576 TRANSACTIONS OF SUB-SECTION B. 



in the number of bacteria, because the two processes went on at about the same 

 rate. Some rearrangements were effected also in the nature of the bacterial flora; 

 for example, the group causing nitrification was eliminated, though no substantial 

 change was effected in the distribution of the other types. The bacteria which 

 remained were chiefly of the class which split up organic nitrogen compounds 

 into ammonia, and as the nitrate-making organisms which normally transform 

 ammonia in the soil as fast as it is produced had been killed off by the treatment, 

 it was possible for the ammonia to accumulate. The question now remaining 

 was, What had given this tremendous stimulus to the multiplication of the 

 ammonia-making bacteria? and by various steps, which need not here be enume- 

 rated, the two investigators reached the conclusion that the cause was not to be 

 sought in any stimulus supplied by the heating process, but that the normal soil 

 contained some negative factor which limited the multiplication of the bacteria 

 therein. Examination along these lines then showed that all soils contain unsus- 

 pected groups of large organisms of the protozoa class, which feed upon living 

 bacteria. These are killed off by heating or treatment by antiseptics, and on their 

 removal the bacteria, which partially escape the treatment and are now relieved 

 from attack, increase to the enormous degree that we have % specified Accord- 

 ing to this theory the fertility of a soil containing a given store of nitrogen 

 compounds is limited by the rate at which these nitrogen compounds can be 

 converted into ammonia, which, in its turn, depends upon the number of bacteria 

 present effecting the change, and these numbers are kept down by the larger 

 organisms preying upon the bacteria. The larger organisms can be removed by 

 suitable treatment, whereupon a new level of ammonia-production, and therefore 

 of fertility, is rapidly attained. Curiously enough one of the most striking of 

 the larger organisms is an amoeba akin to the white corpuscles of the blood — the 

 phagocytes, which, according to Metchnikoff's theory, preserve us from fever 

 and inflammation by devouring such intrusive bacteria as find entrance in the 

 blood. The two cases are, however, reversed : in the blood the bacteria are 

 deadly, and the amoeba therefore beneficial, whereas in the soil the bacteria are 

 indispensable and the amoeba become noxious beasts of prey. 



Since the publication of these views of the functions of protozoa in the soil 

 confirmatory evidence has been derived from various sources. For example, 

 men who grow cucumbers, tomatoes, and other plants under glass are accustomed 

 to make up extremely rich soils for the intensive culture they practise, but, 

 despite the enormous amount of manure they employ, they find it impossible to 

 use the same soil for more than two years. Then they are compelled to intro- 

 duce soil newly taken from a field and enriched with fresh manure. Several of 

 these growers here have observed that a good baking of this used soil restores its 

 value again; in fact, it becomes too rich and begins to supply the plant with an 

 excessive amount of nitrogen. It has also been pointed out that it was the custom 

 of certain of the Bombay tribes to burn vegetable rubbish mixed as far as possible 

 with the surface soil before sowing their crop, and the value of this practice in 

 European agriculture, though forgotten, is still on record in the books on Roman 

 agriculture. We can go back to the Georgics again, and there find an account of 

 a method of heating the soil before sowing, which has only received its explana- 

 tion within the last year, but which in some form or other has got to find its way 

 back a^ain into the routine of agriculture. Indeed, I am informed that one 

 of the early mysteries, many of which we know to be bound up with the practices 

 of agriculture, culminated in a process of firing the soil, preparatory to sowing 

 the crop. 



My time has run out, and I fear that the longer I go on the less you will feel 

 that I am presenting you with any solution of the problem with which we set 



out 'What is the cause of the fertility of the soil?' Evidently there is no 



simple solution; there is no single factor to which we can point as the cause; 

 instead we have indicated a number of factors any one of which may at a given 

 time become a limiting factor and determine the growth of the plant. All that 

 science can do as yet is to ascertain the existence of these factors one by one 

 and bring them successively under control; but, though we have been able to 

 increase production in various directions, we are still far from being able to 

 disentangle all the interacting forces whose resultant is represented by the crop. 



