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SOURCES AND FUNCTIONS OF INGREDIENTS OF PLANTFOOD.

The Sources of the Nitrogen of Vegetation.

An address "On Some Points in Connection with Vegetation," delivered at South Kensington by Dr. J. H. Gilbert, has been reprinted in the American Journal of Science (vol. xiii., pp. 20-32, 99-111, 181-195). This treats of the subject of the nitrogen of vegetation in general, and of agricultural production in particular, especially as viewed in the light of the results of the well-known experiments at Rothamstead, England, in which Dr. Gilbert has, in connection with Mr. J. B. Lawes, been engaged for some thirty-three years or more. As a summarizing of those results, by themselves and in comparison with those of other experimenters, it forms a most valuable contribution to our still extremely incomplete knowledge of the ways of supply of nitrogen to crops, and will form a convenient basis for our résumé of this topic.

Sources of Nitrogen Removed from the Soil by Crops without Nitrogenous Manure.

Crops grown on soils to which no nitrogen is applied in manure remove considerable nitrogen. This nitrogen may be accounted for by (1) the combined nitrogen coming down. in rain, snow, etc., which does not exceed eight or ten pounds per acre yearly where observations have been made; (2) the condensation of ammonia of the air within the pores of the soil; and (3) previous accumulations within the soil.

Whether the excess of nitrogen taken by the plant over and above what is brought to the soil by atmospheric precipitation, condensation of ammonia by the soil, etc., comes entirely from the previous stores in the soil; or whether, and in how far, it is otherwise supplied-i. e., through assimilation of atmospheric nitrogen by plants and soils—is a much-vexed question, on which the summarizing of the results of Messrs. Lawes and Gilbert's experiments throws a good deal of light. Certain it is that as crops are removed year after year their amounts decrease, and that at the same time the store of nitrogen in the soil diminishes wherever accurate tests have been made. In the Rothamstead ex

periments it is noticeable that the "root crops "-turnips and sugar-beets-fell away more in their yield, and exhausted the nitrogen of at least the superficial layers of the soil, more than the gramineous crops wheat and barley, and, indeed, more than any other crop.

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Leguminous Crops and Soil Nitrogen.

Referring to the fact that leguminous crops contain much more nitrogen than gramineous crops, Dr. Gilbert says that we have no evidence leading to the conclusion that this increased assimilation [of nitrogen] is done at the expense of the nitrogen of the soil." On the other hand, after taking away so much more nitrogen than other crops, they leave a good deal more behind. In discussing the well-known fact that leguminous crops receive less benefit than others from nitrogenous manures, he cites the results of a number of comparative soil-analyses after clover and after barley. These "all concurred in showing an appreciably higher percentage of nitrogen, especially in the surface soil (nine inches deep) of the land from which the clover had been removed than that from which the barley had been taken; and this was so, although in every case all visible vegetable débris had been carefully picked out. Here, then, the surface soil, at any rate, was positively enriched in nitrogen by the removal of a very highly nitrogenous crop."

Assimilation of Nitrogen by Leguminous Crops Aided by Potassic Fertilizers.

In experiments on the mixed herbage of grass land continuing through twenty years, "complex mineral manure without potash increased the yield over unmanured plots 41 per cent.; the same with potash increased it 67 per cent. The extra increase with potash was due almost entirely to leguminous plants which the potash brought in. The average annual yield of nitrogen per acre without potash was 38.1 pounds, with potash 56 pounds. Dr. Gilbert concludes that "mineral manures, and especially potassic manures, increase in a striking degree the growth of crops of the leguminous family grown separately, and coincidently the amount of nitrogen they assimilate over a given area."

We may add that this observation, which is well substan

tiated by the Rothamstead experiments, is quite in accord ance with the fact that, as considerable experience has shown, the German potash salts are particularly useful for clover, beans, vetches, and other leguminous crops.

Is the Free Nitrogen of the Air a Source of the Nitrogen Assimilated by Plants?

The theory that plants can directly assimilate the free nitrogen of the air was long since disposed of by the experiments of Lawes and Gilbert, and of Boussingault. It has still been maintained that free nitrogen could be oxidized. by the aid of ozone, and the resulting compounds directly absorbed by plants. Six methods for this oxidation of nitrogen have been alleged: (1) by ozone assumed to be evolved by plants; (2) by contact of nitrogen with bodies undergoing oxidation; (3) during electrical discharges in the air; (4) the combining of ozone with nitrogen in the presence of water; (5) the same in presence of bases, as alkalies, lime, baryta, etc.; and (6) through evaporation and condensation of water in presence of air. Ville supposes that ozone is evolved by plants and combines with nitrogen in solution in their juices. It has been quite generally assumed that nitrogen outside the plant tissues could be oxidized by ozone exhaled by the plant and then absorbed by the leaves. From experiments at Rothamstead, as well as from those of Boussingault, Dr. Gilbert strongly doubts both the above assumptions, and adds that "whether such actions take place or not, it is at any rate certain that in our own experiments we have not been able to persuade plants to avail themselves of this happy faculty of producing their own nitrogenous food."

