186 University of California Publications in Agricultural Sciences [Vol. 1 



sity, had been in grain culture for thirty or more years. The 

 humus in its subsoil is nearly double that of the soil and even in 

 the fourth foot is equal to that of the soil itself. A sample of 

 virgin soil taken near this spot was found ti^ have 1.25 per cent 

 of humus, which was probably the original amount in the field, 

 thus showing a loss of nearly 30 per cent. The nitrogen in the 

 virgin soils was 0.07 per cent, but in the cultivated was but 0.04 

 per cent, which is a loss of more than 40 per cent of nitrogen. 



It will be noted that the percentage of humus in the Davis soil 

 below the upper foot is greater throughout its twelve feet than 

 in any other of the columns. This may be accounted for by the 

 greater development of the root systems each year and their 

 subseciuent humification. Investigation made by ^Ir. Farrer. 

 formerly of the University Farm, showed that the roots of 

 wheat, barley, and the California poppy, coincident with those 

 of orchard trees, reached water at a depth of twelve or thirteen 

 feet. If. as is probable, this had been the case for many years, 

 the amount of root material for humification has been large 

 enough for these results. The humus nitrogen of the surface 

 foot is greatest in the Woodland and Yuba City soils. 0.07 per 

 cent and 0.06 per cent respectively : but those columns were taken 

 from an uncultivated lot near Woodland and from the AYalton 

 orchard south of Yuba City. 



Previous analyses of other clay-loam soils gave the following 

 percentages of humus in the surface foot: Dixon. 1.71: Kell's 

 place near Yuba City. 1.28. and north of Willows. 3.61. 



The humus in these columns from the seven localities is not 

 especially rich in nitrogen with the exception of the lower por- 

 tion of that from AYalton 's place s^uth of Yuba City: but the 

 amount of humus is so small that the nitrogen given to the soil 

 is very little in amount. The soils from Walton's. AYoodland. 

 and Davis are the only ones of the group whose upper three feet 

 have the normal of 0.05 per cent of humus-nitrogen, that of 

 Davis being 0.08 per cent, or approximately 9000 pounds per 

 acre. 



Black Adobe Clay Soils. — There are several regions of these 

 black clays within the eastern, western, and southern parts of 

 the Sacramento Vallev. One of these reaches from southwest 



