THE SOIL. 21 



CHAPTER IT. 

 THE SOIL. 



THE soil is that part of the earth's surface into which 

 plants send their roots in order to obtain foothold and 

 nourishment. Geologically, soil is a recent rock con- 

 sisting of the products of the "weathering" of older rocks, 

 and mixed with decomposed animal and vegetable matter. 

 A "weathering agent" is something which, by physical 

 or chemical action, aids in the decomposition of mineral 

 and organic matter. Besides desintegrating the hard rocks 

 to make soil, the weathering agents are ever acting on 

 the soil particles, rendering them soluble and available for 

 the feeding of plants. Since the weathering agents make 

 the plant food available, it is necessary to enumerate a few 

 of the most important. WATER is one of the most 

 powerful, and in addition, its presence is often a necessary 

 condition for the action of other agents of decomposition. 

 It decomposes by its power of dissolving substances and 

 acts mechanicallv by friction when in motion. In the soil, 



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water usually contains acids and salts in dilute solution 

 which increase the weathering power of soil water for 

 mineral and organic matter. Owing to the absence of 

 rain in Egypt, weathering agents have less activity here 

 than elsewhere, except where the mineral and organic 

 matters come under the influence of infiltration or irri- 

 gation water. The Nile acts even a better part than that 



