244 SOILS: PROPERTIES AND MANAGEMENT 



Soil moisture, therefore, in proper amounts, becomes 

 one of the controlling factors in crop growth and must 

 be looked to before the maximum utilization of the 

 primary elements can be expected. The amount of 

 water held within the plant is not large, however, in 

 comparison with the amount lost by transpiration, al- 

 though green plants contain from 60 to 90 per cent of 

 moisture. Although the main cause of the high trans- 

 piration of most crops is not traceable to the dilute con- 

 dition of the soil solution, certain regulatory functions 

 may, however, also come into play. 



Because of the readiness with which moisture passes 

 from plants into the atmosphere, large quantities of 

 water must be taken from the soil in order that 

 the plant may maintain its proper turgidity. This 

 excess water is largely lost or disposed of by trans- 

 piration, at the same time performing its regulatory 

 functions. 



168. The water requirement of plants. As might be 

 expected, the pounds of water transpired for every pound 

 of dry matter produced in the crop is very large. This 

 figure, called the transpiration ratio, or water require- 

 ment, ranges from 200 to 500 for crops in humid regions, 

 and almost twice as much for crops in arid climates. 

 An accurate determination of the transpiration ratio of 

 a crop is somewhat difficult, due to the methods of pro- 

 cedure necessary and also to the difficulty of controlling 

 the numerous factors that may vary the transpiration. 

 For really reliable figures the plants must be grown in 

 cans or pots, in order that the water lost may be deter- 

 mined accurately by weighing. If there is no percolation, 

 the water ordinarily lost from a cropped soil includes 

 both that evaporated from the soil surface and that 



