;i2 



NATURE 



[September 8, 1910 



sterile condition, and observations have been from time 

 to time recorded to tlie effect tliat soil wfilch 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 form- 

 ing any explanation ; a fact is not a fact until it fits into 

 a theory. Not only is sterilisation by heating thus 

 effective, but other antiseptics, 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 though 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 .'\lsatian 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 estab- 

 lished bevond question. Approximately the crop becomes 

 doubled if the soil has first been heated to a ternperature 

 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 explanation, 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 found that the treatment did not 

 effect complete sterilisation of the soil, though it caused 

 .It 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 

 employed 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 more than 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 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 nitrifi- 

 cation 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 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 multiplication of the bacteria therein. 

 Examination along these lines then showed that all soils 

 contain unsuspected groups of large organisms of the 



NO. 2132, VOL. 84] 



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. According 

 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 suit- 

 able treatment, whereupon a new level of ammonia pro- 

 duction, and therefore of fertility, is rapidly attained. 

 Curiously enough, one of the most striking of the larger 

 organisms is an amcvba akin to the white corpuscles of 

 the blood — the phagocytes, which, according to Metchni- 

 koff'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 deadlj', and the amoeba therefore 

 beneficial, whereas in the soil the bacteria are indis- 

 pensable, 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 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 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 explanation within the last year, but which in some 

 form or other has got to find its way back again 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 direc- 

 tions, we are still far from being able to disentangle all 

 the interacting forces the resultant of which is represented 

 by the crop. 



One other point, I trust, my sketch may have suggested 

 to you : when science, a child of barely a century's growth, 

 comes to deal with a fundamental art like agriculture, 

 which goes back to the dawn of the race, it should begin 

 humbly by accepting and trying to interpret the long chain 

 of tradition. It is unsafe for science to be dogmatic ; the 

 principles upon which it relies for its conclusions are often 

 no more than first approximations to the truth, and the 

 want of parallelism, which can be neglected in the labora- 

 tory, gives rise to wide divergencies when produced into 

 the regions of practice. The method of science is, after 

 all, only an extension of experience. What I have 

 endeavoured to show in my discourse is the continuous 

 thread which links the traditional practices of agriculture 

 with the most modern developments of science. 



