122 IRRIGATION PRACTICE 



plant itself. A sickly plant evidently does not require, nor 

 can it use, so large quantities of water as a strong, healthy 

 plant. Many farmers fail to understand this simple and 

 almost self-evident law, and therefore apply to a crop 

 poorly developed fully as much water as is applied to 

 one which is growing vigorously. 



89. Root-system. Another factor of importance in 

 determining the rate of loss of soil water due to plants is 

 the development of the root-system. If the roots have 

 been developed near the surface, more water will be used 

 from the top soil than if the roots have been more evenly 

 distributed throughout the soil, and the energy expended 

 in lifting the water from the lower depths is increased. 

 To drive the roots downward, water should not be applied 

 too early in the season, nor should it be applied in such 

 quantities as to make it unnecessary for the lower roots 

 to continue their work. Only when the roots fill the soil 

 to the greatest depth in the most thorough manner, will 

 the soil moisture be used most economically. 



90. Age of plants. The age of a plant naturally 

 determines, largely, the rate at which soil moisture is 

 absorbed. A plant increases very rapidly in dry weight, 

 up to the time of flowering. After this time the increase 

 is slight, and finally diminishes. The rate at which plants 

 use water varies somewhat in the same way. There is a 

 steady increase in the rate at which plants use water 

 from early spring up to flowering; after which there is 

 a diminution, until, when the plant is old, it uses water 

 at a very low rate. A similar relation exists between 

 growth and water-use of biennial crops such as sugar 

 beets. The effect of the age of plants on the rate of loss 

 of soil water is well shown in the following table: 



