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The Pamphlet Collection of Sir Robert Stout: Volume 73



The choice and most valuable parts of food-plants, such as, for example, peas, beans, lentils, clover seed, the grain of wheat, barley, oats, rye maize, and in a less degree rice, potatoes, carrots, cabbages, &c., contain a considerable proportion of nitrogen. This element the plant has to find for itself and build into its tissues—albumen, gluten, legumin, &c. Now as has been already stated, the atmospheric air is composed chiefly of this gas. The weight of the atmospheric nitrogen resting on every acre of land or water, all round the earth, is as much as 25,000 tons; which, if made into ammonia, would produce 31,000 tons of that prince of manures, of which the market value at present is £60 per ton.

It had long been discussed whether plants by means of their leaves, or in any other way, can take their nitrogen direct from the air (I am not now speaking of the 8lb. or so of nitrogen per acre that the annual rainfall washes out of the air in the form of nitric acid and ammonia).

Many experiments on many plants were made by the most [unclear: emine] chemists and botanists of their time to answer this question. And the outcome of all the results obtained is to establish the universal, opinion of all best qualified to judge that, with a few exemptions in the lowest classes of plants (moulds, mushrooms, lichens, and their allies), plants cannot draw page 5 directly on the atmospheric nitrogen for their requirements. There is now known, however (discovered a few years ago), an indirect means of enabling plants to get their nitrogen from this source, which may be explained as follows: It had long been observed that on the roots of many plants of the order of the Leguminosæ (leguminous plants) peculiar warty concretions, or tubercles, or nodules were formed. These knotty growths were regarded as a diseased condition of the plant (something like finger-and-toe in turnips); but this was inconsistent with the fact that the plants whose roots were most affected with these warty growths were always larger, stronger, and in every way better than their clean-rooted neighbours. It had also been known for some time that these nodular warts were rich in nitrogen; but it is to Hellriegel and his recent experiments that we owe the important discovery that these nodules are nothing else than the homes of countless millions of microbes or bacteria of one particular kind, which in these lectures I shall always refer to as the "nitrogen-microbe." These microbes, it is now known, are constantly drawing nitrogen from the air, building it up into albuminous matter, and passing it in that state over to their host as ready-made material for its own growth.

Among his other experiments Hellriegel sowed peas in two similar plots of barren sand. The sand was in both cases quite free from all substances containing nitrogen, but had enough of such other mineral matter or ash ingredients—phosphates, lime, potash, &c.—as is required for the growth of a good crop of peas. Both plots produced young pea-plants, which grew equally well until the store of nitrogen contained in the seeds got used up. Then the young plants began to languish for want of nitrogen, and very soon further growth ceased. At this stage Hellriegel sprinkled on one of the plots some water obtained from a rich loam, in which a strong crop of knotty-rooted peas and clover had been recently grown. The change wrought on the plot so watered was marvellous. The young pea-plants began almost immediately to waken up, and to assume a healthy green appearance. Their growth was thenceforth rapid and luxurious, and they produced abundance of leaves, flowers, and seed. Meantime the starved plants in the neglected plot faded and died. On the roots of the plants in the microbe-tinctured plot Hellriegel found plenty of these same warty nodules. In the words of Professor Storer, of Harvard University, "The loam water contains certain microscopic organisms, which enter the pea roots and develop colonies there, and these colonies of bacteria are speedily enclosed in nodules, which form about them. But the bacteria in the nodules thrive upon the substance of the pea-plant, and upon free nitrogen which they take from the air; and the pea-plant in its turn feeds upon the substance of the bacteria and gets nitrogen enough therefrom for all its needs."

Hellriegel by other experiments proved that it is by the agency of living organisms that this wonderful appropriation of nitrogen from the air is effected. After killing the bacteria in the rich loam-water by boiling it he applied it as before to a young pea crop in a sterilised sand, and found that, as he expected, it had no effect at all on the plants.

Another line of investigation was pursued as follows: (1.) The soil of a plot of ground was analysed, and the total amount of nitrogen which it contained down to the depth the roots would reach was thus ascertained, (2.) A crop of lupines (a leguminous plant) was grown in the soil. (3.) The total quantity of nitrogen in the lupines was determined by analysis. (4.) The soil in which the lupines had grown was again analysed, and its page 6 nitrogen determined. From the results of these analyses it was found that there was much more nitrogen in the lupines and in the soil in which they had grown than the soil contained before the lupine seed was sown. The lupines therefore must have got nitrogen from sources other than the soil. The air was the only other source of nitrogen accessible to them. From the air therefore they must have got their supplies of this essential element, and there were plenty of bacterian nodular warts to account for the agency.

