August I, 1889] 



NATURE 



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the sterile soil with the young plants there was added in a large 

 number of cases a small quantity of an extract of a garden soil ; 

 the extract used contained less than one milligramme of nitrogen ; 

 the oats, rape, and buckwheat remained undeveloped, but the 

 leguminous plants soon became deep green an J grew vigorously. 

 If the soil extract were previously sterilized by heat, it produced 

 no effect. Moreover the soil used in the preparation of the 

 extract was of importance ; with peas any soil extract answered, 

 but not so with lupins and sainfoin ; wiih these plants, to render 

 success certain it was found nece.sary to use an extract of a soil 

 which had previously grown ti'je same plants. Some experi- 

 ments were also made in large sealed flasks, to which carbon 

 dioxide was admitted at intervals ; in these the results were 

 practically the same as in free air, showing that it was not the 

 combined nitrogtn of the air which was absorbed. 



It was also found that whilst on the leguminous plants which 

 developed well, either with or without the addition of soil 

 extract, the characteristic tubercules of papilionaceous plants 

 were well marked, on those which did not develop in the sterile 

 soil, and also on plants grown in sterilized soil to which nitrate 

 had been added, and which plants developed at the expense of 

 the added nitrogen, but did not assimilate free nitrogen, there 

 were no tubercules. Hence there is obviously some connection 

 between ihe production of the tubercules and the assimilation of 

 the free nitrogen. In all cases where free nitrogen is presumably 

 assimilated by the plant, the soil is also enriched in nitrogen, 

 the more so when the plaiit growth is more vigorous, and this 

 excess of nitrogen in the soil is almost entirely in organic 

 combination. 



The general conclusions are that leguminous plants, though 

 they are able to make use of combined nitrogen in similar forms 

 to those the graminaceous plants utilize, yet differ from this latter 

 order of plants in being able to use some other form of nitrogen, 

 not that existing in the soil. This second source of nitrogen 

 must be the free nitrogen of the air, which the Leguminosse 

 utilize through the agency of certain micro-organisms which are 

 in symbiosis with them, and exist in the tubercules of the roots 

 of this order of plants. 



The results obtained by von Wolfif at Hohenheim, from 188^ 

 onwards, are mentioned. Wolfif is not inclined to admit that 

 plants assimilate free nitrogen, but thinks that the only remaining 

 hypothesis is that certain plants can appropriate the combined 

 nitrogen of the air, either directly through their leaves or more 

 probably after absorption by the soil. A porous soil probably 

 absorbs far more nitrogenous compounds from the air than an 

 equal superficial area of dilute acid, as used in experiments by 

 Schlcesing, Kellner, and Miiller. He admits-, however, that it 

 is difficult to see why the grasses are unable to benefit by this 

 equally with the legumes. 



W. O. Atwater has published three papers on various aspects 

 of the subject in the American Chemical Journal. In these papers 

 he gives results of his own experiments and also discusses those 

 of others. He concludes that in many of his experiments with 

 peas, when the growth was normal, half or more of the total 

 nitrogen of the developed plants was obtained from the air. In 

 what way the nitrogen was accjuired, the experiments do not 

 show, but Atwater inclines to the idea that the plants themselves 

 directly acquired the atmospheric nitrogen. The conclusion of 

 this second part of the memoir gives some recent experiments and 

 opinions of Boussingault on the subject. He remained strongly 

 of the opinion that plants were unable to assimilate free nitrogen ; 

 although, as is here pointed out, some of his experiments in 1858 

 and 1859, with lupins, might be considered as leading to such 

 a conclusion. 



The third part of the memoir gives a summary and general 

 considerations and conclusions. 



Regarding the evidence relating to other sources than free 

 nitrogen, Lawes and Gilbert have shown that the amount of 

 nitric acid remaining in a soil is much less after the growth of a 

 crop than under corresponding conditions without a crop. Also 

 that nitrification in soils is more active where leguminous crops 

 are grown than where gramineous plants only are present ; 

 and that deep-rooted leguminous plants like Medicago saliva 

 or Melilotus leiicantka take up more nitric acid from the soil 

 than shallower-rooted leguminous plants like Trifoliiim repens. 

 But the supply of nitric acid in some soils, such as clover- 

 exhausted land or bean- exhausted land, is inadequate to account 

 for the nitrogen taken up by other leguminous crops grown on 

 such land. No very definite conclusions could be drawn from 

 the Rothamsted experiments as to the power of the acid sap of 



roots to take up nitrogenous organic matter from the soil, though 

 it is seen to be not improbable that green- leaved plants can 

 "take up directly, and utilize, amide bodies rendered soluble 

 within the soil by the action of their acid root sap." 



