THE SOIL ITS FORMATION AND COMPOSITION. [fl^ 



It was fonnerly erroneously supposed, that upon tlit* 

 presence of the decaying vegetable matter which gives the 

 dark colour to the soil of the meadow and garden, and of 

 which peat is chiefly composed, that the power of a soil to 

 grow plants depended. This substance you will find fre- 

 quently mentioned in agricultural works under the names of 

 Humus and " vegetable mould ;" and some chemists suppose 

 that by its decay certain acids, rich in carbon, are produced in 

 the soil, which in some forms of combination are capable 

 of bemg taken up by the growing plant, and employed for its 

 nom-ishment. The researches of Liebig, however, and other 

 eminent authorities, are entirely opposed to this opinion, and 

 have demonstrated that for a soil to be capable of support- 

 ing any plant it is necessary that it should contain the 

 inorganic matters that we discover in the ashes of that plant. 

 From what has already been stated you will readily un- 

 deretand the explanation which is given of the beneficial 

 efifects which experience has taught the farmer are derived 

 from decaying vegetable matters in the soil. By the gradual 

 union of the carbon of the dead vegetable with the oxygen 

 of the air, carbonic acid is produced and slowly evolved from 

 every particle of organic matter to which the air has access, 

 and thus a more abundant supply of that source of food to 

 plants is provided. The carbonic acid as it is formed is 

 taken up by the roots of plants, and thus the crop attains a 

 gi'eater development than when dependent upon the ordinary 

 supply of that gas afforded by the atmosphere. "If," says 

 Liebig, in his Letters on Chemistry, "we suppose all the con- 

 ditions for the absorption of carbonic acid present, a young 

 })lant will increase in mass, in a limited time only, in pro- 

 portion to its absorbing surface ; but if we create in the soil 

 :i new source of carbonic acid by decaying vegetable sub- 

 stances, and the roots absorb at the same time three times as 

 much carbonic acid from the soil as the leaves derive from 

 the atmosphere, the plant will increase in weight fourfold. 

 This fourfold increase extends to the leaves, buds, stalks, ttc. 

 and in the increased extent of surface the plant acquires an 

 increased power of absorbing nourishment from the air." 



1 35. What is termed the physical qualities of soils, that is, 

 their tenacity, porosity, power of absorbing and retaining 

 water, &c. materially influence both the labour required for 

 then* cultivation and then- productiveness. The atmosphere 



