224 iiOAv cKO:-s feed. ' 



products of fermentation, a process that goes on in the 

 juices of plants that have become a part of the soil or of 

 a compost. 



These acids can scarcely exist in the soil, except tempo- 

 rarily, as results of fermentation or decay, and tlien in but 

 very minute quantity. They consist of carbon, hydrogen, 

 and oxygen. Their salts are all freely soluble in water. 

 Their relations to agricultural plants have not been studied. 



HlinniS (in jiart). — The gen«?ral nature and origin of 

 humus has been already considered. It is the debris of 

 vegetation (or of animal matters) in certain stages of de- 

 composition. Humus is considerably complex in its 

 chemical character, and our knowledge of it is confessedly 

 incomplete. In the paragraphs that immediately follow, 

 we shall give from the best sources an account of its non- 

 nitrogenous ingredients, so far they are understood, reserv- 

 ing to a later chapter an account of its nitrogenized con- 

 stituents. 



The Non-nitrofifenoiis (ompoiients of Hiiiuiis. — The 



appearance and composition of humus is diiFerent, accord- 

 ing to the circumstances of its forrnati^m. It has already 

 been mentioned that humus is brown or black in color. 

 It appears that the first stage of decom[iositiou yields the 

 brown humus. It is seen in the dead leaves hanging to a 

 tree in autumn, in the upper layers of fallen leaves, in the 

 outer bark of trees, in the smut of Avheat, and in the u}>- 

 per, dryer portions of peat. 



When brown liumus remains wet and with imperfect 

 access of air, it decomposes further, and in time is convert- 

 ed into black humus. Black humus is invariably foimd 

 in the soil beyond a little depth especially if it be com- 

 pact, in the deeper layers of peat, in the interior of com- 

 post heaps, in the lower portions of the leaf-mould of 

 forests, and in the mud or muck of swamps and ponds. 



lllmic Acid aild lliuiiu, — The browu humus contains 



