Soil Conditions and Humus Decomposition 151 



sandy soils the interchange of gases between soil and 

 overlying air is rapid, the supply of oxygen is renewed 

 without great difficulty, and the processes of decay 

 go on uninterrupted. 



In fine-grained, compact soils, the conditions are 

 different. These soils do not part as readily with the 

 rain that falls upon them; their power of lifting water 

 from the subsoil is greater, and a larger portion of their 

 air-space is therefore occupied by water. Furthermore, 

 because of their compactness, the air does not enter 

 them or circulate in them as freely. Hence, the decay 

 processes in such soils are not so intense. In extreme 

 cases, the humus accumulates more rapidly than it 

 is destroyed. This is true of water-logged soils, of swamps 

 and meadows, and also of upland moor and heath soils. 



In the presence of excessive moisture the aerobic 

 bacteria are almost entirely suppressed, and the anaero-' 

 bic species become prominent. The decomposition pro- 

 cess then partakes of the nature of putrefaction, which 

 does not involve as far-reaching destruction of the 

 organic matter. The air being argely or entirely ex- 

 cluded, the carbon of the humus is changed only partly 

 to carbon dioxid, and that at the expense of the oxygen 

 derived from decomposing vegetable matter. Another 

 portion of the carbon passes off as marsh-gas, while a 

 third portion remains in the soil in the form of sour 

 compounds so-called humic acids. The humic acids 

 and other substances formed with them are more or 

 less antiseptic in character. Peat, for instance, is a 

 material that is resistant to decay even after it is re- 

 moved from the swamp. The sour humus of upland 



