THE NUTRITION OF THE ROOT 63 



the carbonate of lime into gypsum, sodium chloride into the 

 sulphate, and the magnesium contained in the insoluble 

 iron and magnesium compound (dolomite) is transformed 

 into the very soluble magnesium sulphate (Epsom salts). In 

 like manner the insoluble tribasic phosphate of lime is con- 

 verted into the soluble phosphate, and clay is split up into 

 a sulphate and a silicate of aluminium and magnesium. 



By this breaking up of substances the number of soluble 

 substances necessary for the active functioning of the roots is 

 greatly increased. To cause them to function most actively, it 

 is necessary to maintain as even a supply of oxygen as possible, 

 and this can only be done by keeping the physical structure 

 of the soil as constant as possible. This is actually brought 

 about by the breaking up of the soil. The hardest mineral 

 cannot withstand a continued action of the carbonic acid. We 

 find large blocks of granite sticking out of the ground, the 

 surface of which seems eaten away, and when touched they 

 often break to pieces. The eroded surface is caused by the 

 action of the carbonic acid on one of the constituents of the 

 granite, the feldspar, from which it extracts in the first place 

 the potassium and the sodium. The feldspar is thus transformed 

 into clay, from which the more resistant pieces of quartz and 

 mica project. The water containing carbonic acid now sepa- 

 rates from the silicates the monoxides (ferrous oxide, lime, 

 magnesium, potassium, and sodium oxides), and by uniting 

 with them it liberates some hydrated alumina, which does 

 not readily combine with carbonic acid, and also a small quan- 

 tity of silica. This potassium containing feldspar (orthoclas, 

 3SiO^(K20 4- AlgOg)) thus furnishes a large amount of the 

 necessary potassium. The rain then washes away the above- 

 mentioned monoxides, and also the sesqui-oxides (ferric 

 oxide and alumina), until only a soft finely-divided mass 

 remains, which forms the almost indestructible framework 

 of the soil, consisting of fine granules of quartz sand and 

 silicates. 



To give some idea of the permeability of the soil for 

 air, which is constantly changing as the constitution of the 

 soil is being changed by the breaking down of its original 

 constituents, we append a table drawn up by Ammon. 



