THE BIOLOGY OF THE SOIL 101 



varies witli the climate, the season, the physical state of 

 the soil, its humus-content, its chemical reactions, its 

 aeration, and the presence or absence of deleterious 

 micro-organisms or their products. Continued cropping 

 does not necessarily exhaust a soil, and when it does it is 

 probably due not so much to the depletion of the nutritive 

 salts as to its failing powers of nitrification. Nitrate- 

 bacteria can only flourish in a well-aerated and alkaline 

 soil. Hence the importance of digging deep and liming 

 well. Under conditions of bad culture, even with no lack 

 of manure, the soil becomes sour and foul. The aerobic 

 bacteria diminish, and in their place anaerobic bacteria 

 and other denitrifying organisms make their appearance 

 and impoverish the soil. In some cases it has been shown 

 that certain toxic substances, acids and aldehydes, are 

 excreted by the plants themselves; and it is alleged by 

 some authorities that with continued cropping these may 

 accumulate to such an extent as to thoroughly poison 

 the ground. This has not been proved, but even if it is 

 so good cultivation wiU remove them. Moreover, nitro- 

 gen is the most easily lost of all plant-foods. Owing to 

 the extreme solubility of most of its compounds, it is 

 easily washed down beyond the roots, or it may be dis- 

 sipated into the air as gas by the agency of anaerobic 

 bacteria. To retain the nitrogen in the soil it is again 

 necessary to use plenty of lime. Lime, and in its less 

 active form chalk, is, in fact, the most important of aU 

 manures. It not only favours nitrification by correcting 

 acidity, but it liberates plant-food from manures and 

 minerals, at the same time fixing it in a form which pre- 

 serves it in the soil and makes it available for plants. 



The amelioration of the soil for the production and con- 

 servation of nitrogen likewise favours the availability 

 of other chemical snfcetances needed by plants, particu- 

 larly phosphoric acid and potash. Labomr and lime, 

 together with large and regular supplies of organic matter 

 in the form of manures, will secure conditions of fertility 

 in practically all soils. 



