16 University of California Publications in Agricultural Sciences [Vol. 1 



GENERAL DISCUSSION 



As I have already pointed out in an earlier publication,- the 

 slow formation of clay substances in soils of the arid region, 

 owing to the peculiar climatic conditions there obtaining, is 

 doubtless responsible for a much greater degree of aeration in 

 soils because of the larger volume of pore spaces made possible 

 through a lack of large quantities of cementing substances. Thus 

 when soils first begin to form from disintegrating rock we have 

 much more complete aeration with an encouragement for bac- 

 teria, probably the earliest inhabitants of the soil, to penetrate 

 to greater depths. Such penetration on the part of bacteria is 

 invariably accompanied by the production of more favorable 

 physical and chemical conditions in the soil for the roots of 

 plants. These in their turn, through physical and chemical 

 changes which they bring about in the soil in their search for 

 water and food, make better conditions for a deeper penetration 

 of bacteria and so through mutual aid the latter and the higher 

 plants are able, under our arid climatic conditions, to make the 

 deeper layers of soil a more congenial medium for each other. 

 The changes thus brought about result in a more uniform tex- 

 ture of soils at great depths, uniformity of chemical composi- 

 tion, including humus content, in all the soil layers, and a 

 much closer approximation of the bacterial flora in the lower soil 

 layers to those of the upper laj'ers than can be found in the 

 average soils of the humid region, where climatic conditions are 

 unfavorable to good aeration, because tendencies opposite to those 

 above described for our soils are in operation. An estimate of 

 the biological condition of our deep soils was thus similarly made 

 by Hilgard on a priori considerations and the investigations 

 above recorded serve, in general, to confirm liis surmise. 



Viewing the subject in its entirety, we find that the organ- 

 isms forming ammonia in soils penetrate to greater depths than 

 the nitrifying or nitrogen-fixing bacteria studied. While am- 

 monification is iisually most vigorous in the surface, four to six 

 feet, it is none the less very pronounced in the lower layers from 

 six to ten feet in depth in all of our normal deep soils. Hardpan, 



- Lipman, C. B., New Facts about Bacteria of California Soils, Science 

 K S., June 11, 1909. 



