174 IRRIGATION PRACTICE 



at the same thickness while the plant is growing. The 

 young plant requires less water per day than the older 

 plant, but the ease with which the water may be obtained 

 should be practically the same for young and older plants, 

 as long as they are growing vigorously. Such a condition 

 of uniform water content in the soil is practically impos- 

 sible unless water is added daily to the soil to replace that 

 lost by evaporation and transpiration. The intermit- 

 tent nature of irrigation, fundamentally characteristic, 

 implies a period of high moisture percentage imme- 

 diately after an irrigation, gradually diminishing until, 

 just before the following irrigation, the soil is often very 

 dry. Nevertheless, the irrigation farmer must attempt to 

 apply irrigation water in such a way as to leave the plant 

 in an approximately uniform moisture environment 

 throughout the season. Therefore, irrigation must be 

 more abundant and frequent at periods of high transpira- 

 tion. So far as the soil is concerned, the intermittent 

 nature of irrigation is highly favorable in producing a 

 condition favorable to plant-growth. 



The discussion of the time of applying irrigation water 

 may be surveyed as follows: 



1. Irrigation when crop is not growing. 



(a) Fall irrigation. 



(6) Winter irrigation. 



(c) Early spring irrigation. 



2. Irrigation when crop is growing. 



(a) For germination. 



(6) Use of early spring floods. 



(c) Irrigation at different periods of crop growth. 



(x) Annuals. 



(y) Biennials. 



(?) Perennials. 



