18 Irrigation and Drainage 



tributing nothing to the physiological processes which are involved 

 in the production of the harvest sought. 



Irrigation and land drainage are, each of them, methods of 

 treatment of field conditions which aim to modify and control the 

 quantitative relations of the water which the soil shall contain, 

 and hence it becomes a matter of importance to know how much 

 water is necessarily involved in the production of a given amount 

 of a given crop. Much work has been done by various investi- 

 gators bearing upon this problem, but in all of those cases the 

 work has been by methods and appliances which have placed the 

 plants experimented with under such conditions that the roots 

 were forced to develop in a volume of soil which was much smaller 

 than field conditions usually afford. In the writer's work, how- 

 ever, he has aimed to give the plants more nearly the normal 

 amount of root room ; and in one series has aimed, also, to so 

 place the experiment that the plants should be growing as 

 nearly as possibly under the meteorological conditions of the field 

 crop. 



The apparatus used for this work is illustrated in Fig. 1, 

 where, for the first trials, 50-gallon vinegar casks were used for 

 pots in which to place the soil. But after the first year's work 

 these were abandoned, and there were substituted for them, for 

 the field work, galvanized iron cylinders 18 inches in diameter and 

 42 inches deep. These were placed in pits in the ground in the 

 field, as illustrated in Fig. 1, so that the tops of the cylinders 

 were at the level of the top of the field soil, and so that the cylin- 

 ders in which the experimental plants were growing stood in the 

 field surrounded by the crop of the same kind growing under field 

 conditions. The object of placing the experiment in this manner 

 was to secure for the plants, as nearly as possible, the meteorologi- 

 cal conditions of the field, and these conditions were quite closely 

 realized in all particulars except the one of soil temperature. In 

 this particular the cylinders, being necessarily isolated from the 

 body of the field soil in order that they might be weighed at any 

 time, allowed the soil to take more nearly the temperature of the 

 atmosphere than was true of the deeper layers of soil in the field, 

 and also to be subject to wider diurnal changes in the lower por- 



