93 



time it is to be irrigated and. by the amount it should contain in order 

 that plants may do their work to the best advantage 



The maximum capacity of upland field soils for water ranges from 

 about 18 per cent, of their dry weight for the light sandy types to about 

 30 per cent for the heavy clayey varie ies, while the amoun s of water 

 these soils should contain in order that plants may thrive in them best 

 is from 12 to 14 per cent, for the former and from 18 to 20 per cent, 

 for the latter. The growth of plants will be seriously checked in sandy 

 soils when the water content falls below 8 per cent., and in heavy, 

 clayey types when it falls below 1 4 per cent, of the dry w eight of i he soil. 



The dry weight of a light sandy soil and subsoil will average about 

 105 pounds per cubic foot, and the heavy, clayey tpye about 80 pounds 

 per cnbic foot. Hence the maximum amount of wat r per cubic foot of 

 soil would be about 24 pounds for the clay and 18.9 pounls for the 

 sand. This being true, 4.6 inches of water on the level would com- 

 pletely saturate the surface foot of heavy clay soil, were it entirely dry 

 to begin with, while 3.6 inches would place the sandy soil in a 

 similar condition. 



But since water should be applied as soon as the water content of the 

 sandy soil falls to 8 per cent, and that of the clayey soil to L4 per cent, 

 it follows that under these conditions 10.5 pounds of water, or 2 in- 

 ches, is the maximum amount which would be needed to fill the sur- 

 face foot of sandy soil and 12.8 pounds, or 2.46 inches, is enough to 

 fill the surface foot of clay soil. 



If we consider the seco' d foot of soil to have been dried out to a 

 corresponding extent, and that it is desirable to saturate this with water 

 also, then the amounts just stated would need to be doubled, 4 inches 

 being demanded for the sandy soil and 4.92 inches for the clayey soil. 

 It is quite certain, however, that such an application of water to a field 

 at one time would result in the percolation of a considerable amount of 

 this water below the depth of root action, and hence in a considerable 

 loss of it unless a large crop were growing upon the land at the time. 

 It appears, therefore, that the amounts of water which may be applied 

 to a field at one time will lie between 2 and 5 inches in depth over its 

 whole surface. 



How often this watering may need to be repeated, it is not possible 

 to state in anything like definite terms, but practical experience shows 

 that as a rough average the intervals between watering where maximum 

 yields are sought can not much exceed 7 to 14 days, the time being 

 shortest when the crop is making its most vigorous growth. 



In experiments at the Wisconsin Station during 1895 corn was 

 irrigated once about every 7 to 9 days, applying at each time 4 . 43 

 inches of water. The corn, however, was planted very thickly upon 

 the ground, the rows being only 30 inches apart and the hills 15 inches 

 apart in the row, with from 2 to 5 stalks in each hill. The first 

 irrigation was given June 26 and the last August 15, the total amount 

 of water applied being 26.6 inches. The yield produced was 11,125 

 pounds of water-free substance per acre. 



In the case of the water meadows of Europe very little attention is 

 paid to the natural rainfall, the irrigation waters being applied when- 

 ever it is possible to do so, and, whatever rains fall are counted as so 

 much additional gain. It is true, however, that on most lands with 



