24 THE CHEMISTRY OF THE FA KM. 



dantly in the surface soil ; the depth to which its action 

 extends depends on the porosity of the subsoil. Nitrifi- 

 cation only takes place in a moist soil, sufficiently porous 

 to admit air. It is also necessary that some base should 

 be present with which the nitric acid may combine : this 

 condition is usually fulfilled by the presence of carbonate 

 of calcium, nitrate ot calcium being produced. Nitrifica- 

 tion is most active at summer temperatures; it ceases 

 apparently near the freezing-point. 



Oxidation is most active in soils under tillage. In 

 arable land the production of available plant food is at its 

 maximum, and so is also the waste by drainage. The 

 nitrogenous humic matter of arable land is maintained 

 only when the new supply from crop residues and organic 

 manures is equal to the amount annually oxidised. In an 

 untilled pastur.e or forest soil, on the other hand, a con- 

 siderable accumulation of organic matter may take place, 

 the annual residue of dead roots and leaves being generally 

 in excess of the means of oxidation. In a peat bog oxida- 

 tion is further checked by a high water level, which ex- 

 cludes air from the soil ; here an unlimited accumulation 

 of organic matter may take place if plants capable of grow- 

 ing under the circumstances are present. 



6. Movements of Salts in Soil. If water is allowed 

 to drain through a soil it carries with it a part of the 

 readily soluble matter which the soil contains. The sub- 

 stances chiefly removed by the water will be carbonate 

 of calcium, and the nitrates, chlorides, and sulphates of 

 calcium and sodium. When heavy rain falls these sub- 

 stances are washed into the subsoil, and partly escape by 

 the nearest outfall into the springs, brooks, nnd rivers. 



