1914] Loughridge: Humus and Nitrogen in California Soil Columns 265 



Tracy, Biggs, and south of Dixon, which have less than 1 per 

 cent. A very striking example of this was found in a sample 

 of adobe soil sent for examination some time ago from near the 

 hills in the Santa Clara Valley, which although almost jet black 

 had but a trace of humus. 



It is further interesting to note that in the Berkeley and 

 Walnut Creek columns there is more than 1 per cent of humus 

 in each of the upper six and five feet below the surface respec- 

 tively; in each of the upper three feet of the Santa Clara and 

 San Ramon columns and in the upper two feet of the Willows 

 column. In the Dixon, Tracy, and Biggs columns there is less 

 than 1 per cent in the first foot. 



In the fourth foot of the Santa Clara, seventh foot of the 

 Berkeley, and third foot of the Yuba City there are very sharp 

 drops in the percentages, and this is seen also in the lighter color 

 of the respective columns at those depths. These indicate that 

 the main root systems of the plants were confined to the upper 

 part of these soils, while in other columns there was less of root 

 mass and a greater downward development. In all except where 

 ground water was found above the twelfth foot, the humus 

 clearly could be found at greater depth, especially in the Berkeley 

 column, which had 0.49 per cent in the twelfth foot. 



The Santa Clara soil has in its first foot the highest per- 

 centage of humus of any soil of the state thus far examined, 

 excepting the tule marshes; the same is true if we compare the 

 average of the three upper feet and also that of the upper six 

 feet, which are respectively 3.62 and 1.99 per cent. The general 

 average of humus in the first foot of the eleven columns is 1.61, 

 for the second 1.20, and for the third foot only 0.92 per cent. 



Relation of Humus Percentage to Color and other 



Soil Characters 



There are types of land having other than textural charac- 

 teristics concerning whose relation to humus content some ques- 

 tion may arise, and we have segregated some of the more 

 important of these in the following table. They refer to the 

 uplands and lowlands, the several color characters and to strong 



