io6 THE MOVEMENTS OF SOIL WATER [chap. 



little impeded by skin friction. Drainage will therefore 

 be good, but since the soil particles possess so small a 

 surface that can be wetted, the soil will therefore retain 

 very little moisture, sometimes no more than lo per cent, 

 when fully saturated but drained. Similarly, the 

 particles being large the water-films round them are 

 not extensive, and will not possess much power of lifting 

 water against gravity ; when evaporation is going on, the 

 water will rise quickly because it is little impeded, but 

 to no great distance. The capillary tubes formed by 

 the interspaces between the sand grains are large, so 

 motion up them is quick, but for only a short distance. 

 In the opposite way, in a very finely grained soil like a 

 clay, percolation will be slow, because the downward 

 motion is hindered by the clinging of the extensive 

 surface to the water, and by the fineness of the inter- 

 spaces through which it has to ooze. This extensive 

 surface, however, retains a considerable proportion of 

 water, up to 40 per cent, when drainage is over but 

 losses by evaporation have not begun. Again, although 

 the pull exerted by so great a surface is enormous, it is 

 very ineffective in drawing up water rapidly, because of 

 the drag exerted by the particles on the water with 

 which they are in such close contact. When the clay 

 has been tempered and kneaded so as to reduce it to 

 its maximum of fine division and also to pack the 

 particles together as closely as possible, water will not 

 move through it at all, because the water particles are 

 held so tightly by the clay particles with which they are 

 in contact that to all intents and purposes they cannot 

 move at all. For practical purposes the most valuable 

 soils are those which are fine-grained enough to possess 

 considerable surface, capable of retaining water against 

 drainage, and also of lifting it from the wetLer layers 

 below when the proportion has been reduced by 



