chairman's address. 575 



can further postulate only a very small fixation of nitrogen to balance the other 

 comparatively small losses in the drainage water or in the weeds that are removed ; 

 but on a neighbouring plot which has been left waste for the last quarter of -a 

 century, so that the annual vegetation of grass and other herbage falls back to the 

 soil, there has been an accumulation of nitrogen representing the annual fixation 

 of nearly a hundred pounds per acre. The fixation by the azotobacter has been 

 possible on this plot, because there alone does the soil receive a supply of carbo- 

 hydrate, by the combustion in which the azotobacter obtained the energy necessary 

 to bring the nitrogen into combination. On the unmanurod plot the crop is so 

 largely removed that the little root and stubble remaining does not provide 

 material for much fixation. 



Though numerous attempts have been made to correlate the fertility of the 

 soil with the numbers of this or that bacterium existing therein, no general 

 success has been attained, because probably we measure a factor which is only 

 on occasion the determining factor in the production of the crop. Meantime our 

 sense of the complexity of the actions going on in the soil has been sharpened by 

 the discovery of another factor, affecting in the first place the bacterial flora in 

 the soil and, as a consequence, its fertility. Ever since the existence of bacteria 

 has been recognised attempts have been made to obtain soils in a sterile condition, 

 and observations have been from time to time recorded to the effect that soil 

 which has been heated to the temperature of boiling water, in order to destroy any 

 bacteria it may contain, had thereby gained greatly in fertility, as though some 

 large addition of fertiliser had been made to it. Though these observations have 

 been repeated in various times and places they were generally ignored, because 

 of the difficulty of forming any explanation : a fact is not a fact until it fits into 

 a theory. Not only is sterilisation by heating thus effective, but other anti- 

 septics, like chloroform and carbon bisulphide vapour, give rise to a similar result. 

 For example, you will remember how the vineyards of Europe were devastated 

 some thirty years ago by the attacks of phylloxera, and (hough in a general way 

 the disease has been conquered by the introduction of a hardy American vine 

 stock which resists the attack of the insect, in many of the finest vineyards the 

 owners have feared to risk any possible change in the quality of the grape through 

 the introduction of the new stock, and have resorted instead to a system of 

 killing the parasite by injecting carbon bisulphide into the soil. An Alsatian vine- 

 grower who had treated his vineyards by this method observed that an increase 

 of crop followed the treatment even in cases where no attack of phylloxera was 

 in question. Other observations of a similar character were also reported, and 

 within the last five years the subject has received some considerable attention until 

 the facts became established beyond question. Approximately the crop becomes 

 doubled if the soil has first been heated to a temperature of 70° to 100° for 

 two hours, while treatment for forty-eight hours with the vapour of toluene, 

 chloroform, &c, followed by a complete volatilisation of the antiseptic, brings 

 about an increase of 30 per cent, or so. Moreover, when the material so grown 

 is analysed, the plants are found to have taken very much larger quantities of 

 nitrogen and other plant foods from the treated soil ; hence the increase of 

 growth must be due to larger nutriment and not to mere stimulus. The explana- 

 tion, however, remained in doubt until it has been recently cleared up by 

 Drs. Russell and *Hutchinson, working in the Rothamsted laboratory. In the 

 first place, they found that the soil which had been put through the treatment 

 was chemically characterised by an exceptional accumulation of ammonia, to 

 an extent that would account for the increased fertility. At the same time 

 it was also found, that the treatment did not affect complete sterilisation of the 

 soil, though it caused at the outset a great reduction in the numbers of bacteria 

 present. This reduction was only temporary, for as soon as the soil was watered 

 and left to itself the bacteria increased to a degree that is never attained 

 under normal conditions. For example, one of the Rothamsted soils em- 

 ployed contains normally about seven million bacteria per gram — a number which 

 remains comparatively constant under ordinary conditions. Heating reduced the 

 numbers to 400 per gram, but four days later they had risen to six 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 ammonia in the treated soils was accounted for by this increase 



p p 2 



