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



upper foot there is more than 10 per cent; but here again the 

 amount of humus is small. The Chico column should also be 

 mentioned, as from the fifth to the twelfth the humus of each 

 foot contains from 10 to 14 per cent of nitrogen, except the ninth 

 which has 9.5 per cent. 



In the one thousand determinations of nitrogen there were 

 but sixty-four instances, or about 6.4 per cent, where the humus 

 contained more than 10 per cent of nitrogen : fourteen of these 

 have from 15 to 20 per cent and six have above 20 per cent. The 

 general average of all, including the marshes, is 5.92 per cent 

 for the first foot, 5.60 per cent for the upper three feet and 

 5.57 per cent for the entire depth of twelve feet. 



The humus of the surface foot of the composite of Southern 

 California soils is richer in nitrogen than that of any of the 

 other regions, though nearly equalled by that of the tule marshes. 

 That of the lava-bed valleys is poorer in nitrogen than any. 



When we consider the upper three feet of the composite 

 columns we find that the humus of the San Joaquin Valley is 

 much the richest and that of the lava beds again the poorest. 

 Four of the eight columns fall below the general average for the 

 state. The same is true for the averages of the twelve foot 

 columns. 



The differences in the percentages of nitrogen in the humus 

 of the upper three feet and of the total column are not very 

 great, especially in the valleys of the Coast Range, where the two 

 are very nearly the same; in the Sacramento Valley, Southern 

 California lava beds and the desert plains the average in the 

 column is considerably less. 



Organic Nitrogen in the Soils. — It is well to remark here that 

 0.10 per cent of humus-nitrogen in the upper foot of a humid 

 soil is regarded as an ample supply for fertility, while in the 

 arid region with its deeper soil, deeper humus and deeper root 

 penetration, one-half of that amount, or 0.05 per cent in each 

 of the three upper feet is considered sufficient for many years, 

 because the roots are in a deeper feeding area than in the humid 

 region, and bacterial activity is greater and deeper. This per- 

 centage would mean 2000 pounds of nitrogen per acre in each 

 foot. 



