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TEXTBOOK OF BOTANY 



crease or decrease the available organic compounds and the water- 

 holding capacity of the soil, as well as its texture and penetrability. 



The interstices among the irregular soil particles make up from a third 

 to a half of the soil volume. After a rain they are partly or entireh filled 

 with water (Fig. 130). The air in these interstices constitutes a "soil 



Fig. 130. Arrangement of soil particles and interstices in a cultivated field. Photo 



by L. D. Baver. 



atmosphere." Water movement in these spaces is for the most part by 

 capillarity, and to a slight extent by gravity. The lateral movement is 

 almost as rapid as the downward movement. 



Following a rain the water gradually approaches an equilibrium in 

 which similar parts of the soil mass have about the same water content. 

 Water vapor is also diffusing through the soil mass, and some of it may 

 diffuse out of the pores at the soil surface. Soluble salts are likewise 

 diffusing in the soil solution from regions of high concentration to 

 regions of low concentration. Thev may be carried about also during 

 mass movement of the water in tlie soil. Owing to electrical forces, 

 mineral ions may also become adsorbed (electrically held) on the sur- 

 faces of soil particles. 



