SOIL AND SUBSOIL. 



I2g 



in drying into a small sheet of lignitized wood ; the original trunk, 

 projecting from a bed of sand some forty feet below the surface, being 

 so porous and spongy that when wet it flattened somewhat by its own 

 weight ; it was connected with the little sheet of lignite by a spirally 

 twisted, tapering stipe. 



Here evidently the proportion of lignite formed was a very minute 

 one, doubtless because of the long leaching to which the trunk had 

 been subjected. It thus seems impossible, as in the case of humus, 

 to assign any definite proportion as between woody matter and coal 

 formed from it. 



Normal humification takes place only under the influence of 

 moderate temperature. When the temperature is too low, bac- 

 terial and fungous growth are repressed or arrested ; when too 

 high, the fungous vegetation assumes a different phase, the 

 result of which is the almost total oxidation of the organic 

 matter, sometimes so accelerated as to initiate rapid com- 

 bustion (" fire-fanging" of dung) ; leaving in any case but a 

 trifling organic residue of very high ash contents. 1 



Eremacausis. In the absence of a sufficient degree of mois- 

 ture to co-operate with the other agencies of humification, the 

 final result in the soil is practically the same as in the " fire- 

 fanging " of dung. The organic matter is almost wholly de- 

 stroyed by direct oxidation (eremacausis) with or without 

 the aid of minute organisms; leaving essentially only the ash 

 behind to be reincorporated with the soil. This is to a very 

 great extent the predominant process in the arid regions of the 

 Globe ; most of the soils formed in these climates being, there- 

 fore, very poor in humus-substances, and deriving it almost 

 entirely from the decay of roots only. 



The extent to which the humus of a soil may be derived from 

 the vegetable debris falling or growing upon the surface, varies 

 greatly with the climatic conditions as well with the nature of 

 the soil. In the forests of humid climates with loamy soils, not 

 only does the autumnal leaf-fall, as well as decaying twigs and 

 trunks, become obviously incorporated with the surface soil as 



1 A striking illustration of this is afforded by Naegeli's experiment of enclosing 

 several loaves of bread in a loosely closed tin-box. After eighteen months there 

 remained only seventeen per cent of air-dry mouldy matter, totally destitute of 

 starch. 



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