It is now claimed that all leguminous plants can be supplied in this manner with nitrogen from the air through the medium of bacteria. This order of plants includes, among others, peas, beans, lupines, lentils, lucerne, tares, all the clovers, vetches, gorse, broom, the acacia, laburnum, &c.

It is not a little strange that it is only on plants of this order that the nitrogeniferous bacteria grow. Their works are never found on the grasses—wheat, oats, barley, rye, maize, &c.—nor on potatoes, turnips, cabbages, carrots, &c. These crops, however, benefit indirectly by their labours when they are grown in soil previously enriched in nitrogen by these fertilisers, as after a long course of clover eaten on the ground, or after a heavy crop of young leguminous plants ploughed in and rotted in the ground. The conditions favourable to the growth and activity of the nitrogen-microbe are as follow: (1.) Darkness. Bright sunlight kills them; they therefore work under the surface, at a depth of from Sin to 2ft. (2.) Warmth. Blood-heat seems to be the most favourable temperature, but they can live and work at higher and lower temperatures, but probably not much below 50deg. Fahr. nor much above 120deg. Fahr. Hence it is in summer and in warm climates that they are most active. (3.) Moisture. They cannot work in a soil that is quite dry, nor do they thrive in a soil that is very wet. Standing water would, of course, shut out the air, and they would thereby be deprived of the nitrogen that is necessary to them. Water also by its evaporation cools the soil, and on that account is obnoxious to these microbes. A well-drained and well-ploughed, thoroughly broken-up soil, giving free access to air and the solar heat, are what they require in that respect. (4.) A slightly alkaline substance, such as limestone or slaked lime or wood-ashes, in the soil. Too strong an alkali, like quicklime in large quantities and quite fresh, is injurious to them; so is the sourness of wet clay lands or lands standing for weeks under water in a wet season, for in such lands acids are generated which are the very opposite of the alkaline substances that these microbes require.

A mixture of powdered limestone, therefore, or slaked lime worked into the soil in the late spring would afford them a favourable base for their operations.

This important discovery of Hellriegel's explains and accounts for the long-known fact that old clover pastures when again broken up yield large crops of cereals (wheat, oats, &c.), the explanation being, of course, that on these old pastures the microbe has been at his colonising, nitrogen-gathering work, and storing this element away in the roots of the leguminous clover; whence, by the action of other kinds of microbes, it is converted into the ammonia and nitric acid that are such valuable fertilisers. It has been observed that as a rule leguminous plants (clover, peas, beans, &c.) do not thrive well on new land just reclaimed. This is partly due, no doubt, to the absence of the nitrogen-microbe from such fresh lands, as there had been no leguminous plants on which it could have operated. The remedy that suggests itself would be the impregnation of such new land by watering it with loam water from rich, old paddocks on which large crops of peas or lucerne had been grown, or by page 7 mixing some of the loam from such old paddocks with any manures applied to the land. To get the benefit of this discovery, farmers must—(1) lime their limeless lands to provide the mild alkali in which these microbes work, and to sweeten the soil by neutralising the acids it contains. (2.) They must drain the wet land, and so provide for the removal of any standing water which shuts out the nitrogen of the air, which is so necessary to the growth and propagation of these minute organisms. (3.) The land must be maintained in an open, free, friable, porous condition—and, therefore, well tilled—to let the nitrogen of the air down to the dark region of their fertilising operations.

This revolutionary discovery of Hellriegel's will probably soon find a wide application in many ways to increase the fertility of the soil. Nitrogen in the form of ammonia or nitrate is far the most expensive item in manures, and any means that are available for procuring it so cheaply from the air will not be neglected.

[Continuing his remarks on the nitrogen-microbe, the lecturer announced that during the week two of his students had analysed a quantity of the nitrogenous warty concretions taken from the roots of an acacia. The substance was first freed from earthy matter, and as well as possible from the fragments of the roots on which it grew. It was then dried at the temperature of boiling water, and mixed in a mortar with about eight times its weight of soda lime. The mixture was then put into a glass tube closed at one end, and then subjected to a dull-red heat in a combustion-furnace. The gases were collected in a known quantity of sulphuric acid. The heat of the furnace and the soda-lime had converted all the nitrogen into ammonia, which was fixed by the acid. The sulphuric acid was lastly neutralised by the cautious addition of standard potash, and the ammonia determined. The results proved that the nodules contained a little over 4 per cent, of nitrogen. If this nitrogen were present, as is probable, in the form of albuminous matter, the quantity of it found in the sample analysed would make about 28 per cent, of albumen in the nodules.]

