1914] Loughridge: Humus and Nitrogen in California Soil Columns 253 



Table 45. — Composite Averages of Agricultural Eegions (in percentages) 



Range of most annual plant roots. 



posite columns except that of the Sierra Nevada foothills, the 

 lava-bed meadows and the tnle marshes, where, because of the 

 underlying country-rock on the one hand and of water on the 

 other, the depths are limited to eight, ten and seven feet respec- 

 tively. Seventy-five of the columns have a depth of twelve feet, 

 eleven others a depth of ten feet, and eight a depth of nine feet. 

 In some of the columns the amount of humus is so high in the 

 twelfth foot, especially in that of the valleys of the Coast Range, 

 as to leave no doubt of its being found at a greater depth had 



