The Micro-Organisms of the Soil

CHAPTER I.. The Development of the Idea of a Soil Population.

THE DEVELOPMENT OF THE IDEA OF A SOIL POPULATION.

From the earliest times agriculturists have been familiar with the idea that decomposition of vegetable and animal matter takes place in the soil, and that the process is intimately connected with soil fertility.

By the middle of the nineteenth century three different ways were known in which the decomposition occurred. One had been since early times specially associated with soil fertility, in that it gave rise to humus, the black sticky substance in farmyard manure or in soil--which was supposed up to 1840 to be the special food of plants. No good account of the process or of the conditions in which it occurred is, however, given by the older writers.

A second resulted in the formation of nitrates. This process became known as nitrification: it was described by Georgius Agricola (1494-1555) in his book “De Re Metallica,” and it was of great importance in the seventeenth and eighteenth centuries, because it was used for the manufacture of gunpowder in the great wars of that period. The conditions for the making of successful nitre beds were so thoroughly investigated that little fresh knowledge was added to that of 1770 until quite recently. This process, however, was not usually associated with soil fertility, although both Glauber (1656) and Mayow (1674) had insisted on the connection.

See the remarkable collection of papers entitled “Instructions sur l’établissement des nitrières,” publié par les Régisseurs-généraux des Poudres et Salpêtre. Paris, 1777.

A third type of decomposition was brought into prominence by Liebig in 1840. Reviewing the decomposition of organic matter in the light of the newer chemistry, he concluded that the process was a slow chemical oxidation, to which he gave the name “Eremacausis.” He recognised that humus was formed, but he regarded it only as an intermediate product, and emphatically denied its importance in soil fertility. The true fertility agents, in his view, were the final products--CO₂, potassium and other alkaline salts, phosphates, silicates, etc. He went on to argue brilliantly that instead of applying farmyard or similar manures to the soil it was altogether quicker and better to apply these mineral compounds obtained from other sources than to wait for the slow process of liberation as the result of decomposition. For some reason, difficult to understand, he overlooked nitrification and the part that nitrates might play in soil fertility. Lawes and Gilbert were much attracted by this new idea; they showed that it was incomplete because it took no account of the necessity for supplying nitrogen compounds to the crop. When ammonium salts were added to Liebig’s ash constituents the resulting mixture had almost as good a fertilising effect as farmyard manure. Lawes at once saw the enormous practical importance of this discovery, and set up a factory for the manufacture of artificial fertilisers. He did not, however, follow it up more closely on the scientific side.

The numbers refer to the short bibliography on p. 18.

Both Lawes and Gilbert were in constant touch with the idea of decomposition in the soil, and they attached so much importance to nitrogen compounds in plant nutrition that it is not easy to understand how they missed the connection with nitrification. But they did so, and like other English and German workers of the day, considered that plant roots assimilated their nitrogen as ammonia. For the first ten years of the history of Rothamsted only few experiments with nitrates were made, and not till thirty-five years had elapsed were they systematically studied.

It was by Boussingault and in France that the connection between nitrification and soil fertility was first recognised. The news came to England, but it was not accepted, although Way, one of the most brilliant agricultural chemists of his time, showed that nitrates were formed in soils to which nitrogenous fertilisers were added, and that they were comparable in their fertiliser effects with ammonium salts. “The French chemists,” he wrote in 1856, “are going further, several of them now advocating the view that it is in the form of nitric acid that plants make use of compounds of nitrogen. With this view I do not at present coincide, and it is sufficient here to admit that nitric acid in the form of nitrates has at least a very high value as manure.” Indeed, Kuhlmann went so far as to argue that the nitrates found in the soil were there reduced to ammonia before assimilation by plants could take place. The water-culture work of the plant physiologists of the ’sixties finally showed the correctness of the French view.

