THE SOIL. 45 



possible, and in these, and other ways, it helps in the 

 preparation of plant food. On the other hand, it has 

 certain undesirable functions ; in draining it removes plant 

 food from the soil ; its evaporation cools the soil ; its 

 rising in the soil tends to the deposition of salts in the 

 surface soil ; its application washes the smallest particles 

 into and fills up the air spaces thereby increasing the 

 tenacity, and decreasing the ventilation and drainage of 

 the soil. 



Soil obtains its water from rain, irrigation or infil- 

 tration. Rain water, when its regular supply can be 

 depended on, is the most satisfactory source of soil water. 

 Where rain generally falls in small quantities there 

 is seldom that great dilution of plant food and loss 

 of good material through excess of drainage consequent 

 on the application of a heavy flooding by irrigation. The 

 rain in falling does much good in cleaning the leaves and 

 stems of the plants. In consequence of the supply of 

 water being small the soil particles do not run, and the 

 soil remains loosely packed. Unfortunately rains cannot 

 usually give a regular or convenient supply of water and, 

 of course, the amount of rain falling in Egypt is insuffi- 

 cient for the growth of crops. 



By irrigation, a regular supply of water can be fed to 

 the crops, and further, in Egypt, the Nile water is the 

 only source from which fresh soil can be obtained. For 

 improving sandy soil this deposit of mud is very valuable. 

 Well water is naturally devoid of suspended matter and 

 does little to enrich the soil. The great disadvantage of 

 irrigation is the running and packing of the soil particles 



