1 7G University of California Publications in Agricultural Sciences [Vol. 1 



ground during and after a rain. ... In humid climates and in a ground 

 fairly stocked with these worms the soil thus brought up may amount to 

 from one-tenth to two-tenths of an inch annually over the entire surface; 

 so that in half a century the entire surface foot might have been thus 

 worked over. Aside from the mechanical effect thus achieved in loosen- 

 ing the soil, and the access of air and water permitted by their burrows, 

 the chemical effects resulting from their digestive processes, and the final 

 return of their own substance to the soil mass; also their habit of draw- 

 ing after themselves into their burrows, leafstalks, blades of grass, and 

 other vegetable remains, renders their work of no mean importance both 

 from the physical and chemical point of view. . . . The work of earth- 

 worms is especially effective in loamy soils and in the humid regions. 

 In the arid region and in sandy soils generally the life conditions are 

 unfavorable to the worm, and the perviousness elsewhere brought about 

 by its labors already exists naturally in most cases. 



The amount and nature of humus depends much upon climatic 

 factors as is especially noted in arid and humid regions. In the 

 humid region with its frequent rainfall and a comparatively 

 shallow soil, the vegetable material (roots, leaves, etc.) is held 

 near the surface and to this depth the humus is limited ; while 

 in the very deep and warm soils of the arid region the penetration 

 of plant roots is to depths of fifteen to thirty feet, and as a 

 consequence of their decay humus is found to depths of twelve 

 or fifteen feet and in some cases much deeper. 



It is this deep distribution of humus throughout the agricul- 

 tural regions of California that is treated of in detail in this 

 bulletin, showing a fundamental difference between the soils of 

 humid and arid regions. 



HUMUS IN CALIFORNIA SOILS 



The study of the soils of California was begun by Professor 

 Hilgard immediately after entering upon his duties in 1874 as 

 Professor of Agriculture in the LTniversity of California, and 

 his first report, made in 1877, contains physical and chemical 

 analyses together with descriptions of a number of soils, as well 

 as the results of alkali investigations. The analysis of a soil for 

 its humus content is first given in the report of the Experiment 

 Station for 1879, and since then nearly every annual report to 

 and inclusive of 1904 contains soil descriptions and analyses, the 

 percentage of humus being given for the first foot or sometimes 