Our authors in conclusion point out that, since experimenting 

 in free air instead of in closed vessels, as in Boussingault's and 

 their own researches, has become common, there has been a 

 great accumulation of evidence tending to indicate the fixation 

 of free nitrogen. The modes of explanation of the gain of 

 nitrogen are : that it has been absc^rbed from the air, either 

 by the soil or by the plant ; that there is fixation of free nitrogen 

 within the soil by the agency of porous and alkaline bodies ; 

 that there is fixation in the soil by the agency of electricity ; 

 that there is fixation by the plant itself ; that there is fixation 

 under the influence of micro-organisms within the soil. The 

 balance of recorded evidence is undoubtedly in favour of the last- 

 mentioned mode of explanation. "Indeed, it seems to us," 

 say Lawes and Gilbert, "that, if there be not experimental 

 error, there is fixation of nitrogen within the soil, under the 

 influence of micro-organisms, or other low forms of life." But 

 they think that final judgment must be held in abeyance for 

 the present. Most of their own and Boussingault's previous 

 experiments excluded, by their conditions, the action of electricity 

 or of micro- organisms. 



They then consider some of the facts of agricultural pro- 

 duction in their bearing on the question as to how far the 

 establishment of the reality of the fixation of free nitrogen is 

 necessary to the solution of problems of agricultural production. 

 They point out that the loss of nitrogen in ordinary farm practice 

 is not so great as Berthelot and others have assumed ; the annual 

 loss of nitrogen by cropping in Great Britain, for example, is 

 probably under 20 pounds per acre. The loss by drainage may 

 in some cases be considerable, and in special cases there may be 

 loss by evolution of free nitrogen. Probably the loss of free 

 nitrogen from the plant itself during growth, which is assumed 

 by some, does not occur. The accumulation of combined nitrogen 

 which occurs in the surface soil of pastures is not conclusively 

 explained, but it may have a subsoil origin, and this assumption 

 has as much evidence in its favour as that it has an atmospheric 

 origin. In the soil and subsoil of Rothamsted, to a depth 

 reached by the deeper-rooting plants, there is 20,000 pounds of 

 combined nitrogen per acre ; in very many of the soils of this 

 country there is more, though in some less than this : the accu- 

 mulation of nitrogen in the surface soil may well be due to 

 nitrogenous crop-residue, the nitrogen of which comes principally 

 from the subsoil. Again the natural fertility of most soils is 

 without doubt due to the accumulation of ages of natural vegetation 

 with little cr no removal ; and the amount of nitrogen even now 

 brought into combination under the influence of electricity, over 

 a given are?, would be sufficient, with growth and little or no 

 removal, to account for the accumulations in natural prairie or 

 forest lands even of the richest. 



The Rothamsted experiments have shown that after growing 

 crops for many years without nitrogenous manures there has 

 always been a diminution of nitrogen in the top soil ; this has 

 been found to be the case with diverse crops, including gramineous, 

 cruciferous, chenopodiaceous, and also leguminous crops, and 

 with a four-course rotation of crops. There has not been com- 

 pensation of nitrogen from the air, or at all events to the extent 

 of the annual losses. ' ' The agricultural production of the present 

 age is, in fact, as far as its nitrogen is concerned, mainly dependent 

 on previous accumulations ; and as in the case of the use of coal 

 for fuel there is not coincident and corresponding restoration, so- 

 in that of the use or waste of the combined nitrogen of the soil^ 

 there is not evidence of the coincident and corresponding restora- 

 tion of nitrogen from the free to the combined state." 



It is not yet conclusively proved that the whole of the nitrogen 

 of leguminous plants comes from the subsoil ; it is equally not 

 proved that it comes from the air ; though in the case of crops 

 belonging to other natural orders it may be affirmed that atmo- 

 spheric nitrogen is not the source. May it be that the development 

 of organisms capable of bringing free nitrogen into combination 

 within the soil is favoured by leguminous growth and crop- 

 residue, as there can be little doubt is the case with the organisms 

 which produce nitrification ? 



Frank has shown that on the roots of certain trees, especially 

 the Cupuliferoe but also on willows and some Coniferse, is a 

 fungus-mantle which is believed to be in true symbiosis with the 

 higher plant ; and it may well be supposed that the fungus partly 

 assists the tree by bringing the organic nitrogen of the soil into 



