Apr. 1. 1918 Capacities of Soils for Irrigation Water 33 



not quite 5 feet, since some soil was lost in smoothing the column. Com- 

 pacting in the cylinder was very largely, if not entirely, overcome. 

 Measurements of the length of the column in the cylinder gave results 

 varying from 6yi to 5^ inches. These variations are believed to be due 

 to the difficulty found in inserting the spatula at the 6-inch point without 

 deviating from the horizontal plane rather than to any change in structure 

 of the soil. 



Only one sample was taken at each depth; consequently probable- 

 error calculations can not be made. Very gratifying agreement exists 

 between the results obtained by use of the iron cylinder and those 

 obtained by the paraffin-immersion method, as presented in Table XII. 

 Especially is this true of the averages for the upper 5 feet, wherein the 

 difference is entirely insignificant. Moreover, the results obtained by 

 the paraffin-immersion method and the iron-tube method, which were 

 secured in March, 191 6, confirmed beyond doubt the correctness of the 

 results obtained by the rubber- tube method in October, 191 5, at the end 



of the irrigation season. 



SUMMARY 



(i) This paper reports some observations upon the capacities of 

 certain soils under different conditions to retain water and also develops 

 a new method of determining the volume weight of soils in place, repre- 

 senting some phases of a six-year study of the economical duty of water 

 for alfalfa in Sacramento Valley. 



A relation between the depth of water necessary to add a given per- 

 centage of moisture to a certain depth of soil of given volume weight 

 is expressed mathematically and graphically. 



The observations of capacity of soils to retain water are based on 

 9,584 moisture determinations in the upper 6 feet of soil, 672 in the 

 depth from 7 to 9 feet, and 192 in the tenth to twelfth foot sections, 

 making 10,448 in all. 



Volume-weight determinations upon which the pore-space values 

 largely depend and by which the percentages of water v/ere converted 

 to inches of water per foot of soil were made upon the soils in place to a 

 depth of 6 feet. 



(2) The observations indicate that percentages of pore space which 

 are filled by the water that a soil holds immediately after irrigation 

 increases with the increase in fineness of soil texture. Variations from 

 40 per cent in silt-loam soils having fine sandy-loam subsoils, 5 1 per cent 

 in the silt loams, 58 per cent in the clay loams, to 66 per cent in the clay 

 soils have been noted. 



(3) The ratio of the maximum capillary capacities of soils, as deter- 

 mined in a lo-inch tube in the laboratory, to that of the same soils 

 observed in the field after irrigation varied from 1.78 ±0.06 to 1.98 ±0.14. 



41811°— 18 3 



