298 THEORIES OE EERTILISER ACTION [chap. 



growth of the crop. The injurious action may even 

 arise from the growth of a different crop, as in the well- 

 known experiments at the Woburn Fruit Farm, where 

 Pickering has shown that the roots of grasses exert a 

 positively injurious effect, distinct from competition for 

 food, water, or air, upon fruit trees growing in the same 

 soil. 



Assuming that the persistence in the soil of obscure 

 diseases appropriate to the particular plant can be 

 neglected as the cause of these phenomena, there still 

 remains some unexplained factor arising from a plant's 

 growth which is injurious to a succeeding crop, and this 

 may either be the excreted toxins of Whitney's theory 

 or may be some secondary effects due to the competi- 

 tion or injurious products of the bacteria and other 

 micro-flora accumulated in the particular soil layer in 

 which the roots of the crop chiefly reside. Experi- 

 mental evidence is as yet wanting as to these highly 

 complex interactions between the higher plants and 

 the micro-flora of the soil, but Russell and other 

 observers have shown how greatly a disturbance of the 

 normal equilibrium of the flora of the soil may affect its 

 fertility, as measured by the yield of a higher plant. 

 Partial sterilisation, such as is brought about by heating 

 the soil to 98° for ten hours, will double the yield of the 

 succeeding crop and will show a perceptible beneficial 

 effect up to the fourth crop after the heating; and 

 exposure to the vapours of volatile antiseptics like 

 toluene or carbon bisulphide, which are afterwards 

 entirely removed by exposure, will increase the yield in 

 a similar but smaller degree; even drying the soil 

 appears to have an influence upon its fertility. 



It is in this direction perhaps that the clue may be 

 found to the unexplained benefits of the rotation of 

 crops, and to some of the other facts difficult of ex- 



