HUMUS. 53 



most generally found in the easily decomposable carbonate of 

 lime or magnesia. 



Carbonate of lime is also active in the changes which accom- 

 pany the application of many manures to the soil, notably so 

 in the case of sulphate of ammonia. 



Humus, the organic matter of the soil, is of great impor- 

 tance both from 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 im- 

 portance as affecting the absorption 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* has shown that humus, or rather calcium hu- 

 mate, is a colloidal body possessing greater cementing pow r er 

 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 retaining water. On the 

 other hand, it has been shown that in clay humus materially 

 lessened the plasticity and coherence. 



The chemical nature of humus is still very imperfectly 

 known. According to Mulder,! from 2-5 to 4-0% 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, nlmin, 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 J by treating peat with 

 a solution of potassium carbonate and precipitating with 

 hydrochloric acid. After repeated purifications an amorphous 

 substance corresponding in composition to the formula CaoH^O^ 

 was obtained; it still, however, contained -179% of nitrogen. 

 It is described as being soluble in 8300pts. of cold or 625pts. 



* Compt. Bend. 74, 1408 ; J.C.S. 1872, 839. t Annalen 36, 243. 



: Lamlw. Versuch. Station, XIV. 248; J.C.S. 1872, 521. 



