188 rniversity of California Publications in Agricultural Sciences [Vol. 1 



of Yuba City northward, passing west of Gridley and beyond 

 Biggs. A column was taken from this belt, a few miles south- 

 west of Yuba City, the surface soil of which contained 39 per 

 cent of clay. Below this upper foot the color changed from black 

 to gray. Another column was taken by ]\Ir. F. E. Johnson from 

 near Biggs. This contained 50 per cent of clay in its upper 

 foot and the black color changed to gray below the third foot. 



A region of black clay lies three miles east of Willows, Glenn 

 County, whose surface foot contains 47.46 per cent of clay, and 

 the black color extends through five feet. A column of this was 

 taken to the depth of twelve feet. 



A few miles south of Dixon. Solano County, there is a large 

 body of very black adobe clay having 61.75 per cent of clay, 

 and the black color passes through three feet, changing to gray 

 below. This soil contains the highest percentage of clay thus 

 far found in any black adobe in the state. 



The black adobe clays, with the exception of that from east of 

 AYillows. are a disappointment with respect to their humus 

 content, for we had anticipated finding fully 1.5 per cent in 

 each of the upper three or four feet, or hoped that they would 

 at least come up to the average of the other 109 colunms of the 

 collection, whose summation for three feet is 2.81 per cent. It 

 is evident from the results that a black color does not always 

 mean a high percentage of humus, for the densely black adobe 

 of Biggs and that south of Dixon each had less than the much 

 lighter colored soils elsewhere, and less than one-half that of 

 the reddish alluvial soil of Chico Creek. The samples from Biggs 

 and Yuba City are from the same belt of black adobe, though 

 many miles apart, and each shows very small percentages below 

 the first foot. Clearly, these clays would be greatly benefited in 

 texture and richness by a good system of green-manuring. 



The humus in the adobe soils from the several localities and 

 even in the respective depths of each column is not uniformly 

 rich in nitrogen, as will be seen by reference to the table. It is 

 sometimes richest at a depth of several feet, and frequently 

 there is a sudden and great diminution in an adjoining level, 

 the cause of which is not apparent. 



