THE BIOLOGY OF SOIL. 59 



oxidised simpler compounds, and that the 

 following chief changes result. The ammonia 

 and some other nitrogenous bodies remain behind 

 in the soil, as also do the phosphoric acid and 

 much of the potash ; whereas large quantities of 

 nitric and nitrous acids, together with much 

 sulphuric acid, chlorides, and calcium salts pass 

 avvay in the drainage. These facts are obviously 

 highly important in agriculture. 



Experiments on sewage farms have shown also 

 that the upper soil retains most of the bacteria of 

 the sewage, Koch found at Osmont, near Berlin, 

 that whereas the different sewage waters contained 

 numbers so enormous that each cubic centimeter 

 probably held 38,000,000 germs, the different 

 drainage waters held only 87,000 per c.cm. ; and 

 the whole process of water-filtration through sandy- 

 soils depends on these well-known facts. 



Recent experiments in connection with soil- 

 filtration, however, bring out the further facts that 

 the oxidations which organic matters undergo in 

 the soil and without which they are useless to 

 the higher plants are enormousl}' enfeebled if 

 the upper layers of soil are sterilised, so as to 

 deprive them of the myriads of aerobic bacteria, 

 fungi and yeasts which they normally contain, 

 and there can no longer be any doubt as to the 

 importance of the biology of the soil in connection 

 with the preparation of materials suitable for 

 absorption in solution by the root-hairs of agri- 

 cultural and other plants. 



The researches of the last ten years have 



