ESSAY. 



13 



or the perfecting of the structure of plants. Humus, in 

 fact, is the pabulum from which vegetation is directly 

 fed. Every soil capable of growing crops contains it- 

 It may generally be known by the dark or black color 

 which it gives to the upper layer of the soil. If we dig 

 a hole in any cultivated field we shall find that this dark 

 colored layer extends as deeply as the land has been 

 lately plowed. That portion of soil immediately under- 

 neath this layer is commonly of the same character as 

 the upper layer itself, except that it is nearly destitute 

 of this vegetable matter. Humus is a compound and 

 very complex substance, made up of various acids and 

 salts, all of which must be rendered soluble before 

 being taken into the circulation of plants by absorption 

 through the roots. It is probable that most of the 

 alkaline substances used as manures, such as those 

 consisting largely of potash, lime and ammonia, act 

 mainly by rendering humus soluble, so that although 

 not in themselves nutriment in the same sense in which 

 humus is, yet by combining with insoluble humus 

 compounds, they render them soluble, and then the 

 new soluble compound becomes food. It is important 

 to bear this distinction in mind, that, although all 

 substances included under the general term manure 

 contribute to promote fertility in the soil to which they 

 are applied, yet humus acts directly and principally, 

 while most others mainly as adjuncts to that. For 

 instance, the addition of soluble humus to a blowing 

 sand, makes it at once capable of supporting vegetation, 

 while without humus, none of the so called artificial 

 manures would produce a like condition. Ashes would 

 not cause a barren sand, free from humus, to be fertile, 

 neither would lime, or in fact, any inorganic matter. 



