SOIL AND SUBSOIL. 



135 



The large amounts of the soluble alkalies potash and soda 

 thrown down with the humic matters are very striking, as is 

 the very large proportion of phosphoric acid. Lime and mag- 

 nesia had, of course, been mainly eliminated by the prelimi- 

 nary acid treatment. 



Functions of the Unhumificd Organic Matter. The unhu- 

 mified plant debris in the soil are not to be regarded as useless, 

 even aside from their potential conversion into active humus. 

 Not only do these remnants of vegetation lighten the soil, 

 rendering it more pervious to air and water, but in their pro- 

 gressive decay they give off carbonic gas, which is active in 

 soil-decomposition ; and they serve as nourishment to the soil 

 bacteria upon which its thriftiness so greatly depends. See 

 below, chapter 9. 



The Nitrogen-Content of Humus. Since soil-humus is 

 doubtless the chief depository of soil-nitrogen, and the main 

 source from which, through the process of nitrification, the 

 nitrogen-supply to plants is usually derived, its content of that 

 element is a matter of great interest. It has been customary 

 to estimate approximately the nitrogen-content of soils by the 

 proportion of humus-substance present ; and as the light tints 

 of the soils of the arid region indicate a small humus-content, 

 a scarcity of nitrogen seemed to be also indicated for these 

 lands. As this in a number of cases did not seem to accord 

 with actual experience, an investigation of the subject was 

 made at the California experiment station, 1 with the results 

 shown in the subjoined table. In considering these results it 

 must be kept in mind that while arid conditions can rarely be 

 fulfilled in the humid region, humid conditions are quite fre- 

 quently locally represented in the arid, in lowlands and on high 

 mountains; while moderately moist benchlands represent the 

 semi-arid regime. 



1 Hilgard and J,i_t]\t. On the \itrogen-content of Soil-humus in the Humid and 

 Arid regions. Rep. Cal. Kxp't Station for 1892-4; Agric. Science, April, 1894; 

 Wollny's Forsch. Geb. Agr. I'hys., 1894. 



