128 



SOILS. 



them can resume their original soil-functions without prolonged weather- 

 ing. 



Amounts of Humus and Coal Formed from Vegetable Matter. Only 

 very general and indefinite estimates can be given of the amount of 

 humus or coal formed from a given quantity of vegetable matter, since 

 these must vary according to the conditions under which the transforma- 

 tion occurs. The greater or less access of air and of moisture, the 

 temperature and pressure under which the process occurs, will modify 

 very materially the quantitative as well as the qualitative result. In 

 the hot arid regions the fallen leaves may wholly disappear by oxida- 

 tion on the surface of the ground, while under humid conditions they 

 are mostly incorporated with the surface soil. If we assume that in 

 the humification of plant debris (estimating their average nitrogen con- 



FIG. 14. Section of lignitized log showing contraction into solid lignite on drying. 



tent at i%), no nitrogen is lost, it would seem that in the humid 

 region one part of normal soil-humus might be formed from 5 to 6 

 parts of (dry) plant debris ; while in the extreme regime of the arid 

 regions, from 18 to 20 parts of the same would be required. But as 

 most probably some nitrogen also is lost in the process of humification, 

 a considerably larger proportion of original substance may be actually 

 required. 



As to coal, it is usually assumed that it requires about 8 parts of 

 vegetable matter for one of bituminous coal. Much higher estimates 

 are made by some, and an observation made by the writer at the Port 

 Hudson bluff, Mississippi, in 1869, would seem to justify such estimates. 

 The above figure, from a sketch made at the time, shows the pro- 

 portions to which a pine log about eight inches in diameter had shrunk 



