E. W. Hilgard — Peculiarities of Rock - Weathering. 269 



the humus acids of the soil might exert " aufschliessende " 

 action also, seems at least probable from its effect in " rusting " 

 metals, and from that of lactic acid in the animal stomach. 

 The sulphate is probably very inert in this respect. 



An interesting and important difference exists in the nitro- 

 gen-content of the arid and humid soils, respectively. The 

 average humus-content of the properly arid soils is materially 

 less than that of the humid, as might be expected from the much 

 smaller amount of vegetable debris that finds its way into the 

 soil. As a matter of fact, the leaves of all the spring vegeta- 

 tion of the arid region remain outside of the soil and are oxi- 

 dized away during the hot summers, leaving little more than 

 their ash behind. It follows that the humus of the arid soils 

 is almost wholly derived from the decay of roots alone ; and 

 the deep penetration of the latter, already referred to, explains 

 the occurrence of humus down to the great depths shown in 

 the small table below, which gives the humus percentages 

 found in a bench soil in which hop roots penetrated over 

 twelve feet. 



Humus in Russian River bench soil, from, one to twelve feet depth. 



Depth, feet 1 2 3 4 5 6 7 8 9 10 11 12 

 Humus #,1-21 1-16 1-14 1-17 '74 -60 -47 '78 -54 '52 -53 '44 



But the total humus percentage in arid soils is usually so 

 small that in the humid region it would be considered wholly 

 inadequate. So it would be, from two causes : first, that it is 

 there among the first things needful for the maintenance of 

 proper tilth in the clayey lands, while in the pulverulent soils 

 of the arid region this is much less necessary ; second, because 

 we find the nitrogen-content of arid humus to be on the aver- 

 age three or more times higher than is the case in the humid. 

 Hence the adequacy of the minute amounts of humus found 

 in many of the most esteemed arid soils ("25 per cent and even 

 less) ; to which may be added the at least probable greater 

 intensity of nitrification when no great excess of carbon and 

 hydrogen compounds is present to compete with the activity 

 of the nitrifying organisms ; an activity which, moreover, can 

 evidently progress at much greater depths in the pulverulent, 

 pervious arid soils. 



The physical and chemical details of this subject are quite 

 complex, so that a full discussion is necessarily lengthy. But 

 the summary here given is probably enough to show its wide 

 theoretical as well as practical interest, and the need of caution 

 in applying the maxims of the humid region in arid climates. 



