2 Journal of Agricultural Research voI.xiii.no. r 



powers of the same soils under field conditions because, first, they con- 

 sider a very short column of soil which is acted upon by special capillary 

 forces; and, second, the samples of soil used have in most cases volume 

 weights which are much lower than those obtaining in the undisturbed 

 condition. It is desirable, especially where irrigation is practiced, to 

 have accurate knowledge of the maximum water-holding capacity of the 

 soil in place. Burr (5) found the maximum capacity of a fine, sandy loam 

 (loess) to be 16 to 18 per cent of the weight of the dry soil. Quantities 

 of water found by various investigators after heavy irrigations or rain- 

 fall (2, 7, 8, 9, II, 12) seem to be in agreement with the results of Burr's 

 experiment. Indirectly, therefore, the maximum water capacities of 

 soils in place have been determined by a number of workers under various 

 conditions. 



The optimum quantity of water to add to a given depth of soil in a 

 single irrigation is dependent on the moisture content of the soil before 

 irrigation and its maximum water capacity. 



Let P = the percentage of water to be added ; 



I^ = the weight of soil to be moistened; 



w = the weight of water to be applied. 



ThenPXl^ = w (i) 



Since the quantity of water applied to a soil is usually expressed in 

 depth 9ver the surface, it is desirable to so express the quantity here 

 needed. 



Therefore let A = the area of land to be irrigated ; 



Z) = the depth of soil which needs water; 

 Vw = the volume weight of the soil ; 

 d = the depth of water to be added ; 



Then W = VwAD and w = AdXi 



Consequently, by substituting for W and w in equation one, their 

 values as above, we have 

 PXVwAD = Ad and d = PXVwXD (2) 



By taking D as 12 inches and assuming several values of Vw, figure i 

 has been prepared from equation 2 . For a given value of Vw the number 

 of inches of water necessary to add a given percentage of moisture to i 

 foot of soil may be readily determined. 



EXPERIMENTAL CONDITIONS 



The investigations here reported were made under the following con- 

 ditions: First, on a number of alfalfa fields in Sacramento Valley repre- 

 sentative of the best practice there; second, on 0.25-acre plots of the 

 irrigation tract at the University of California farm at Davis ; and third, 

 on the 0.4-acre plots of a temporary experimental tract about 4 miles 

 northeast of Willows, also in Sacramento Valley. 



The soil of the typical farms ranges from silt loam underlain with fine 

 sandy loam to heavy clays. The upper 2 feet of the Yolo loam in the 



