370 



SCIENCE 



[N. S. Vol. XXXII. No. 820 



tains normally about seven million bac- 

 teria per gram — a number whicli remains 

 comparatively constant under ordinary 

 conditions. Heating reduced the numbers 

 to 400 per gram, but four days later they 

 had risen to sis million, after which they 

 increased to over forty million per gram. 

 When the soil was treated with toluene a 

 similar variation in the number of bacteria 

 was observed. The accumulation of am- 

 monia in the treated soils was accounted 

 for by this increase in the number of bac- 

 teria, because the two processes went on at 

 about the same rate. Some rearrange- 

 ments 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 bac- 

 teria which remained were chiefly of the 

 class which split up organic nitrogen com- 

 pounds into ammonia, and as the nitrate- 

 making organisms which normally trans- 

 form ammonia in the soil as fast as it is 

 produced has been killed off by the treat- 

 ment, it was possible for the ammonia to 

 accumulate. The question now remaining 

 was, What had given this tremendous stim- 

 ulus to the multiplication of the ammonia- 

 making bacteria? and by various steps, 

 which need not here be enumerated, 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 multi- 

 plication of the bacteria therein. Exami- 

 nation along these lines then showed that 

 all soils contain unsuspected 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 bac- 

 teria, which partially escape the treatment 

 and are now relieved from attack, increase 



to the enormous degree that we have 

 specified. According to this theory the fer- 

 tility 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 bac- 

 teria 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 suit- 

 able treatment, whereupon a new level of 

 ammonia-production, and therefore of 

 fertility, is rapidly attained. Curiously 

 enough one of the most striking of the 

 lai'ger organisms is an amoeba akin to the 

 white corpuscles of the blood — the phago- 

 cytes, which, according to Metchnikoff's 

 theory, preserve us from fever and inflam- 

 mation by devouring such intrusive bac- 

 teria 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 con- 

 firmatory evidence has been derived from 

 various sources. For example, men who 

 grow cucumbers, tomatoes and other plants 

 under glass are accustomed to make up ex- 

 tremely 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 com- 

 pelled to introduce 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 



