74 



SOIL PHYSICS AND MANAGEMENT 



matter in soils. The presence of water in large amounts arrests 

 decomposition, as in swa'mps, by excluding oxygen, while its presence 

 in moderate quantities stimulates nitrification in drained land. In 

 its movement downward through a soil, water not only carries 

 soluble compounds with it, but moves the fine particles downward, 

 thus producing differences in physical composition in the different 

 strata. 



(a) Arid Soils. The agencies of disintegration predominate 

 over those of decomposition in arid regions As a result the soils are 

 characterized by large amounts of original minerals that have been 

 changed very little. 



Mineral Content of Soils l 



The great agency in chemical changes of rocks is water, and its 

 deficiency in arid regions has protected the minerals from those 

 profound changes that take place in humid regions. The minerals 

 have been broken down into rather fine material, but not into clay, 

 which results largely from decomposition. Silt and various grades 

 of sand predominate. Their mineral content is indicated in the 

 above table. 



The low rainfall renders any large amount of leaching impos- 

 sible, so that the soluble salts formed during the limited decomposi- 

 tion remain in the soil. They may be moved downward to some 

 extent by the water, but when evaporation takes place they are 

 brought to the surface again. If excessive evaporation occurs these 

 salts may be brought to the surface in sufficient quantities to be 

 quite injurious as " alkali." 



Soluble Salts in Soils * 



Per cent 



Arid 



Prairie (subhumid and humid). 

 Timber (humid) 



0.333 

 0.048 

 0013 



