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THE BOOK OF NATURE STUDY 



wetter to drier places, whether the motion has to be directed 

 upwards, downwards, or sideways. And the greater the surface 

 exposed, as in the fine grained soils, the greater will be the 

 water-moving power ; but when the particles are very small 

 indeed, as in a clay soil, the rate of motion becomes extremely 

 slow, because of the friction of the water moving between the 

 very fine grained particles. It is this capillary power in soils 

 which keeps plants growing during a drought ; they would 

 quickly use up all the water which the soil immediately round 

 the roots had retained were not the surface soil constantly 



2 456 HOURS 



FIG. 77. Rate of rise of water in different soils, due to capillarity or surface tension. 



lifting water from the wetter layers below. Sandy soils suffer 

 badly from drought, because both water retaining power and 

 capillary lifting power are low ; clay soils can stand a short 

 drought because they retain so much water, but when this is 

 exhausted they can only renew their supplies very slowly, and 

 also begin to lose further by the big cracks that form. The 

 best soils for keeping crops growing during drought are loams 

 mainly composed of very fine sand ; their water-retaining power 

 may not be high, but the particles possess sufficient surface to 

 lift water rapidly, and yet are not so small as to offer resistance 

 to its movement. 



