THE PHYSICAL PROPERTIES OF THE SOIL. 855 



On examining a moist coluiim of soil, in which movement of water 

 has ceased, it is found that the qnantity of water increases on descend- 

 ing, and that above a certain height tlie qnantity of water remaina 

 constant. The reason of this is tliat in the Tipi)er part the water is 

 held only by surface attraction and the capillarity of the very fine 

 interstices, while the water flows out of the larger spaces. In descend- 

 ing the column increasing amounts of water are held even by the^ 

 larger interstices until finally if the particles are sufficiently tine the 

 water is held even in the largest interstices and the soil is saturated. 

 For this reason we distinguish between an absolute or minimum water 

 capacity, representing the quantity of water held in the upper part of 

 a sufiiciently high column of soil by adhesion and in the finest cajnl- 

 laries, and a complete or maximum saturation, representing the quan- 

 tity of water held by the soil when all the intergranular spaces are 

 filled.i 



The variations in maximum capacity for water in different soils, with 

 the exception of extremely coarse samples, are comparatively sniall^ 

 because the spaces are approximately equal. (Compare p. 764.) On the 

 other hand, the volume per cent of water corresponding to the minimum 

 water capacity is remarkably variable in natural soils, and generally 

 shows a considerable difference from the maximum capacity. The values 

 for the two kinds of water cajiacity approximate only in soils which 

 have very small particles and are rich in clay and humus. From these 

 facts it will be seen that the maximum is of much less interest than the 

 minimum water capacity in forming a judgment of a soil. 



The minimum water capacity increases, other conditions being equal^ 

 with the fineness of the soil particles and the proportion of colloid sub- 

 stances present. It is diminished considerably by the formation of aggre- 

 gates and increased by compressing the soil. Stones diminish the water 

 capacity. With an increase of temperature the water capacity dimin- 

 ishes owing to the decrease in the viscosity of the water. Eepeated 

 freezing and thawing of the mass cause changes in the mechanical 

 structure of clay soils which result in a lowering of the water cai^acity.^ 



Permeability of the soil for tvater. — This is very closely connected 

 with the properties already described. It is this property of soils 

 therefore which jn^events their supersaturation. 



Other things being equal, the quantity of water which filters through 

 a soil may be considered a measure of its permeability. Experiments 

 on this property have shown that the permeability of the soil for water 

 increases with an increase in the size of the grains and a decrease of 

 the quantity of colloid substances present. Clay, humus, and very 



lA. Mayer, Landw. Jahrb., 1874, p. 753. 



*G. Schiibler, Grundsiitze der Agriculturcbemie, 2, 1838. C. Tromnier, Die Boden- 

 kunde, 1857. Meister, 1. c. W. Schumacher, 1. c, p. 86. A. von Liebenberg, 1. c 

 F. Haberlandt, 1. c, 1, p. 9. A. Mayer, Fiihliiig'8 landw. Ztg., 1875, p. 18. U. voa 

 Klenze, 1. c, p. 122. E. Wollay, Forach. Gob. agr. Phys., 8, p. 177. 



