274 Universify of California Publications in Agricultural Sciences [Vol. 1 



5.92 per cent; that of each of the upper three feet is 5.60, and 

 a little less for the entire twelve feet. It varies from 1 to 20 

 per cent in individual soil layers. 



11. The organic nitrogen in the soil derived from the humus 

 and dependent on the amount of the latter, varies from almost 

 nothing in the lower depths of the soil to as much as 0.13 per 

 cent in the upper three feet. The average of the first foot of the 

 soil columns is 0.07 per cent; for each of the three upper feet, 

 the range of most annual plant roots, it is 0.05 per cent. The 

 investigations of Professor Lipman of this station show that 

 nitrifjdng bacteria are present and active in California soils to 

 depths of six feet and ammonifying bacteria are present through 

 a depth of twelve feet, thus making available to plants the 

 nitrogen content of the humus to these depths. 



12. Humus contains soluble mineral plant food in combina- 

 tion, the phosphoric acid being present in the humus of Cali- 

 fornia soils to the extent of from 0.01 per cent to 0.08 per cent 

 throughout the entire depth to which humus reaches, though 

 usually greatest in the upper few feet. 



13. Humus is sometimes less in the first foot than in the 

 second because it is gradually destroyed by cultivation and 

 summer fallowing of the soil, but may be replaced and increased 

 by proper methods of green-manuring, or the turning-under and 

 humification of legumes. 



14. Arid soils have an immense advantage over those of the 

 humid region of the United States because of this distribution 

 of humus and its nitrogen, as well as of mineral plant food, 

 through a depth of many feet, as it gives a greater depth of soil 

 and induces a deeper root penetration for plants and trees into 

 a greater feeding area and where there is more moisture. The 

 wonderful endurance of drought on the part of California soils 

 is due to this. 



15. The practical value and hence the commercial valuation 

 of land in California cannot be based alone upon the nature of 

 the surface soil and subsoil as in the humid region, but chiefly 

 upon its texture and depth, and upon the freedom with which 

 plant roots are able to penetrate to many feet and secure moisture 

 and the abundant food supply that exists at those depths in all 

 of California soils. 



