viii] CARBON DIOXIDE IN SOIL GASES 153 



oxidising action of the decaying bacteria, oxygen is 

 always being replaced by carbon dioxide, until, cis the 

 various determinations of the composition of the air ex- 

 tracted from an alluvial pasture soil indicate, there may 

 be 10 per cent of carbon dioxide present, despite the 

 exchanges that are always going on at the surface where 

 the soil becomes open to the air. Now the carbon 

 dioxide will dissolve in the soil water in proportion to 

 its concentration in the air with which it is in contact, 

 hence a solution with a fair amount of power to attack 

 minerals is formed as soon as the rain-water has been 

 lying for some little time in the soil and has taken up 

 its proper proportion from the soil air with which it is 

 in contact In a few cases, as with peaty soil, it is 

 possible that the organic matter may yield certain acids 

 to the soil water which give it an extra solvent power, 

 and the bleaching of stones and sand in peaty soils is 

 sometimes taken as a proof that this happens ; but it is 

 doubtful whether carbon dioxide would not equally 

 effect the solution, aided perhaps by reducing actions 

 caused by the organic matter. In the same way it has 

 already been stated that the supposed solvent action of 

 roots may be put down to the carbon dioxide they 

 excrete, rather than to any fixed sap acids that exude 

 from the roots and then attack the soil particles with 

 which they are in contact. That roots have a powerful 

 solvent effect, may be learnt from various direct experi- 

 ments ; in one case it was shown that plants growing in 

 a mixture of sand and ground rock phosphate could get 

 a good supply of phosphoric acid, whereas similar plants 

 starved when growing in the sand alone, though they 

 were supplied with water that had previously filtered 

 through a second plot of sand and phosphate. In 

 another experiment plants were grown in powdered 

 granite, and when this was washed after growth was 



