IMPORTANCE AND PROPERTIES OF THE SOIL 



171 



substances is favoured and accelerated l . Carbonic acid is formed in abundance by 

 oxidation and decomposition in a soil rich in humus, and hence the air permeating 

 it contains more carbon dioxide than does atmospheric air. In an ordinary soil 

 the very variable percentage of this gas will average from 0-2 to i-o per cent, at 

 a depth of i metre, and at 6 metres may rise to as much as 8 per cent., or in certain 

 soils even higher than this 2 . Along with this increased amount of carbon dioxide, 

 which is in itself injurious to ordinary aerobic plants, the amount of oxygen present 

 decreases, but not below 13-6 per cent, in Fleck's researches (see Sachsse, I.e.) on 

 the air present in a garden soil. Nevertheless, in dense soils or masses of mud 

 where the aeration may be very imperfect, and especially when micro-organisms are 

 present, the amount of oxygen may decrease so markedly that anaerobic bacteria 

 are able to develop. In such media, roots are unable to grow unless they contain 

 sufficiently large air canals to supply them with the necessary oxygen, and even then 

 the decomposition products produced in 

 sour soil frequently exert an injurious or 

 fatal effect upon roots growing in it. 



Solvent action of roots, rhizoids, &<;. 

 The solvent action of roots is admirably 

 indicated by the natural etchings frequently 

 found upon calcareous stones lying in the 

 soil ; such etchings may be readily obtained 

 by causing roots to grow upon polished white 

 or black slabs of marble 3 (Fig. 1 7). Sachs 

 recommends that a marble plate should be 

 placed in a pot filled with earth or saw- 

 dust, and that seedlings should be grown 

 in it, so that the roots come into contact with 

 the marble plate and spread out horizontally 

 over its surface. The same result can also 

 be readily obtained by placing the young 



radicle of Pisum, Phaseolus, &c., when 5 to 10 cm. long, upon the polished upper 

 surface of the slab, covering it with a few .layers of wet filter-paper, and placing 

 a glass plate over the whole to keep the root adpressed to the slab. The slab may 

 be placed in a glass dish under a bell-jar, with the end of the filter-paper dipping 

 into a nutrient solution which thus moistens the root (Sect. 73). The projecting 

 plumule grows upwards, and after two to six weeks the roots can be seen to have 

 formed a roughened etching, or even distinct furrows upon the plate. After a still 

 longer period, much deeper furrows may be produced, such as often occur in nature. 



In a similar manner Sachs produced etchings on plates of dolomite, magnesian, 



FIG. 17. A natural root etching found upon 

 a piece of Solenhofer slate. (Nat. size ) 



1 Cf. Sachsse, I.e., p. 181, and the literature there given. 



2 Sachsse, I.e., p. 142 ; Ebermayer, Forsch. a. d. Geb. d. Agriculturphysik, 1890, Bd. XIII, p. 15. 



3 First correctly interpreted by Liebig, Ann. d. Chem. u. Pharm., 1858, Bd. cv, p. 139. 

 De Candolle (Physiol., 1833, T. I, p. 186) had already called attention to the corrosions of rocks 

 due to lichens. After the death of the root, the pattern may apparently continue for a time to be 

 etched more deeply by the action of putrefactive products and of the organisms then present in 

 abundance. Also Sachs, Bot. Zeitung, 1860, p. 117. 



