1914] Loughridge : Kumus and Nitrogen in Calif ornia Soil Columns 267 



humus iu both classes is in the decay and humifieation of plants 

 whose roots had penetrated to depths of many feet. 



The small differences throughout the composite columns would 

 also show that the alluvial lands and the deltas possess no material 

 advantage, except perhaps in moisture conditions, over the higher 

 lands which are of good depth and free from hardpan. 



Relation of Humus to Soil Color. — It has already been pointed 

 out that a black color does not necessarily mean the presence of 

 a high percentage of humus, and this is again emphasized in the 

 composite columns of many soils of three colors taken from the 

 main agricultural regions of the state. In these the surface foot 

 of the black has but little more humus than that of the gray 

 soil (1.19 and 0.94 per cent respectively). But still more sur- 

 prising is the fact that, although the upper three feet of the 

 black lands are almost invariably very dark and even black, 

 they contain less humus than the upper three feet of the gray 

 sandy lands; and that throughout the entire composite columns 

 the percentage of humus is greater in the gray than in the black 

 soils with the exception of the first foot. There are, of course, 

 black clay lands of Santa Clara, Arroyo Grande, and other 

 localities which have high percentages, but these are offset by 

 those with very small amounts. 



The composite of eleven red lands embracing mesa and hill 

 lands and the more level lands on the eastern side of the great 

 valley contains less humus than either the black or the gray, 

 except in the lower six feet. 



The gray lands of the state seem therefore to have greatly the 

 advantage over both the black and red classes in larger humus 

 content and in its better distribution throughout the depth of 

 twelve feet. 



Humus in Strong Alkali Soils. — This column is a composite 

 of four alkali soils from the San Joaquin Valley, including the 

 Tulare Lake bed, and shows a deficiency in humus throughout. 

 Alkali salts are usually held in the upper four feet of these 

 valley soils and it is there where they would prevent the growth 

 of vegetation with accompanying root systems whose decay would 

 result in the production of humus. Hence w^e find but little more 

 than mere traces of humus below the fifth foot. 



