1914] Loughridge: Huinus and Nitrogen in California Soil Columns 189 



The humus of the adobe soils contains on an average about 

 5.5 per cent of nitrogen in the first foot and 4.9 per cent in 

 the entire column. The amount in the top soil is greatest in the 

 Willows adobe (0.09 per cent), and least in the Biggs soil (0.08 

 per cent), or an average of 0.06 for four soils. This is equivalent 

 to 2400 pounds of organic nitrogen per acre in the surface foot 

 which, under the proper ammonifying and nitrifying conditions, 

 is gradually converted into an available supply for plant use. 



The humus of the soil near Dixon is richest in nitrogen both 

 in the surface foot and in the upper three feet, though the amount 

 of humus is not so great as in that from Willows. 



The amount of humus must be at least 1 per cent, and its 

 nitrogen content must be fair to give to a soil the 0.05 per cent 

 which is regarded as sufficient for present fertility, and we find 

 that of these four localities the soil from Biggs falls short in the 

 first foot and that from Yuba City in the second and third foot 

 respectively. 



The percentage in the soils is naturally less and less down- 

 ward as the humus diminishes, and there are but traces of 

 nitrogen in the lower half of each column. The natural nitrogen 

 supply for the roots of crops must clearly come from the upper 

 three feet in each locality, that from Willows being equivalent 

 to about 8000 pounds per acre in three feet depth. 



Red Mesa and "bedrock" Lands.— These form a wide border 

 along the eastern and northern sides of the valley, reaching 

 south into San Joaquin County, and are characterized by usually 

 shallow soils underlaid by either heavy, compact red clays or by 

 cemented beds of gravel and grit, forming a hardpan or bedrock 

 at depths of from two to five feet below the surface. Because 

 of the shallowness of these lands, this region is represented by 

 but three columns; one from the bluff of Oat Creek southwest 

 from Red Bluft', and one-half mile west of the Corning road; 

 another from near Sheridan, Placer County, representing the 

 lands on the eastern side of the valley; and the third from near 

 Acampo in San Joaquin County, taken by Mr. F. E. Johnson. 



The red soils from Sheridan and from the mesa southwest 

 from Red Bluff show very low percentages of humus, not only 



