27 



result is not obviously explainable if we assume that the denitrifying 

 action of fresh horse dung in the soil is to be attributed to the organisms 

 which the manure supplies, for there is no reason to suppose that these 

 -organisms would die in a soil which naturally contains them ; but the re- 

 sult is at ono * understood if we grasp the fact that denitrification is de- 

 termined by the presence of fermentable organic matter. 



Several results which greatly puzzled the German investigators ap- 

 pear capable of explanation, if we regard them from the point of view 

 just taken. Thus, Wagner found that when dung was preserved by the 

 addition of superphosphate, or kainit, it possessed a greater denitrifying 

 power than when these substances were omitted. His temperature de- 

 terminations, in fact, show that these substances decreased the fermen- 

 tation in the manure heap. 



The much better results obtained by the German experiments from 

 sheep manure than from horse or cattle manure are apparently to be 

 explained by the fact that the sheep manure contain a much greater pro- 

 portion of easily nitritiable nitrogenous matter, and a distinctly smaller 

 proportion of organic matter per unit of nitrogen. The German inves- 

 tigators are fully alive to the importance of the first point, but give no 

 weight to the second. The far smaller denitrifying power of sheep 

 manure is, however, quite in accordance with its much smaller propor- 

 tion of fermentable organic matter per unit of nitrogen. Its compara- 

 tive freedom from denitrifying action certainly does not support tb.8 idea 

 ihat this action is due to the presence of a special organism derived from 

 the animal intestine, for in this respect sheep dung must stand on a par 

 with the horse or cow dung. 



Attempts to destroy the denitrifying organisms in dung and thus 

 alter its action on nitrates, have met with no success. Wagner sifted 

 a quantity of horse dung, and destroyed the living organisms in one por- 

 tion by treatment with bisulphide of carbon. After removal of the 

 bisulphide, the denitrifying power of the treated horse dung was 

 compared with that of the untreated. It was of course expected 

 that, the denitrifying organism having been destroyed, the treated 

 horse dung would be found comparatively harmless when applied 

 to the soil along with nitrate of sodium or other nitrogenous 

 manure. To the surprise of the experimenter, however, the treated 

 manure proved more injurious that the untreated. We could not have 

 an experiment showing more conclusively that the presence or absence 

 of organisms in the manure is a matter of indifference so long as the 

 necessary organisms are present in the soil, (i) 



(1) In Maercker's Second Report, which has come to hand after the above 

 was written, experiments are described in which wheat and oat straw were 

 soaked for two days in 1 per cent, sulphuric acid, with a view of destroying the 

 denitrifying organisms present. The straw thus treated was found, however, 

 when mixed with soil, to reduce the yield of nitrate of sodium to the same ex- 

 tent as the untreated, straw. Two other germicides were tried, with a similar 

 failure of result- Straw sterilised by steam had a worse effect on crops than 

 unsteamed straw j the prejudicial effect in this instance is considered by 

 Maercker as due to the formation of acid humus. All attempts to prove that 

 the depressing action of straw and horse dung is due to the organisms whioh 

 they contain have thus failed. 



