they are 

 too well aerated. Fig. 42 shows the effect of organic matter 



