138 SOILS. 



It thus appears that on the average the humus of the arid 

 soils contains about three and a half times as much nitrogen as 

 that of the humid ; that in the extreme cases, the difference goes 

 as high as over six to one (see Nos. 37 and 704) ; and that in 

 the latter cases, the nitrogen-percentage in the arid humus con- 

 siderably exceeds that of the albuminoid group, the flesh-form- 

 ing substances. 



It thus becomes intelligible that in the arid region a humus- 

 percentage which under humid conditions would justly be con- 

 sidered entirely inadequate for the success of normal crops, may 

 nevertheless suffice even for the more exacting ones. This is 

 more clearly seen on inspection of the figures in the third 

 column, which represent the product resulting from the multi- 

 plication of the humus-percentage of the soil into the nitrogen- 

 percentage of its humus ; as appears in comparing the respective 

 averages, or Nos. 1167 and no and others. An additional 

 consideration is the probable greater ease with which the nitri- 

 fying bacteria can act upon a material so rich in nitrogen. 



We must not, then, be misled by the smallness of many 

 humus-percentages in the arid region, into an assumption of a 

 deficiency in the supply of soil-nitrogen. 



Decrease of Nitrogen- Content in Humus with Depth. Since the 

 oxidation of the carbon and hydrogen in the humus- substance, and the 

 consequent increase of its relative nitrogen-content, are manifestly de- 

 pendent upon the presence of air and heat, it is reasonably to be 

 expected that the nitrogen- percentage of the humus should decrease 

 with the depth of the soil. That this is really the case is plainly shown 

 in the subjoined table, which gives the humus-percentages and the 

 nitrogen-content of the humus from the surface foot down to twelve feet, 

 in a soil on the bench of the Russian River, Cal., which is sub-irrigated, 

 and liable to more or less rainfall during the summer. It will be seen 

 that not only does the absolute humus-percentage decrease quite regularly 

 down to seven feet, at which point there evidently was at one time a 

 strong root development, causing a notable increase of the humus-con- 

 tent ; from which again there is a regular decrease down to the twelfth 

 foot. It will be noted that the nitrogen-percentage in the humus, 

 while not decreasing with the same regularity as the humus-content 

 itself, yet exhibits a general recession from 5.30 to 1.15 in the ninth 

 foot, to which direct oxidation doubtless never penetrates. 



