CHAP. XII WATER IN THE SOIL 47 



and upon the nature of the particles. Quartz-sand raises water rapidly ; 

 clay and other very fine grained soils raise it slowly ; calcareous sand 

 and humus fairly rapidly. But the height to which it is raised is least 

 in sand (only about forty centimetres above the water-table in fine sand, 

 according to Ramann ^), is greater in clay, and greatest in peat. (The 

 widely accepted view that bog-mosses in heath-moors raise water out 

 of the ground is nevertheless incorrect.) ^ If the grains of soil be more 

 than two to three millimetres in size, the pores are too large to act as 

 capillary tubes. 



The power of the soil to raise water is particularly of importance 

 when evaporation from the surface of the soil is great. It may be added 

 that it is more advantageous for a soil poor in water to have a small 

 than to have a great power of raising water, because in the former case 

 the soil is not so easily dried up. 



By the water-capacity of soil we mean its power to take up and retain 

 liquid, so that none of it sinks into deeper layers of soil. This is measured 

 by the quantity of water that a given weight, or better a given volume, 

 of soil can retain. It depends upon the adhesion of water to the particles 

 of soil, and varies with the capillary power of the soil and with the natuie 

 of the particles. 



The water-capacity is greater the more numerous and narrow are 

 the capillary spaces in soil and the more uniform their size, because the 

 adhesion-surface is thereby increased. Quartz-sand with particles one 

 to two millimetres in size can retain only one-tenth of the amount held 

 when the size of the particles is 0107 milUmetre.^ 



Research ^ has shown that the water-capacity is smallest in quartz- 

 sand, greater in calcareous sand, still greater in clay or in fine, pure 

 calcareous soil, and greatest in humus soil. In the last, the amount of 

 water is reinforced by the presence of imbibition-water, which occurs in 

 organic bodies ; of all soils peat has the greatest water-capacity. 



Some kinds of soil display so strong an adhesion to water that when 

 ^his is added the interstices between their sohd components are widened 

 and thus their volume is increased, that is to say, these soils swell ; on 

 the contrary, when deprived of some water they shrink ; in this way 

 a modification in the characters of these soils takes place ; when wet 

 they are soft and partially plastic, when dry they are hard and brittle. 

 These statements hold good in reference to clay and peat. 



In general, soil is not saturated with water (with the obvious exception 

 of swamps and similar spots in the vicinity of ground-water) ; in soil 

 clad with vegetation the maximum capacity is never attained because 

 the plants are continuously expending water in transpiration. 



The drying of soil depends on various factors : the above-mentioned 

 characters of the soil, the consumption of water by plants and animals, 

 and evaporation. 



Evaporation obviously has a profound influence on the amount of 

 water in soil and consequently on the economy and constitution of vegeta- 

 tion. Soil retains a certain quantity of water when exposed to the most 

 intense natural evaporation. The force with which water is held fast 



' Ramann, 1893, J905- ^ '^^''^ ^- '-'riihner, igoi ; C. A. Weber, iqoj. 



' See Livingston, ic/31, 1903, 1905. 

 * Schiibeler, 1886-8. 