Even when the importance of nitrification was realised its mechanism was not understood: some thought it was chemical, some physical. Again the explanation came from France. Pasteur in 1862 had expressed the view that nitrification would probably be a biological action, since purely chemical oxidation of organic matter was of very limited occurrence. “Pénétrés de ces idées,” as Schloesing tells us, he and Müntz in a memorable investigation cleared up the whole problem, and in 1877 opened the way to a most fruitful field of research. The formal description is given in his papers in the “Comptes Rendus,” but a more lively account is given in his lectures before the École d’application des Manufacteurs de l’état, which, though not printed, were collected and issued in script by his distinguished son, and a copy of this work is among the treasures of the Rothamsted Library.

He had been asked to study the purification of sewage, and he and Müntz showed that it was bound up with nitrification. The process was slow in starting, then it proceeded rapidly. Why, they asked, was the delay? There should be none if the process were physical or chemical, and the fact that it occurred strongly suggested biological action. The process was stopped by chloroform vapour, but could be restarted after the removal of the vapour by the addition of a little fresh soil.

The importance of this work in connection with soil fertility was immediately realised by Warington, who had recently come to Rothamsted. He quickly confirmed the result, and made the valuable discovery that two stages were involved--the conversion of ammonia to a nitrite by one organism, and of the nitrite to nitrate by another. He made long and persistent attempts to isolate the organisms from the soil, using the best technique of his time, but though he found many bacteria none of them could nitrify ammonium salts; yet the soil did it easily. For years he continued his efforts to find the nitrifying organism, but always failed. His health was not good, his life at Rothamsted was not happy owing to disagreements with Gilbert, and although his other research work was succeeding, this investigation on which he had set his heart was not coming out; bacterial technique was not yet sufficiently far advanced. Ten bitter, disappointing years passed, and the crown of disappointment came when Winogradsky, a young bacteriologist in Paris, changed the technique and succeeded at once in isolating both the nitrite and the nitrate-forming organisms.

The numerous bacteria found by Warington in the soil suggested the presence of a soil population, and this idea was greatly strengthened by another line of investigation which was being followed up in France. Boussingault had shown that soils absorb oxygen and give out carbon dioxide; Schloesing extended this discovery, as also did Wollny. It was concluded that oxidation was the result of the activities of the soil organisms in decomposing the organic matter of the soil, and thus preparing the way for the nitrifying organisms.

A third important function of soil bacteria was revealed by Berthelot. It was known that considerable loss of nitrogen from the soil took place as the result of the conversion of nitrogen compounds into nitrates, which were subsequently washed out in the drainage water. It followed inevitably that the stock of nitrogen compounds in the soil must long ago have become exhausted had there been no addition of nitrogen compounds to the soil. Berthelot argued that there must be fixation of atmospheric nitrogen, and, following the prevailing trend of thought in France, he attributed it to bacteria. He confirmed the anticipation by exposing soil to air in such conditions that dust, rain, etc., were excluded, and he found an increase in the percentage of nitrogen.

Looking back over the work, it is difficult to understand the result. The fixation of nitrogen is a process that absorbs energy, and should have necessitated some source of energy, which apparently was not supplied. But in spite of this drawback the investigation was immediately fruitful in that it gave the key to another problem which had long puzzled agriculturists.

It had long been known that the growth of leguminous crops, unlike that of others, enriched the ground, and Lawes and Gilbert had shown that this was due to an increase of soil nitrogen. But no explanation could be found till Hellriegel and Wilfarth solved the problem. In studying the nitrogen nutrition of gramineous and leguminous crops, they discovered that the gramineous plants died in absence of nitrate, and in its presence made growth which increased regularly with nitrate supply; while leguminous plants sometimes died and sometimes flourished in absence of nitrate, and behaved equally erratically with increasing nitrate supply. When the plants flourished nodules were invariably present on the roots, but not otherwise. They concluded, therefore, that the nitrogen nutrition of leguminous plants differed from that of the gramineæ, and depended on some factor which sometimes came into their experiments and sometimes did not, and, in any case, was associated with the nodule. Knowing that the nodules on the roots of leguminous plants contained bacteria-like bodies, and remembering Berthelot’s results, they explored the possibility of bacterial fixation. They sterilised the sand and found that peas invariably failed to develop nodules and died, but after adding a little garden soil nodules were found and vigorous growth was obtained.

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