42 THE MECHANICAL ANALYSIS OF SOILS [chap. 



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 increase in the finer fractions 

 if a soil be washed with dilute acid before it is separ- 

 ated 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 

 are found to contain both carbon and nitrogen, occasion- 

 ally in quantities comparable with those found in the 

 soil itself. This is only the case with the sedimentary 

 rocks and particularly the indurated clays, the carbon 

 and nitrogen in fact only represent the organic 

 matter in the original deposit in a more or less 

 mineralised condition. But since these carbon and 

 nitrogen compounds are only slightly affected by any of 

 the weathering processes by which soil is made, they 

 must pass into the soil and |here become merged with 

 the organic matter of more recent origin. Such material, 

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

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