It has long been known that a big crop of peas or beans is a severe scourge to some kinds of soil, and this seems to be inconsistent with the Hellreigel theory. When properly understood, however, it is a corroboration of that theory. The explanation is this: (1.) Given an old field of not more than average fertility, the soil containing in an available form a fair but not an excessive proportion of the mineral food of plants, such as phosphates, lime, potash, &c., sufficient for a good crop of wheat, oats, or potatoes. (2.) A crop of peas or beans is grown on this field. The nitrogen-microbe, recognising its opportunity and rising to the occasion, draws upon the stores of nitrogen in the atmosphere, and feeds the young plants with an ungrudging abundance of that important element. Stimulated by this generous nourishment, the plants grow rapidly and vigorously into a big strong crop.

But as the peas or beans require also a due proportion of phosphates, potash, and lime, these must keep pace with the quick step of the nitrogen poured into the plants by the microbes; and, as the microbes take all the labour and responsibility of providing the nitrogen in rich profusion, they save the plant the trouble of collecting and assorting it. The plants, therefore, hurry out of the soil the requisite phosphates, potash, and lime step for step with the nitrogen brought by the microbes. The result is that the soil is for a time impoverished in these mineral ingredients to an undue extent by the forced production of a larger crop of peas or beans than the mineral constituents in it could legitimately and unaided supply. Had there been no nitrogen-microbes in that field there would have been page 8 only a very ordinary crop, and the mineral food in the soil would not have been drawn on to such an extent; or had other than a leguminous crop been grown this outside microbe influence would not have been summoned, and the phosphates, potash, and lime would have had a less drastic call to meet. Now, change the conditions and assume that the field was, to begin with, rich in phosphates, lime, and potash, but very poor in mineral nitrogen or vegetable nitrogen. Such a field for the want of nitrogen could not produce a good grain crop—wheat, barley, oats-all of which require ample supplies of nitrogen in an available form. "The minimum rules the crop," just as the strength of the weakest link in the chain is the strength of the chain. In the supposed case the minimum lies in the nitrogen, and the wheat crop will be poor, because there is not enough nitrogen in the soil for a rich one; and wheat, not being a leguminous plant, is not visited by the nitrogen-microbe, and cannot therefore take nitrogen from the air. But now grow a leguminous crop—say, peas or beans—and follow this with a wheat crop, and the result will be astonishing. The leguminous crop will have laid away stores of the needed nitrogen in its roots and other débris, and by the fermenting action of other orders of microbes this will be converted into ammonia and nitrates and nitrites—all rich in available nitrogen, in the best condition for the wheat crop that is to follow. A big crop of peas or beans will therefore much increase the fertility of such a soil by raising the "minimum" (strengthening the weakest link in the chain) for the grain crop that is to follow.

Considerations of this kind also explain the well-known fact that rich ammoniacal manures, such as Peruvian guano, scourge some kinds of soil They do so, just as the nitrogen-microbe does it, by unduly stimulating the growth of, say, turnips, which require a large supply of potash. If the soil is not particularly rich in potash, a big crop of turnips—carted off, eaten elsewhere, and nothing returned to the soil—will inevitably impoverish it in that valuable mineral constituent. Potash forced out of the ground by the stimulated turnip crop will become the "minimum," and until it is restored only minimum crops will follow.

It is not at all surprising that the British farmer should have long ago—before Hellriegel and his nitrogen-microbe were heard of—discovered as the result of experience that a crop of beans is just the thing to grow after a big crop of wheat, and also before a crop of wheat or other cereal. The preceding grain crop removed the available nitrogen from the soil for its own special fare; the crop of beans drew on the atmospheric supplies of nitrogen, storing up that element in its roots, and therefore also in the soil. The succeeding grain crop (which could not possibly have done well in the nitrogen-impoverished soil immediately after the first crop) now grows luxuriantly, feeding on the nitrogen restored to the soil by the intermediate crop of beans. Quite consistently with all this result of experience, we find in the published returns of rotations followed on the heavy, well-farmed lands of Wiltshire, Bast Lothian, Ayrshire, Derby, and other districts, beans taking their turn between two grain crops (wheat for choice), In these cases the practice suggested by Hellriegel's discoveries has been anticipated and justified by experience,