THE PHYSICAL PROPERTIES OF THE SOIL. 859 



limit, above whfch it does not change with an increase in the depth of 

 the soil. This limit varies considerably in different soils. It lies at 

 that point where the capillary rise of water from the lower moist layers 

 of the soil toward the surface ceases. It increases, therefore, with the 

 ability of the soil to conduct water by capillarity, and vice versa. Varia- 

 tions in the humidity increase as the thickness of the soil diminishes. 



INFLUENCE OF THE POSITION OF THE SOIL. 



Inclination. — A level soil is, as a rule, moister than an inclined soil. 

 If inclined, the moisture decreases with an increase in inclination. In 

 the case of inclined soils the difference in moisture in the higher and 

 lower parts of the soil increases with inclination. 



Exposure. — In soils having different exposures, those facing the north 

 have most moisture, then those toward the west, then the east, while 

 those facing the south are driest. These differences increase with the 

 angle of exjiosure.^ 



INFLUENCE OF THE COVERING OP THE SOIL. 



The moisture content of a soil covered with plants during the period 

 of growth is always lower, under like conditions, than one left bare. 

 This is due to the rapid transpiration of water by living plants. This 

 drying up of the soil, which is also noticed in timber lands, is greater 

 in proportion to the development of the plants, the thickness of the 

 stand, and the length of the period of growth. The influence on the 

 humidity of the soil diminishes as the plants die off on approaching 

 maturity, and is greatest when development of the aerial organs is 

 greatest. 



A covering of dead plants, straw, stable manure, etc., or of such 

 materials as stones, gravel, wood, etc., increases the moisture of a soil 

 during the warm seasons, because such a covering affords protection 

 against evaporation. In general the moisture of the soil increases with 

 the thickness of such a covering. 



We see, then, that as a rule soils shaded by growing plants contain 

 least moisture, soils covered with stones and other lifeless materials 

 contain most, while those that are bare stand intermediate.^ 



1 E. Wollny, Forsch. Geb. agr. Phys., 9, p. 3; 10, p. 3. 



* G. Willielni, Uer Eoden uiul das Wasser, Vienna, 1861; Wochenbl. fiir Forst- land 

 Landw. in Wiirttemberg, 1866, p. 174; Landw. und foist. Ztg., 1867, p. 31; Fiih- 

 ling's landw. Zfcg., 1876, p. 40; Wiener landw. Ztg., 1874, p. 159. E. Eisler, Che- 

 miscber Ackersmann, 1870, p. 131. W. SclinniacLer, Fiibling's landw. Ztg., 1872, p. 

 604; 1873, p. 683. A. Vogel, Abliandl. der k. bayerischen Akad. der Wissenschaiten, 



11. Classe, vol. 10, part II, 1867. J. N. Woldrich, Ztsclir. osterr. Ges. Meteorol., 6, 

 No. 8. E. Ebermayer, Die physikaliscben Einwirkungen des Wakles auf Luft nnd 

 Boden, Berlin, 1873; Forsch. Geb. agr. Fliys., 12, p. 147. E. Wollny, Der Einfluss 

 der Pflanzendecke nnd der Bescbattung auf die physikaliscben Eigenscbaften nnd die 

 Frncbtbarkeit des Bodens, Berlin, 1877; For.scl). Geb. agr. I'liy-s., 10, pp. 261, 415; 



12, p. 21; 13, p. 134; 14, p. 138; 17, p. 171. R. Lorenz von Liburnau, Wald, Klima 

 und Wasser, 1878. E. Ramanu, Die Waldstreu, 1890; Forsch. Geb. agr. Phys., 11, 

 p. 299; Forstlicbe Bodenkunde, 1893, p. 255. 



