III. THE SOIL 47 



where the amount of organic matter is excessive. The acidity is often 

 apparent in such soils by the bleaching effect they have upon the sand 

 and gravel upon which they rest. In many cases all the iron, etc., to 

 which the gravels and sands owe their colour, is found to be washed 

 out by the acid drainage from the peaty soil. 1 Then still more im- 

 portant is the part played by calcium carbonate in the process of nitri- 

 fication (vide Chap. IV). Some basic material is essential for the 

 continuance of this process, and the base is most generally found in 

 the easily decomposable carbonate of lime or magnesia. 



Calcium carbonate, owing to its ready solubility in water contain- 

 ing dissolved carbon dioxide and to its tendency to react with other 

 substances produced in, or added to soils, is liable to suffer many losses 

 or to undergo many movements and changes in soils. 



It is active in the changes which accompany the application of 

 many manures to the soil, notably so in the case of sulphate of am- 

 monia. 



Humus, the organic matter of the soil, is of great importance on 

 account of both its physical and chemical properties. 



As has already been stated, it is a light, bulky substance, having a 

 high specific heat, great capacity for holding water, and a dark colour. 

 This last property is of considerable importance as affecting the ab- 

 sorption of the sun's heat ; dark soils are found to become heated much 

 more readily by the sun than light-coloured ones, while their radiating 

 powers, by which they are cooled at night, are practically the same, 

 the radiation being of obscure heat, while the absorption was of the 

 intense radiant heat. 



Schloesing 2 has shown that humus, or rather calcium humate, is 

 a colloidal body possessing greater cementing power than clay in the 

 proportion of about 11 to 1. It is thus highly important in sandy 

 soils as a cementing material, as well as on account of its power of re- 

 taining water. On the other hand, it has been shown that in clay, 

 humus materially lessens the plasticity and coherence. 



The chemical nature of humus is still very imperfectly known. 

 According to Mulder, 3 from 2 '5 to 4'0 per cent of nitrogen is present. 

 Many experimenters have obtained from the dark brown substance 

 known as humus, several distinct bodies, amongst others humic acid, 

 humin, ulmic acid, ulmin, crenic acid and apocrenic acid ; but little 

 definite knowledge is possessed of the character and composition of these 

 acids. 



Humic acid was obtained by Detmer 4 by treating peat with a 

 solution of potassium carbonate and precipitating with hydrochloric 

 acid. After repeated purifications, an amorphous substance corres- 



1 Another possible explanation of this bleaching action is that the organic matter 

 draining from the peat reduces the ferric oxide to ferrous oxide, which is converted 

 first into carbonate and then into the soluble bicarbonate by the carbon dioxide also 

 abundant in the drainage water. 



2 Compt. Eend., 74, 1408; Jour. Chem. Soc., 1872, 839. 



3 Annalen, 36, 243. 



4 Landw. Versuehs, Stat., 14, 248 ; Jour. Chem. Soc., 1872, 521. 



