84 RELATIONS OF SOIL TO WATER 



water contents is expressed in pounds per cubic foot as when 

 it is expressed as per cents, by weight. 



We have further to bear in mind the fact already noticed 

 (Table VII), that a sand gives up its water far more completely 

 to a crop than will clay or humus. This is in great measure due 

 to the smaller internal surface of the sandy soil. In the case 

 of a sandy and clay soil, both containing 7 per cent, of water, 

 the water in the sand occurs as a much thicker film, being 

 spread over a much smaller surface ; it is thus in a condition 

 better suited for absorption by the roots of plants. The same 

 weight of water in the clay soil is held as an extremely thin 

 film, being spread over an enormous surface. The thinner is 

 the water film the more firmly is it held by the solid particles 

 of the soil, till it finally becomes quite incapable of assimilation 

 by plants. The greater availability of the water in a sandy 

 soil will also be partly due to the absence of the colloid bodies 

 which occur in clay and humus, as these, as we have already 

 seen, have the property of firmly retaining water. 



We have further to take into account the wider distribution 

 of the roots in a sandy soil, and the greater freedom with 

 which water moves within it. 



The facts just mentioned will not unfrequently turn the 

 balance in favour of a sandy soil as a supplier of water to 

 the crop. King (The Soil, 161) mentions the case of a sandy 

 and clay soil at Wisconsin having water capacities of 1 8 and 

 26 per cent. Maize grown on these soils reduced the water 

 in the sand to 4-17, and in the clay to 11-79 P er cent. 

 Calculating from these data, he tells us that the sandy soil 

 had yielded the crop 13-83 Ib. of water per cubic foot of 

 soil, while the same volume of the clay soil had yielded only 

 12-5 Ib. Experience in the United States supplies abundant 



