CHAPTER VII 

 THE WATER-COST OF DRY MATTER 



THE steady transpiration stream of water, passing from 

 the soil through all growing plants to be evaporated at the 

 leaves, is responsible for the largest loss of soil moisture. 

 This loss is, also, the most difficult to control; for, as 

 shown in previous chapters, direct evaporation from the 

 soil may be largely prevented by simply stirring the top 

 soil, but many complex factors are involved in the loss 

 of water by transpiration. Many experiments have been 

 made to determine the relative quantities of water lost 

 by evaporation and transpiration. While no absolute 

 numbers of general application have been obtained, yet, 

 when the land is not cultivated to prevent evaporation, 

 one and one-half times as much water evaporates ordi- 

 narily from the vigorous, growing plant as from the soil. 

 When the soil is well tilled, and direct evaporation thus 

 reduced, the water lost by transpiration is often five to 

 ten times greater than the quantity lost by evaporation 

 from the corresponding area of soil. 



This great loss of soil moisture by transpiration is a 

 matter of much concern to the farmer, who pursues his 

 work in the hope of reaping the largest harvest from 

 the land, water and labor employed. Especially, where 

 water is scarce or irrigation is practised, the important 

 question is whether the increased yield is in proportion 

 to the quantity of water passing through the crop by 

 transpiration. If the yield increases in proportion to the 



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