i.] RESISTANCE OF SOILS TO DROUGHT 113 



In any case the freedom of movement of water 

 ithin the soil by capillarity is a factor of great irnport- 

 nce in bringing to the roots the water that is close 

 t hand. In a drought it is always the gravels and 

 aarse sands which suffer first, and this not so much 

 ecause they start with less water, for we have already 

 ^en that what they absorb they can give up almost 

 'holly to the plant, whereas a clay, which absorbs much 

 lore, can only hand over about the same proportion to 

 le plant as the sand did, so much being held as hygro- 

 :opic moisture. The plant suffers because the small 

 urface of the soil particles gives the coarse-grained 

 and or gravel a very limited power of moving the 

 ubsoil water to the roots of the plant. Should a 

 rought continue, the clay soils begin to suffer next, 

 )r though they start with large supplies of water and 

 ave an extensive surface of soil particles, yet water can 

 e moved so slowly through the very fine pore spaces 

 lat the supply cannot keep pace with the loss by 

 'anspiration and evaporation. The soils which are 

 jast affected by drought are the deep loamy sands of 

 ery uniform texture, fine-grained enough to possess 

 considerable surface, and yet not too fine to interfere 

 nth the free movement of soil water. The root range 

 1 these soils is always greater than in clay soils because 

 f their better aeration. The winter penetration of 

 oots into a clay soil is often limited by its temporary 

 aturation with water, and this comparative deficiency 

 a the depth of soil traversed by and supplying water to 

 he roots largely accounts for the manner in which 

 rops on clay soils suffer in a protracted drought. The 

 ,'estern soils which the American writers describe as 

 apable of growing wheat with a winter rainfall of 

 o to 12 inches and an unbroken summer drought of 

 hree months' duration, are deep, fine-grained, and 



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