CROP DEVELOPMENT UNDER IRRIGATION 159 



largest possible proportion of the material formed in the 

 plants may be retained in above-ground parts which are 

 harvested. In the case of root crops, the reverse is desired. 

 The roots possess the highest value, the tops the smallest 

 and therefore the largest possible proportion of the plant 

 constituents should be found in the roots at the time 

 of harvest. 



Moreover, it is of interest and often of importance to 

 know in what way the general growth of the plant is 

 affected by varying methods of irrigation. To understand 

 thoroughly the principles underlying the art of .irriga- 

 tion, it is not sufficient to know how much crop by weight 

 is produced by given quantities of water applied in given 

 ways, but it is equally important to know in what way 

 the various parts of the plant are affected in their growth 

 by such variations in irrigation. 



105. Response to irrigation. The plant responds 

 quickly to irrigation. In irrigation, water is applied at 

 infrequent intervals. At first the soil is very wet; then it 

 gradually dries, until it reaches the lento-capillary point 

 or even the wilting point; then it is again wet, again dry, 

 and so on throughout the season. 



All the life processes of plants growing on irrigated 

 land become very active as soon as water is applied to 

 the soil. Under conditions of irrigation, therefore, the 

 plant is somewhat intermittent in its growth. Assimila- 

 tion and all other processes favoring growth are espe- 

 cially rapid after each irrigation, gradually diminishing in 

 intensity and almost ceasing before the next irrigation. 



In the Utah experiments, for instance, it was found 

 that during the first week after irrigation of peas, more 

 than 500 pounds of dry matter were added to the weight, 

 and of oats, more than 700 pounds of dry matter were 



