PHYSICAL PEOPEBTIES OF SOILS. 45 



comes dry, water begins to ascend, because of the 

 capillary power of the soil. The water brings with it 

 some of the soluble plant food in the soil, thus bring- 

 ing to the surface roots of plants . a good supply of 

 food. The water as it evaporates cannot carry the 

 dissolved mineral plant food into the air, so that, so 

 long a3 evaporation goes on, the surface soil accumu- 

 lates these salts. In regions in which there is little or 

 no rain there is a considerable accumulation on the 

 surface. The alkaline plains of our western territo- 

 ries are a good illustration, but the best is found in 

 the great beds of nitrate of soda on the west coast of 

 South America. In caves or under old houses the 

 soil often contains a large supply of saltpetre and 

 other salts. 



Although there is often some loss from plant food 

 being carried off in drainage water the upward and 

 downward mjDvement of water in soils is helpful in 

 several ways. The water not only does a great work 

 in making plant food available in the soil and bring- 

 ing it into contact with th.e roots of plants, but it also 

 carries into the soil carbonic acid, ammonia and nitric 

 acid from the air. Often the rain is warmer than the 

 soil and thus increases the temperature of the latter. 

 As the water goes down it is followed by the air and 

 this aids in decomposing the mineral matter of the 

 soil. 



While water, especially when charged with gases, 

 has the power of dissolving some of the mineral mat- 

 ter of the soil, and has a tendency to carry with it 

 the finely divided matter and all dissolved solids, the 

 soil has the important property of attracting and 

 holding the dissolved solids it finds in water. The 



