190 SOIL CONDITIONS AND PLANT GROWTH 



for these remarkable facts. The whole subject deserves serious 

 attention from some competent chemist. 



It was somewhat hastily inferred that organic matter 

 would have a retarding effect in the soil just as it has in culture 

 solutions. From the outset, however, certain facts were 

 known to be against this view : thus, there was a good deal 

 of organic matter in the old nitre beds (235) and also in 

 rich gardens, and yet nitrification went on vigorously in both 

 cases. An exception was therefore made in favour of " humus " 

 (208). Later on Adeney (2), and again Miss Chick (66), found 

 another exception : the organic matter of the filter beds used 

 in sewage purification. Richards finds that nitrification pro- 

 ceeds very vigorously during the activation of sludge by 

 aeration. Coleman has now shown (68), and Stevens and 

 Withers (271^) have confirmed it, that only in culture solutions 

 is organic matter injurious : in the soil it does no harm, and 

 may even help the process. Thus quantities of dextrose that 

 stopped nitrification entirely in Winogradsky and Omelianski's 

 culture solutions were found to act beneficially in soil under 

 normal conditions of temperature and moisture content. The 

 discrepancy cannot y^t he explained. Sucrose, lactose ; and 

 certain other non-nitrogenous compounds had no effect, but 

 nitrogenous compounds were distinctly injurious. 



The organisms will not tolerate an acid medium ; a suf- 

 ficient excess of calcium carbonate is therefore necessary both 

 in culture solutions and in soils. Nor will they tolerate free am- 

 monia. In culture solutions the nitrate producer is somewhat 

 sensitive even to ammonium salts, indeed both Warington 

 (296) and Omelianski (219$) suppressed it by maintaining a 

 sufficient concentration of ammonium sulphate ; Lohnis has 

 shown, however (i8i<z), that it is more tolerant in the soil. 

 Some substance toxic to them is produced when soil is heated 

 to 98 C. or more, and in such soils they cease to act. Neither 

 nitrosomonas nor nitrobacter has been observed to form spores, 

 or to survive temperatures above 45 C., or treatment with 

 mild antiseptics like carbon disulphide and toluene. But 

 so widely distributed are they and so readily can they spread 



