BY R. GREIG-SillTH. 809 



■and the number soon rises above the former leveL Tliere is, in 

 ■consequence, a greater breaking down of the organic matter, and 

 more of the simpler manurial constituents are available for the 

 crop. Heat undoubtedly causes some chemical decomposition, 

 ifor the matter soluble in water is increased. 



The most suggestive explanation is, that the partial sterilisa- 

 tion removes a number of microbes which are of little use econo- 

 mically, and which compete for the food-supply with the more 

 useful kinds. With the former out of the way, the latter increase 

 in numbers, and their activity would account for the increase of 

 crop. But the removal of the inactive bacteria does not explain 

 the increase in the number of the bacteria over the normal. 

 Kussell, working in conjunction with Hutchinson, suggests that 

 the reason for the absence of increase in the bacterial content of 

 a normal soil, is that the protozoa and amceb^, which occur in 

 soils, are phagocytes and consume the bacteria. The partial 

 sterilisation disturbs the balance which formally existed between 

 •the phagocytes and their prey, and, in the absence of the protozoa, 

 the bacteria are enabled to increase. 



If the soil is considered as consisting of a solution of nutritive 

 matter and of soil-fragments, there is reason to expect the presence 

 of bacterial toxins. When bacteria are added to a nutritive 

 medium, they increase up to a maximum, and are then slowly 

 destroyed by the accumulation of their excreted products. To 

 all intents and purposes, these products are toxins. Many cells 

 survive, but the majority are destroyed, and this is just what we 

 find in soil. But in the soil there may be many toxins, as there 

 are many classes of bacteria. If some of the bacteria are 

 destroyed, as, for example, by heat, the remainder, on account of 

 the freedom from competition, will multiply up to a certain 

 limit, for the pre-existing toxin will keep the numbers down_ 

 Should, however, the toxin be also destroyed, the road is cleared 

 for the bacteria to equal, and then to exceed the original number 

 of those previously existing in the soil. 



Whitney and others had an hypothesis that the soil-toxins were 

 <ierived from plants, and were destroyed by fertilisers and by 



