N.] THE NATURE OF HUMUS 47 



matter in the surface soil. Many of the fermentation 

 changes that also take place in this vegetable matter 

 give rise to acids, which in their turn combine with 

 the calcium carbonate. So rapid are these removals of 

 calcium carbonate that it is difficult to understand how 

 any of it persists in the surface layers of many soils, the 

 subsoil of which shows that they must have been initially 

 poor in chalk, were there not some compensating 

 agencies at work. Amongst these agencies must be 

 reckoned the calcium salts in plants, which in many 

 cases are drawn up by deep-seated roots from 

 the subsoil and become calcium carbonate on the 

 ultimate decay of the plant tissues. 



In a normal soil the particles of calcium carbonate 

 are of all sizes, many of the finer particles of silt and 

 clay are loosely cemented together by calcium carbonate, 

 as may be seen by the increased proportion of the finer 

 fractions that is found if a soil be washed with dilute 

 acid before it is separated by sedimentation. 



Humus. On examining many rocks taken from 

 such depths that they have undergone none of the 

 weathering processes which convert them into soil, they 

 ire found to contain both carbon and nitrogen, occasion- 

 illy in quantities comparable with those found in the 

 soil itself. This is only the case with the sedimentary 

 ocks and particularly with indurated clays ; the carbon 

 md nitrogen in fact only represent the organic 

 natter in the original deposit in a more or less 

 nineralised condition. But since these carbon and 

 litrogen compounds are only slightly affected by any of 

 he weathering processes by which soil is made, they 

 nust pass into the soil and there become merged with 

 he organic matter of more recent origin. Such material, 

 lowever, plays a very unimportant part in the soil, and 

 ve may pass on at once to the debris of vegetation of 



