62 THE PHYSIOLOGY OF PLANTS 



A passing decrease of the permeability is caused by the 

 frost, which causes the water contained in the interstices of 

 the soil to expand and thus decreases the pores. The water 

 itself, in such a case, looses its mobility. If at this time coarse 

 organic substance is introduced, so as to enlarge the interstices, 

 we at the same time introduce a source of carbonic acid, which 

 lasts as long as oxygen has access to keep up the decomposi- 

 tion. A purely mineral soil contains air of about the same 

 composition as that of the atmospheric air. But even if the 

 oxygen of the air is excluded, decomposition still takes place, 

 partly at the cost of the existing organic substance itself, or 

 with the aid of oxygen gained from the reduction of inorganic 

 substances. This process, however, is comparatively unpro- 

 ductive. 



Although the oxygen contained in the soil is of chief 

 importance for the aeration of the roots, the carbonic acid 

 present in the air and water of the soil is chiefly concerned 

 in the chemical changes, and therefore indirectly in the phy- 

 sical constitution. 



Distilled water, of course, can only take from the soil the 

 soluble substances, and give them off again when it evaporates. 

 The cai-bonic acid, however, contained in the water enables 

 it to dissolve substances which are insoluble in pure water 

 — as, for instance, the carbonates of lime and of magnesium, 

 ferrous carbonate, and manganous carbonate. The simple 

 carbonate of lime is transformed into bicarbonate ; the phos- 

 phates of lime and magnesium become soluble in presence of 

 carbonic acid, and the same is the case with phosphates and 

 silicates of iron, ferrous oxide, and the silicates of potassium, 

 sodium, lime, and magnesium. 



The carbonic acid of the air, in conjunction with the oxygen, 

 acts in the same way as the carbonic acid of the water. It 

 enables the oxygen to transform ferrous and manganous oxides 

 into the higher oxides, and the resulting ferric and manganic 

 oxides, as they take up more space than the lower oxides, help 

 to break up the mineral substances of the soil. The yellow or 

 brownish iron pyrites (FeS.,) is changed by the oxygen, in pre- 

 sence of water, into ferrous sulphate and free sulphuric acid. 

 When these products become dissolved in the water, they change 



