September 8, 19 lo] 



NATURE 



ment is, however, a little more suited to controversy than 

 to real life ; it is too fiercely logical for the things them- 

 selves, and depends upon various assumptions holding 

 rigorously, whereas we have more reason to believe that 

 they are only imperfect approximations to the truth. 

 Still, this view does merit our careful attention, because 

 it insists that the chief factor in plant production must 

 be the supply of water to the plant, and that soils differ 

 from one another far more in their ability to maintain a 

 good supply of water than in the amount of plant food 

 they contain. Even in a climate like our own, which the 

 text-books describe as " humid " and we are apt to call 

 " wet," the magnitude of our crops is more often limited 

 by want of water than by any other single factor. The 

 same American investigators have more recently engrafted 

 en to their theory another supposition, that the fertility 

 of soil is often determined by e.xcretions from the plants 

 themselves, which thereby poison the land for a renewed 

 growth of the same crop, though the toxin may be harm- 

 less to a different plant which follows it in the rotation. 

 This theory had also been examined by Daubeny, and the 

 arguments he advanced against it in 1845 are valid to 

 this day. Schreiner has, indeed, isolated a number of 

 organic substances from soils — di-hydroxystearic acid and 

 picoline-carboxylic acid were the first examples — whicli he 

 claims to be the products of plant growth and toxic to 

 the further growth of the same plants. The evidence of 

 toxicity as determined by water-cultures requires, however, 

 the greatest care in interpretation, and it is very doubtful 

 how far it can be applied to soils with their great power 

 of precipitating or otherwise putting out of action soluble 

 substances with which they may be supplied. Moreover, 

 there are as yet no data to show whether these so-called 

 toxic substances are not normal products of bacterial 

 action upon organic residues in the soil, and as such just 

 as abundant in fertile soils rich in organic matter as in 

 the supposed sterile soils from which they were extracted. 



As, then, we have failed to base a theory of fertility 

 on the plant food that we can trace in the soil by analysis, 

 let us come back to Jfayow and Digby, and consider again 

 the nitre in the soil — how it is formed and liow renewed. 

 Their views of the value of nitrates to the plant were 

 justified when the systematic study of plant-nutrition 

 began, and demonstrated that plants can only obtain their 

 supply of the indispensable element nitrogen when it is 

 presented in the form of a nitrate ; but it was not until 

 within the last thirty years that we chained an idea as 

 to how the nitre came to be found. The oxidation of 

 ammonia and other organic compounds of nitrogen to the 

 state of nitrate was one of the first actions in the soil 

 which was proved to be brought about by bacteria, and 

 by the work of Schloesing and Muntz, Warington and 

 Winogradsky, we learnt that in all cultivated soils two 

 groups of bacteria exist which successively oxidise 

 ammonia to nitrites and nitrates, in which latter state the 

 nitrogen is available for the plant. These same investi- 

 gators showed that the rate at which nitrification takes 

 place is largely dependent upon operations under the control 

 of the farmer ; the more thorough the cultivation, the 

 better the drainage and aeration, and the higher the 

 temperature of the soil, the more rapidly will the nitrates 

 be produced. As it was then considered that the plant 

 could only assimilate nitrogen in the form of nitrates, 

 and as nitrogen is the prime element necessary to nutri- 

 tion, it was then an easy step to regard the fertility of the 

 soil as determined by the rate at which it would give rise 

 to nitrates. Thus the bacteria of nitrification became re- 

 garded as a factor, and a very large factor, in fertility. 

 This new view of the importance of the living organisms 

 contained in the soil further explained the value of the 

 surface soil, and demolished the fallacy which leads people 

 instinctively to regard the good soil as lying deep and 

 requiring to be brought to the surface by the labour of 

 the cultivator. This confusion between mining and agri- 

 culture probably originated in the quasi-moral idea that 

 the more work you do the better the result will be ; but 

 its application to practice with the aid of a steam plough 

 in the days before bacteria were thought of ruined many 

 of the clay soils of the Midlands for the next half-century. 

 Not onlv is the subsoil deficient in humus, which is the 



NO. 2132, VOL. 84] 



accumulated debris of previous applications of manure and 

 vegetation, but the humus is the home of the bacteria 

 which have so much to do with fertility. 



The discovery of nitrification was only the first step 

 in the elucidation of many actions in the soil depending 

 upon bacteria— for example, the fixation of nitrogen itself. 

 A supply of combined nitrogen in some form or other is 

 absolutely indispensable to plants and, in their turn, 

 to animals ; yet, though we live in contact with a vast 

 reservoir of free nitrogen gas in the shape of the atmo- 

 sphere, until comparatively recently we knew of no natural 

 process except the lightning flash which would bring such 

 nitrogen into combination. Plants take combined nftrogen 

 from the soil, and either give it back again or pass it on 

 to animals. The process, however, is only a cyclic one, 

 and neither plants nor animals are able to bring in fresh 

 material into the account. As the world must have started 

 with all its nitrogen in the form of gas, it was difficult to 

 see how the initial stock of combined nitrogen could have 

 arisen ; for that reason many of the earlier investigators 

 laboured to demonstrate that plants themselves were cap- 

 able of fixing and bringing into combination the free gas 

 in the atmosphere. In this demonstration thev failed, 

 though they brought to light a number of facts which were 

 impossible to explain, and only became cleared up v?hen, 

 in 1886, Hellreigel and Wilfarth showed that certain 

 bacteria, which exist upon the roots of leguminous plants, 

 like clover and beans, are capable of drawing nitrogen 

 from the atmosphere. Thus they not only feed the plant 

 on which they live, but they actually enrich the soil for 

 future crops by the nitrogen they leave behind in the roots 

 and stubble of the leguminous crop. Long before this 

 discovery experience had taught farmers the very special 

 value of these leguminous crops ; the Roman farmer was 

 well aware of their enriching action, which is enshrined 

 in the well-known words in the Georgics beginning, " Aut 

 ibi fiava seres," where Virgil says that the wheat grows 

 best where before the bean, the slender vetch, or the 

 bitter lupin had been most luxuriant. Since the discovery 

 of the nitrogen-fixing organisms associated with legu- 

 minous plants, other species have been found resident in 

 the soil which are capable of gathering combined nitrogen 

 without the assistance of any host plant, provided only 

 they are supplied with carbonaceous material as a source 

 of energy whereby to effect the combination of the 

 nitrogen. To one of these organisms we may with some 

 confidence attribute the accumulation of the vast stores of 

 combined nitrogen contained in the black virgin soils of 

 places like Manitoba and the Russian steppes. .^t 

 Rothamsted we have found that the plot on the permanent 

 wheat field which never receives any manure has been 

 losing nitrogen at a rate which almost exactly represents 

 the differences between the annual removal of the crop 

 and the receipts of combined nitrogen in the rain. We 

 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 tlie 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 has been possible by the azotohacter on this 

 plot, because there alone does the soil receive a supply of 

 carbohydrate, by the combustion in which the azotohacter 

 obtained the energy necessary to bring the nitrogen into 

 combination. On the unmanured 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 

 fertilitv. Ever since the existence of bacteria has been 

 recognised, attempts have been made to obtain soils in ri 



