GENESIS OF THE SOIL AND ITS POSSIBILITIES 201 



matter and consequently the rate of decay in similar 

 conditions at any observed periods is nearly the same. 

 In humus which is produced above the water-level 

 Kostyschoff states that all trace of the vegetable struc- 

 ture is destroyed by the leaves being gnawed and passed 

 through the bodies of caterpillars and wire-worms. 

 Under the water-level the vegetable structure is pre- 

 served and peat results. The decay of humus is most 

 rapid in drained and open soils. For this reason the 

 presence of clay in a soil promotes the accumulation of 

 humus. Inferior organisms are the means of diffus- 

 ing organic matter through the soil. The mycelia of 

 fungi grow on a dead root for instance, ramify later- 

 ally and thus carry organic matter outward and suc- 

 ceeding organisms extend this action and the soil be- 

 comes darkened in proportion. Humic acid in black 

 soil is almost exclusively in combination with lime. 



A more common view of the difference between the 

 formation of humus above and below the water-level is 

 that above the water-level there is a very free access of 

 air and even the harder parts of the leaf skeleton can 

 be oxidized through the agency of bacteria, while under 

 the water-level there is a very limited supply of air 

 and this oxidation cannot proceed as rapidly. The 

 harder parts of the leaf skeleton are preserved, and 

 from the freer access of air humus is oxidized more 

 readily in drained and open soils, and accumulates in 

 clay soils where there is less circulation of air. 



The quantity of humus in the soils of arid regions 

 is by no means so great as in those of abundant rain- 

 fall. 



Few of the upland arid soils of the Pacific Coast con- 

 tain over A per cent, of humus, that is the " matiere 

 noire " of Grandeau. This difference according to Hil- 



