CHAPTER IV 



WHAT WATER DOES IN SOILS 



How much water do plants need? Living plants 

 contain from 60 to over 90 per cent of water or 

 sap. The soil must be able to supply them, through 

 the roots, with enough water to make up what they 

 lose through the leaves (page 76). It has been found 

 that to produce a wheat plant, over 300 times its weight 

 (when dry) of water is needed; barley and oats need 

 even more. A mustard plant needs about 900 pounds 

 of water for every pound of the dry plant. An oak 

 tree has been calculated to lose during the growing 

 season some 500 pounds of water for each pound of dry 

 leaves formed. A full-sized orange tree has been found 

 to have about 40 pounds of leaves, weighed dry. Sup- 

 posing the evaporation to be about the same as that 

 of an evergreen oak, we find that orange trees evapo- 

 rate about 2,000,000 tons of water per acre every year 

 from 100 trees. This is equal to nine inches of 

 rainfall. 



All this water must be furnished by rain or by irri- 

 gation. Hence during the long rainless summers of 

 the arid regions, irrigation is necessary to get good 



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