Apr. I, 1918 



Capacities of Soils for Irrigation Water 



21 



found was 26.8 per cent and the maximum quantity of water found in the 

 soil after irrigation was 24.0 per cent. 



It is highly desirable to establish, if possible, a relation between some of 

 the more generally used soil constants (3) and the maximum field capacity 

 of the soil. An attempt has therefore been made to compare the maxi- 

 mum capillary capacity as determined in the laboratory to the field 

 capacity. The laboratory determinations were made by the use of brass 

 cylinders 2 inches in diameter and 12 inches long, having perforated 

 bottoms, which were filled on the Bowman soil compactor, placed in 

 water to a depth of 10 inches overnight, and drained for 24 to 48 hours. 



BROKEN UN£ PLCT-C (3-S' IRRIGATIONS) 



SOLID L/A/C ^ PlOT-D ('>-6'IRRI6ATI0N5} 



DOTTCD LINL.—...—MOt5TUR£. CQUIVALCfJT 



Fig. 8. — Graphs showing the comparison of water content before and after irrigation, moisture equivalent, 

 and pore space of soils of plots C and D, Davis, Cal. Each water-content curve in plot C is the average 

 of 21 borings; in plot D, of 28 borings. 



The results reported in Table IX include the hygroscopic water contained 

 in the air-dry sample used. 



The surface foot of the first group of soils listed in Table IX was no 

 doubt fully saturated by the large amounts of water applied. The 

 quantities held per foot of soil vary from 2.58 in the Griffes tract to 

 3.67 in the Geer tract. The ratio of the amount held in the laboratory 

 to that held in the field varies from 1.53 to 2.10 and has a mean of 

 1.78 ±0.06. The probable error of a single observation is ±0.19 inch 

 per foot. Although the second group of soils was irrigated more mod- 

 erately, the average unit application was 8. 11 inches, which is much 

 more than i foot of soil can retain. Therefore the amounts held 



