CHEMISTEY OF, SOILS 111 



but the soluble compounds do not accumulate in soils : 

 they are lost in the drainage water if not absorbed by 

 plants. 



The organic matter of soils is oxidised and' driven off 

 on heating, and the loss on ignition is often put down as 

 organic matter. No just estimate of the amount can, how- 

 ever, be formed in this way because combined water and, 

 in some cases, carbon dioxide from carbonates may be 

 volatilised at the same time. For example, a soil which 

 lost 5 '8 per cent, on incineration, after drying at 100 C., 

 was found to contain less than 1 per cen,t. of true 

 organic matter. If the organic matter were of constant 

 composition the quantity of it present in any instance 

 could be easily deduced from the proportions of organic 

 carbon or hydrogen. Unfortunately this is not the case, 

 but calculations based on the assumption 1 that it contains 

 58 per cent, of the former and 5 per cent, of the latter 

 have given results in accordance with known facts, and, 

 in most cases, are probably not far from the truth. The 

 proportion of humus bears no constant relation to that 

 of the total organic matter and, as it has a much higher 

 agricultural value than fresh vegetable matter, it should 

 be estimated separately. This can be done by Grandeau's 

 method, or some modification of the same, i.e., by weighing 

 the dry residue left after evaporation of the ammoniacal 

 extract. 



Lime. The calcareous matter or " lime " of soils con- 

 sists of calcium carbonate. It should not be confused with 

 other calcium salts also found in soils. The latter have 

 entirely different properties and produce different effects. 



The proportion of lime in soils varies very widely. In 

 some it forms the predominant constituent, while others 



1 The composition of humus differs from that of the total organic 

 matter. About 45 per cent, of carbon and 4'5 per cent, of hydrogen 

 are probably nearer the average for the former substance. 



