November 1, 1895.] 



KNOWLEDGE. 



261 



of the seed they grew from. He regarded this as the 

 minimum of progress ; but, in some of the experiments 

 recorded, it was evident that a later stage of development 

 had to be reached before the power of assimilating the free 

 nitrogen of the air was acquired." 



According to Ville, therefore, a certain amount of 

 nitrogenous food must be supplied to plants grown in an 

 artificial soil, in order to bring them to that stage of 

 growth at which they are able to take up nitrogen directly 

 from the air, the amount required depending upon various 

 circumstances. He showed that a certain minimum of 

 nitrogen (/.<., nitrogenous manure) was necessary, and also 

 that there was a certain maximum which must not be 

 overstepped. If this maximum was exceeded, then the 

 plant never drew on the atmospheric nitrogen at all, but 

 preferred to feed on the more easily assimilable nitrate of 

 soda or other nitrogenous matter in the soil. Dr. Aitken 

 adverts to his having " referred at length to Ville's experi- 

 ments, conducted upwards of forty years ago,andrepublished 

 by the author about thirty years ago, because, although 

 they were pretty generally discredited at the time, and 

 their results were at variance with those of other more 

 distinguished experimenters, they will be found, as I shall 

 show hereafter, to be a remarkable anticipation of the most 

 recent discoveries regarding the relation of plants to 

 atmospheric nitrogen." 



Notwithstanding the fact that Ville's experiments failed 

 to convince the majority of chemists, a suspicion remained 

 in the minds of some that the nitrogen of the air must in 

 some way or other become available for plants, either 

 directly or indirectly. How else was the "balance of 

 nitrogen " in the atmosphere maintained, seeing that there 

 was actual experimental proof that, by the decomposition 

 and combustion of both vegetable and animal matter, a 

 certain proportion of the nitrogenous matter that they 

 contained was constantly being broken up, with the 

 liberation of free nitrogen. And the same thing occurred 

 when nitrates were mixed with soils rich in humus. On 

 the other hand, it was perfectly well known that large 

 quantities of nitrates were lost from the soil in drainage 

 waters. Everything, therefore, pointed to some compen- 

 sating process or processes by which the free nitrogen of 

 the air was taken up by plants, so as to redress the waste 

 of nitrogenous compounds continuously going on through- 

 out the world. Various natural chemical processes have 

 been suggested at different times as affording the means of 

 this redress — for example, the formation of nitrite and 

 nitrate of ammonium in the air through the agency of the 

 electric discharge during thunderstorms. But it became 

 obvious, after due consideration, that these could only 

 compensate a small part of the loss of the combined 

 nitrogen, and that it must evidently be to some action of 

 plant life that we must turn for an explanation of the 

 riddle. This brings us to a very important paragraph in 

 Dr. Aitken's paper: — "It had been known for many 

 centuries that the growing of leguminous crops was a 

 means of enriching the land in such a manner that after 

 clover, vetches, or the like, an increased yield of wheat or 

 other cereal crop could be obtained ; and, since chemistry 

 has come to the aid of agriculture, it has been discovered 

 that the reason why legummous crops favour the growth of 

 succeeding cereals is that they leave the soil richer in 

 nitrogenous matter than it was before, and this despite 

 the fact that the leguminous crops themselves are 

 distinguished among all other crops by the large amount 

 of nitrogenous matter they contain." 



Mention is then made of the important experiment, 

 extending over many years, carried out by Herr Schultz 

 on his property of Lupitz, in Altmark, Germany, which 



did more than anything else to bring home the above truth 

 to agriculturists ; it should just be stated, in passing, that 

 this experiment was originally undertaken with a view 

 of improving poor light soil, and not for any scientific 

 purpose. Herr Schultz came into possession of his 

 property in the year 1855, at which time the land was so 

 poor that it could not grow oats, and, in order to obtain a 

 fair crop of rye, it was necessary to adopt a system of green 

 manuring with lupines. By following Liebig's teaching in 

 applying superphosphate, kainite and marl, all of which 

 are won-nitrogenous manures, he found that the lupines 

 responded wonderfully. And the above-mentioned system 

 of green manuring (i.e., of growing lupines or some other 

 leguminous crop one year, and either ploughing these in, 

 preparatory to taking a crop of rye, or cutting the lupines 

 for fodder) had for its result thnt the soil became stcailih/ 

 richer in nitroijenowi compounds year by year. It is now 

 forty years since this system was begun, and it is still 

 being continued ; and although large crops rich in nitro- 

 genous matter are taken ofl" the ground annually, yet the 

 soil is now three times richer in nitrogen than when Herr 

 Schultz began to work it. "These experiments of Schultz 

 tell us nothing about the source of the nitrogen except 

 that it came from the air, but whether it (this source) 

 was the ammonia of the air or ordinary atmospheric 

 nitrogen could only be a matter of conjecture. Consider- 

 ing, however, the smallness of the store of atmospheric 

 ammonia, and that leguminous plants under manurial 

 treatment show no liking for ammonia salts of any kind, 

 and are apt to be the worse rather than the better for 

 them, even when applied in quantity ten^times less than 

 the equivalent of that contained in the crop, it seemed in 

 the highest degree probable that the source of the great 

 gain of nitrogenous mitter must be sought for in the free 

 nitrogen of the air." 



"The remarkable effects produced by growing lupines 

 and other leguminous plants at Lupitz required to be 

 studied from some other point of view than the merely 

 chemical one " — -i.e., from the biological. 



The next step towards the solution of the question 

 was the discovery iu the year 1877 of the now well- 

 known process of " nitrification " in soils, by two 

 French chemists, MM, Schloesing and Miiutz. Of this 

 discovery — a discovery which, as Dr. Aitken remarks, has 

 worked nothing short of a revolution in our method of 

 viewing the relations of the soil to plant life — nothing more 

 need be said here than that it has been abundantly proved 

 by numerous experimenters that complex nitrogenous 

 matters existing in ordinary soils undergo oxidation by the 

 oxygen of the air, first into nitrites and then into nitrates, 

 by the agency of perfectly definite microbes ; and some of 

 these latter have been isolated independently, after long 

 and patient endeavours, by Messrs. Winogradsky, P. Frank- 

 land, and Warington. The nitrates thus produced by the 

 microbes are then directly available as plant food. 



Messrs. Hellriegel and Wilfarth then made a special 

 study of the small tubercles which had long ago been 

 observed in the roots of the lupine and plants of the same 

 order. A microscopic examination of the tubercles showed 

 that they contained bacteria, and a mass of bodies some- 

 what resembling bacteria and hence called bacteroids, 

 whilst careful experiments proved that these tubercles only 

 made their appearance on the roots of lupines, etc., when 

 the latter were grown iu ordinary soU, while plants grown in 

 sterilized soil were free from them. It thus followed that 

 the bacteria had their origin in the soil. The next point 

 to be determined experimentally was, what is the specific 

 effect of these tubercles, or rather of the bacteroids which 

 they contain, upon the growth of the plant ? The investi- 



