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



PACE 



Soil Columns of "Desert" Plains: four soil columns; analyses 247 



Imperial Valley, Mojave Mesa, Coachella and Owens River 



valleys ... 247 



General Suininary 250 



Individual soil columns having the highest humus content 250 



Columns with 1 per cent m each of upper three or more feet.... 250 

 Columns having the highest humus percentages in first, upper 



three feet, and in entire column respectively 251 



Composite columns of agricultural regions 252 



Nitrogen in the humus and in the soil 258 



Humus in soils of different texture; in columns of black adobe clay. 262 

 Relation of humus percentages to color and other soil char- 

 acters, texture, color, and alkali 265 



Humus phosphoric acid in soils; analysis of humus ash 268 



Comparison of arid and humid soils 270 



Conclusions 272 



INTRODUCTION 



Humus, as distinct from the unhumified organic matter, is 

 now regarded as one of the most valuable ingredients of fertile 

 soils because of its physical effect and because of its content of 

 nitrogen, potash, and phosphoric acid, which while not soluble in 

 water, are rendered available to plant use by the action of soil 

 bacteria. It has no definite chemical composition, for it is pro- 

 duced by the decay of vegetable and animal matters of all kinds 

 in the soil and under certain conditions of warmth and moisture, 

 and is naturally made up of the elements found in those sub- 

 stances; during the process of humification, however, there is a 

 gain in the percentage of carbon and nitrogen because of corre- 

 sponding losses of oxygen and hydrogen. The proportion of these 

 and of mineral matters in the humus varies then with the nature 

 of the substances from which they are derived, and probably 

 with the degree of intensity of the action of each factor in the 

 humification process, in which oxidation because of too great heat, 

 or other changes because of excess of water may be brought about. 



The amount of humus produced from organic matter depends 

 upon the nature and condition of the materials used, and upon 

 its complete humification by the maintenance of proper condi- 

 tions in the soil. Professor Hilgard^ thinks that "in the humid 

 region one part of normal soil humus might be formed from five 

 or six parts of dry plant debris ; while in the extreme regime 

 of the arid regions from eighteen to twenty parts of the same 



1 Soils (Macmillan & Co., 1906), p. 128. 



