Makers of British Botany; a collection of biographies by living botanists

by Unknown

Another set of field experiments of exceptional interest is that relating to the "Mixed Herbage of Permanent Grass Land." The land was divided into twenty plots. Two plots have received no manure from the commencement of the experiment, two have received a dressing of farm-yard manure each year, whilst the remainder have each received a different kind of artificial or chemical manure, the same kind being applied year after year on the same plot, except in a few special cases. Repeated analyses have shown how greatly both the botanical constitution and the chemical composition of the mixed herbage varied according to the kind of manure applied.

The results of these experiments were given under three headings—agricultural, botanical and chemical, and show in an exceptional manner the care of detail to which every investigation was subjected by Gilbert. Some people have thought that this minute attention to detail was carried to excess by Gilbert, and resulted in a bewildering multiplication of numerical statements and figures. One can, however, but admire his love of accuracy and absolute conscientiousness, and if his caution appeared at times to be carried to an extreme, the result has been to make "the Rothamsted experiments a standard for reference, and an example wherever agricultural research is attempted."

One of the most important results of the Rothamsted investigations has been the replacing of the "mineral theory" of Liebig by the "nitrogen theory" of Lawes and Gilbert. Liebig held the view that each crop requires certain mineral elements from the soil, and that crops will not flourish where the appropriate elements are lacking. Every soil contains some element in the minimum. Whatever element this minimum may be it determines the abundance and continuity of the crop. The only fertiliser which acts favourably is that which supplies a deficiency of one or more of the food elements in the soil. The atmosphere, according to Liebig, supplies in sufficient quantity both the carbon and nitrogen required by crops, and the function of manure is to supply the ash constituents of the soil. The exhaustion of soils is to be ascribed to their decreased content of mineral ingredients rather than to decrease in nitrogen.

When careful study of the composition of the atmosphere proved that the amount of ammonia brought down to the earth by rain scarcely exceeds a few pounds per acre annually, Liebig maintained that plants are capable of directly absorbing ammonia by means of their leaves. He pointed out that the beneficial effects of nitrogenous manures are most apparent in the case of cereal crops with a comparatively short vegetation period, and least apparent in the case of leafy crops with a long vegetation period. The long vegetation period of crops like clover allowed time for the utilisation of the ammonia of the air and no artificial supply was necessary. On the other hand, crops with a short vegetation period had a limited time for accumulating ammonia from the air, and responded readily to applications of nitrogenous manures.

Gilbert, early in his work at Rothamsted, noticed that the results of his field experiments were at variance with this "mineral theory," as it was called, of Liebig, and soon found himself involved in a controversy with the great German chemist which was not always free from bitterness. He found that the nitrogen compounds of the atmosphere were sufficient only for a very meagre vegetation. Cereals treated with ammonium salts and other nitrogenous manures showed a far greater increase of produce than when phosphates, potash or other ash constituents only were supplied. "As more nitrogen was assimilated a greater amount of the fixed bases were found in the ash, and he considered that the function of the fixed bases was to act as carriers of nitric acid. These bases—potash, soda, lime and magnesia, were not mutually replaceable, but the predominance of one or the other affected the produce. Luxuriance of growth was associated with the amount of nitrogen available and assimilated, and in the presence of this sufficiency of nitrogen the formation of carbohydrates depended on the amount of potash available." The possibility that the free nitrogen of the air might supply the nitrogenous needs of plants was disproved by growing plants in calcined soil and removing all traces of ammonia from the air before it was admitted into the glass case in which the plants were growing. Determinations were made of the nitrogen in the seed and soil at the beginning of the experiments, and in the plants and soil at their conclusion.

The work on the assimilation of nitrogen by plants extended over three years and was made the subject of a communication to the Royal Society in 1861. The paper, entitled, "The Sources of the Nitrogen of Vegetation; with special reference to the question whether Plants assimilate free or combined Nitrogen," occupies 144 pages of the Philosophical Transactions, and is a brilliant example of the scrupulous accuracy and attention to detail which characterised all Gilbert's work. It is divided into two parts—I. "The General History and Statement of the question."—II. "The Experimental Results obtained at Rothamsted during the years 1857, 1858 and 1859." The authors state in the summary of conclusions that "in our experiments with graminaceous plants, grown both with and without a supply of combined nitrogen beyond that contained in the seed sown, in which there was great variation in the amount of combined nitrogen involved and a wide range in the conditions, character and amount of growth, we have in no case found any evidence of an assimilation of free or uncombined nitrogen.

"In our experiments with leguminous plants the growth was less satisfactory, and the range of conditions possibly favourable for the assimilation of free nitrogen was, therefore, more limited. But the results recorded with these plants, so far as they go, do not indicate any assimilation of free nitrogen. Since, however, in practice leguminous crops assimilate from some source so very much more nitrogen than graminaceous ones under ostensibly equal circumstances of supply of combined nitrogen, it is desirable that the evidence of further experiments with these plants under conditions of more healthy growth should be obtained."

As long as Gilbert's investigations were confined to non-leguminous plants and to leguminous plants grown in calcined soil the "nitrogen theory" was triumphant. When, however, leguminous plants were grown in uncalcined soil or in the open the results were uncertain, and in many cases the manures supplying ash constituents alone proved the most effective. The elucidation of these uncertain results has been a tedious problem, and has taken many years of patient investigation, but gradually the evidence accumulated which led to its solution.

Field and pot experiments in Germany, France, England and the United States in the late seventies and early eighties furnished abundant proof that under certain conditions leguminous plants do obtain nitrogen from the atmosphere, and gradually, from the work of Rautenberg, Frank and others, the idea was evolving that fungi or micro-organisms play some important part in the process.

Gilbert, however, would not listen to any such heresy, as he considered that the question of the assimilation of the free nitrogen of the air by plants had been finally settled by the experiments of 1857-60. It was therefore a most happy chance that Gilbert was present at the scientific congress in Berlin in 1886 when Hellriegel described his experiments on leguminous plants, showing that the formation of nodules on these plants was associated with the fixation of atmospheric nitrogen. In commenting subsequently on these experiments, Gilbert said, "It must be admitted that Hellriegel's results, taken together with those of Berthelot and others, do suggest the possibility that, although the higher plants may not possess the power of directly fixing the free nitrogen of the air, lower organisms, which abound within the soil, may have that power, and may thus bring free nitrogen into a state of combination within the soil in which it is available to the higher plants—at any rate to members of the Papilionaceous family. At the same time, it will be granted that further confirmation is essential before such a conclusion can be accepted as fully established."