THE PHYSICAL PROPERTIES OF THE SOIL. 857 



The influence of the physical structure of the soil on the evaporation 

 is readily understood if the processes takinj^ place are closely examined. 

 If the water present at the surface of a wet soil passes into the atmos- 

 phere in the form of vapor, the pores near the surface are gradually 

 emptied and the water in the next layer forced toward the surface. 

 When this process is communicated to the layers farther down, an 

 upward movement of water by capillarity occurs in the whole mass. 

 In consequence of this process the quantity of water present in the soil 

 diminishes until capillary movement ceases. From this point the water 

 lost from the surface is no longer replaced, and the only remaining 

 movement is that of water from particle to particle on their surfaces, 

 which is very slow. In consequence, the surface of the soil dries np. 

 The uppermost dry layer now hinders the direct influence of the agencies 

 of evaporation, especially that of the wind. Evaporation, therefore, 

 decreases rapidly after the upper layers dry up, and the resistance 

 increases as the drying proceeds downward. 



From the above it will be seen that the evaporation steadily decreases 

 under long-continued dryness, and further, that the more easily the 

 evaporated water is replaced the more rapid is evaj)oratiou and the slower 

 a dry layer is formed, and vice versa. It is clear that saturated (wet) 

 soils, irrespective of their mechanical constitution, allow approximately 

 equal quantities of water to evaporate, and that the rate of evaporation, 

 other things being equal, rises and falls with the humidity of the soil. 

 Hence the evaporation capacity of a soil is intimately connected with its 

 water capacity, and is greater in proportion to the fineness of the soil 

 and to its richness in colloid substances, and vice versa. 



In crumbly soils the evaporation is less rapid than in soils of sepa- 

 rate grain structure because in the former case the noncapillary spaces 

 retard the rise of water. Stones act in the same way. An increase 

 in density increases the capillarity, and consequently the evaporation. 



In soils of different depths evaporation increases with the height 

 of the soil column up to a certain limit and then diminishes again in 

 proportion to the increase in height. If there is water underneath 

 the soil the evaporation decreases as the distance between the surface 

 of this water and that of the soil increases and as the capillary rise 

 diminishes.^ 



' G. Schiibler, 1. c. Meister, 1. c. F. E. Schulze, Beobachtiingen iiber Verdiin- 

 stung, Rostock, 1860. E. Wolff, 1. c, p. 61. W. Scbiimacher, 1. c. and "Der 

 Ackerbau;" 1864, p. 62. J. Nessler, Laudw. Correspoiidenzblatt fiir das Grossber- 

 zogtbiim Baden, 1860, p. 217. H. Hellriegel, Beitriige zu den naturwissenscbaft- 

 licben Gruudlanen des Ackerbaues, Brunswick, 1863, p. 625. P. Wagner, Bericbte 

 iiber Arbeiten der Versucbs station Darmstadt, 1874, p. 87. A. Scbleh, luaugnral Dis- 

 sertation, Leipsic. F. Haberlandt, 1. c, 2. S. W. Jobnson, Rpt. Connecticut Agl. 

 Expt. Sta. for 1877, p. 76. F. Masure, Ann. Agron.,8, p. 161. R. Heinrich, Grund- 

 lagen der BeurtbeiJiing der Ackerkrume, 1882. E. WoUny, Forsch. Geb. agr. Phys., 

 3, pp. 117, 325, 328; 4, p. 360; 5, pp. 21, 157; Landw. Jahrb., 1876, p. 457. C. Eser, 

 Forscb. Geb. a^jr. Pbys., 7, p. 1. 



