DECOMPOSITION OF ROCKF MOUNTAINS. 31% 



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2°. In the presence of air the decomposition of the vegetable matter 

 of the soil proceeds more rapidly — it is more speedily resolved into those 

 simpler forms of matter, carbonic acid and water chiefly (page 152), 

 which are fitted to minister to the growth of new vegetable races. In 

 the absence of the air also, not only does this decomposition proceed 

 more slowly, but the substances immediately produced by it are fre- 

 quently unwholesome to the plant, and therefore fitted to injure, or ma- 

 terially to retard, its growth. 



3°. When the oxygen of the air is more or less excluded, the vege- 

 table matter of the soil takes this element from such of the earthy sub- 

 stances as it is capable of decomposing, and reduces them to a lower 

 state of oxidation. Thus it converts the red or per-oxide of iron into 

 the ^ro^-oxide (p. 211), and it acts in a similar manner upon the oxides 

 of manganese (p. 213). Jtalso takes their oxygen from the sulphates (as 

 from gypsum), and converts them into sulphurets. These lower oxides 

 of iron and manganese are injurious to vegetation, and it is one of the 

 beneficial purposes served by turning up the soil in ploughing, or by 

 otherwise loosening it so as to allow the free admission of atmospheric 

 air, that the natural production of these oxides is either in a great mea- 

 sure prevented, or that when produced they speedily become harmless 

 again by the absorption of an additional dose of oxygen. 



4°. Further, there are few soils which do not contain, in some quan- 

 tity, fragments of one or other of those compound mineral substances of 

 which, in a previous lecture, (xii., p. 257,) we have seen the crystalline 

 rocks to consist — of hornblende, of mica, of felspar, &c., in a decom- 

 posing state. From these minerals, as they decompose, the soil, and 

 therefore the plants that grow in it, derive new supplies of several of 

 those inorganic substances which are necessary to the healthy nourish- 

 ment of cultivated crops. The continued decomposition of these mine- 

 ral fragments is aided by the access of air, and near its surface, in an es- 

 pecial manner, by the carbonic acid which the air contains. A state of 

 porosity, therefore, or a frequent exposure to the air, is favourable to the 

 growth of the plant, by presenting to its roots a larger abundance not only 

 of organic but also of inorganic food. 



5^. Again, that production of ammonia and of nitric acid in the soil, 

 to which I drew your especial attention on a former occasion (pages 157 

 and 160), as apparently of so much consequence to vegetable life, takes 

 place more rapidly, and in larger quantity, the more frequently the land 

 is turned by the plough, broken by the clod-crusher, or stirred up by the 

 liarrow. Whatever amount of either of these compounds, also, the sur- 

 face soil is capable of extracting from the atmosphere, the entire quan- 

 tity thus absorbed will evidently be greater, and its distribution more 

 uniform, the more completely the whole soil has been exposed to its in- 

 fluence. It is for this, among other reasons, that, as every farmer knows, 

 the better he can plough and pulverise his land, the more abundant in 

 general are the crops he is likely to reap. 



6°. Nor lastly, though in great part a mechanical benefit, is it one of 

 little moment that when thus every where pervious to the air, the roots 

 also can penetrate the soil in every direction. None of the food around 

 them is shut up from the approach of their numerous fibres, nor are they 

 prevented, by the presence of noxious substances, from throwing out 



