RELATIONS OJ' SOIL 1:0 watbh 25 



twenty-six years contained at the same time 60'2 of 

 water per 100 of dry soil. 



Farm crops will not grow in a soil permanently 

 satm^ated with water, and from wdiich air is conse- 

 quently excluded ; the most luxuriant growth is 

 obtained in soils one-half or two-thirds saturated. 



A surface soil is seldom saturated, save immediately 

 after heavy rain ; it is the quantity of water which a 

 soil will retain when fully drained which determines 

 its capacity for supplying a crop with water. The 

 amount of water permanently retained by a soil is not 

 determined by the volume of the interspaces, but by 

 the extent of internal surface, the water being held 

 by adhesion on the surfaces of the particles ; the 

 smaller, therefore, are the particles of the soil, or the 

 more porous, the great 3r is the amount of water 

 retained. Two specimens of powdered quartz, one 

 coarse, the other very fine, held when fully saturated 

 more than 40 per cent, their volume of water ; but 

 when drained, the coarse quartz retained only 7'0 per 

 cent., and the very fine quartz 44'6 per cent, of water; 

 the latter lost, in fact, no water by drainage. 



The soils retaining least w^ater when drained are 

 gravel and coarse sand. The amount retained in- 

 creases as the particles become smaller. The presence 

 of colloid bodies, as clay, humic acid and humates, 

 increases the power of retaining water, as such bodies 

 swell np when wetted and hold the water in a jelly- 

 like substance. The addition of humus to soils is 

 thus one of the best means of increasing their power 

 of retaining water. 



The surface soil may be supplied with water from 

 below if a saturated subsoil exists at a moderate dis- 