E. M. Dixon, in a "Report to the Philosophical Society of Glasgow on the Production of Nitric Acid from the Free Nitrogen of the Air," doubts whether the nitrogen is oxidized by ozone in presence of bodies undergoing oxidationmethod (2) above-and as good as denies nitrification by ozone in presence of water (4). Concerning the theory of the formation of nitrite of ammonia (6) claimed by Schönbein to be established by his experiments, and to explain the absorption of nitrogen by vegetation, he remarks, “It is, however, unfortunate. . . [for this theory] that no sooner

did experimentalists begin to purify the air that they used in repeating Schönbein's experiments than the production of nitrite of ammonia suddenly stopped."

Concerning the combination of ozone with free nitrogen in presence of bases, (5), in experiments by Berthelot, ozone prepared from oxygen by the electric discharge did not oxidize nitrogen in presence of chemically pure baryta water; but air ozonized by phosphorus yielded a trace of nitrate. If the phosphorus was free from nitrogen, the oxidation of free nitrogen by ozone in presence of bases would thus seem to be established (Comptes Rendus, lxxxiv., 61).

Is the Combined Nitrogen of the Air the Source of the Assimilated Nitrogen?

It so happens that plants which gather, or are supposed to gather, nitrogen most readily, as root crops (turnips and the like) and leguminous crops (beans, pease, clover, etc.), have obviously a different foliage from the gramineous crops (wheat, barley, oats, etc.), which are supposed to gather it less easily. It has been commonly taught that the "broadleaved crops," as the former are designated, have a power of taking nitrogen from the atmosphere in some manner which other crops possess in less degree, if at all. Dr. Gilbert opposes this view very decidedly: "It may be safely asserted that neither direct experimental evidence nor a consideration of the physics of the subject would lead to the conclusion that the plants which assimilate more nitrogen over a given area than others do so by virtue of a greater power of absorbing from the atmosphere by their leaves combined nitrogen in the form of ammonia." It is worthy of notice that A. Mayer, whose experiments are cited in support of the theory referred to, says himself that "this hypothesis rests more upon practical experience than upon exact experimental testimony," and accepts it to only a very limited degree.

The supposed means by which plants might prepare for themselves, or get already prepared, and absorb through their leaves compounds formed from the free nitrogen of the air, would thus seem probably limited to one.

Nitrification by Electrical Discharges.

The oxidation of free nitrogen by electrical discharges has long since been established. Whether enough of the nitrogen of the air is thus oxidized and then absorbed by plants to be of much consequence is very doubtful. But late researches by Berthelot have revealed a very noteworthy manner in which nitrogen may be absorbed by organic compounds, under the influence of the silent electrical discharge. A large number of compounds were thus found to absorb nitrogen, both that existing in the pure state and that in the air, producing compounds all of which evolved ammonia on being heated, either alone or with soda-lime. Berthelot suggests that similar processes may take place in nature, and that consequently plants may be able to assimilate nitrogen directly from the air. This last assumption is, however, opposed to the results of the best previous investigation upon the subject, which indicate that the free nitrogen of the air is not assimilated by plants (Comptes Rendus, lxxxii., 1283, and Ixxxiii., 677).

Is the Nitrogen Combined under the Influence of the Soil a Source of the Supply Assimilated by Plants?

Does not the soil gather free nitrogen from the air and induce it, in some way or other, to enter into combinations in which it can be used by the plant? This is a question which, though not yet decided, seems, in the light of recent experimenting, to approach an affirmative answer. In the résumé in the Record for 1876, in discussing the probability of the direct assimilation of atmospheric nitrogen by plants, the remark was made that "it would seem more probable that the absorption of free nitrogen by humus and by soils containing humus, which Simon and Truchot claim to have observed, might be induced in the manner described by Berthelot." The experiments of the past year bring no new evidence of importance against this view. On the other hand, it seems more and more probable that the ways in which crops get their supplies of nitrogen are to be sought in the varying capacities of their roots for feeding upon the nitrogen contained in various forms in the soil. Dr. Gilbert concludes the remarkable address to which we have been referring

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