SO FOOD OF THE VEGETATING PLANT. CHAP. IT. 



What im- With regard to the food of plants derived from 

 pyins ' the atmosphere, the supply is pretty regular; at least, 

 in as far as the gases are coneerned ; for they are 

 not found to vary materially in their proportions on 

 any part of the surface of the globe : hut the 

 quantity of moisture contained in the atmosphere is 

 continually varying, so that in the same season you 

 have not always the same quantity, though in the 

 course of the year the deficiency is perhaps made 

 up. From the atmosphere, therefore, there is a re- 

 gular supply of vegetable food kept up by nature 

 for the support of vegetable life, independent of the 

 aid of man : and if human aid were even wanted, 

 it does not appear that it could be of much avail. 



But this is by no means the case with regard to 

 Composi- soils ; for if soils are less regular in their composi- 

 ti n > tne j are at ^ east niore within the reach of 

 human management. We have already seen the 

 materials of which soils are composed : but what 

 are the proportions of the materials in soils best 

 suited for culture ? According to the analysis of 

 Bergman, the soil best suited for culture contains 

 four parts of clay, three of sand, two of calcareous 

 earth, and one of magnesia : and, according to the 

 analysis of Fourcroy and Hassenbratz, 921 6 parts 

 of fertile soil contained 305 parts of carbon, together 

 with 2/9 parts of oil ; of which, according to the 

 calculations of Lavoisier, 220 parts may be regarded 

 as carbon : so that the whole of the carbon contained 

 in the soil in question may be estimated at about 



