l88 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



RECENT INVESTIGATIONS ON THE PRODUCTION OF 

 PLANT FOOD IN THE SOIL.— II. 



By E. J. Russell, D.Sc. 



[Being the Fourteenth Masters Lecture, read June 22, 1915 ; 

 Sir J. T. D. Llewelyn in the Chair.] 



The conclusion to which we were led in our last lecture was that the 

 decomposition of the residue of plant materials in the soil is of funda- 

 mental importance in soil fertility, determining on the one hand the 

 production of necessary plant nutrients and on the other the extent 

 of the accumulation of organic matter in the soil, which in turn 

 determines many of the soil properties. 



We have now to study this decomposition a little more closely, and 

 in particular to see how it is affected by changes in conditions such 

 as commonly occur in nature. 



We have seen that most of the changes can be brought about by 

 a number of organisms. Thus the fixation of nitrogen may be effected 

 by the aerobic Azotobacter or the anaerobic Clostridium. The 

 production of ammonia can be brought about by moulds, by large 

 bacilli, or by small micrococci, organisms differing considerably in their 

 requirements. Thus the continuance of the decomposition is less 

 dependent on the conditions than might a priori be expected, and if the 

 reaction cannot be brought about by one set of organisms it can 

 by another. Changes in conditions may alter the speed of the reaction 

 or they may alter the agents bringing it about, but they have less 

 effect on the nature of the change. 



For example : the bacterial flora in acid soils devoid of calcium 

 carbonate is very different from that in normal soils, but one cannot 

 point to any reaction that is wholly suppressed in consequence. It 

 was once thought that nitrification ceased, but later work shows 

 that this reaction, sensitive as it is, still goes on, although at a greatly 

 reduced speed. 



It has not yet been found possible to connect the change with the 

 agent — to say at any given moment which organisms are playing 

 the most important part at that time. 



The obvious method of studying the changes in the soil i s to 

 observe the growth of plants, but the phenomena involved are too 

 complex to be readily interpreted. 



For our present purpose we can follow the changes in the soil by 

 three methods : 



(1) Measuring the rate at which oxygen is absorbed or carbon 

 dioxide is given off by the soil. 



