278 NATURE OF THE ORGANIC CONSTITUENTS OF SOILS. 



1°. Humus. — The general name of humus is given to the fine, brown 

 light powder which imparts their richness to vegetable moulds and gar- 

 den soils. It is formed from the gradual decomposition of vegetable 

 matter, exists in all soils, forms the substance of peat, and consists of a 

 mixture of several different compounds which are naturally produced 

 during the decay of the different parts of plants. It is distinguished into 

 mildy sour, and coaly humus. 



The mild gives a brown colour to water, but does not render it sour, 

 gives a dark brown solution when boiled with carbonate of soda, evolves 

 ammonia when heated with caustic potash or soda or with slaked lime, 

 and leaves an ash when burned which contains lime and magnesia. 

 The sour gives, with water, a brown solution of a more or less sour 

 taste, [or reddens vegetable blues — see page 45.] This variety is 

 less favourable to vegetation than the former, and indicates a want of 

 lime in the soil. The coaly humus gives little colour to water or to a 

 hot solution of carbonate of soda, leaves an ash which contains little 

 lime, occurs generally on the surface of very sandy soils, and is very un- 

 fruitful. It is greatly ameliorated by the addition of lime or wood 

 ashes. 



2°. Humic acid. — When a fertile soil or a piece of dry peat is boiled 

 with a solution of the common carbonate of soda of the shops, a brown 

 solution, more^ or less dark, is obtained, from which, when diluted muri- 

 atic acid (spirits of salt) is added till the liquid has a distinctly sour 

 taste, brown flocks begin to fall. This brown flocky matter is humic acid. 



3°. Ulmic acid. — If, instead of a solution of carbonate of soda, one 

 of caustic ammonia, (the hartshorn of the shops,) be digested upon the soil 

 or peat by a gentle heat, a more or less dark brown solution is obtained, 

 which, on the addition of muriatic acid, gives brown flocks as before, 

 but which now consists of ulmic acid. 



These two acids combine with lime, magnesia, alumina, and oxide of 

 iron, forming compounds (salts) which are respectively distinguished by 

 the names of humates-ax\d ulmales. They probably both exist, ready 

 formed, in the soil in variable proportions, and in combination with one 

 or more of the earthy substances above mentioned — lime, alumina, &c. 

 They are produced by the decay of vegetable matter in the soil, which 

 decay is materially facilitated by the presence of one or other of these 

 substances, and by lime especially — on the principle that the formation 

 of acid compounds is in all such cases much promoted by the presence 

 of a substance with which that acid may combine. They 2^'''edispose 

 organic substances to the formation of such acids, and consequently to 

 the decomposition by which they are to be produced. These two acids 

 consist respectively of 



Humic acid, Ulmic acid. 



Carbon 63 67 



Hydrogen. ... . . 6 4? 



Oxygen 31 38^ 



100 100 



Some writers upon agriculture have supposed that these acids con- 

 tribute very materially to the support of growing plants. But Liebig 



