574 TRANSACTIONS OF SUB-SECTION B. 



but it was not until within the last thirty years that we obtained an idea as to how 

 the nitre came to be found. The oxidation of ammonia and other organic com- 

 pounds 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 Miintz, 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 

 investigators 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 tempera- 

 ture 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 nutrition, 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 regarded 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 agriculture 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 some of the clay soils of the Midlands 

 for the next half-century. Not only is the subsoil deficient in humus, which is 

 the 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 indispens- 

 able 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 atmosphere, until compara- 

 tively recently we knew of no natural process except the lightning flash which 

 would bring such nitrogen into combination. Plants take combined nitrogen from 

 the soil, and either give it back again or pass it on to animals. The process, how- 

 ever, 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 capable of fixing and bring- 

 ing into combination the free gas in the atmosphere. In this demonstration they 

 failed, though they brought to light a number of facts which were impossible to 

 explain and only became cleared up when, 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 flava 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 leguminous 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. At 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 



