Popular Science Monthly/Volume 82/March 1913/The Utilization of the Nitrogen of the Air
|THE UTILIZATION OF THE NITROGEN OF THE AIR|
PROFESSOR OF THEORETICAL CHEMISTRY IN THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY
A GERMAN geographer has estimated that the world contains 1,700 million people, and that they are increasing at the rate of twelve million a year. During each succeeding decade, therefore, provision must be made for feeding a new population greater than the present population of the United States. This demands an enormous, steadily growing increase in the world's output of agricultural products. How to provide for this increase is one of the largest material problems that confronts our generation and the generations to come. Many factors must contribute to its solution. New land must be brought under cultivation by a wider distribution of population, by increased facilities of transportation, by better utilization of the available water supply through storage and irrigation. A larger yield per acre must be secured by improvement of the varieties of food-yielding plants through biological selection and breeding, through the adoption of more economical methods of farming, and especially through increasing and maintaining the fertility of the land by the scientific use of fertilizers in adequate amount.
This last aspect of the problem is the one with which this article is concerned. It is a vital part of the food problem, one which can not be eliminated by advances in any of the other directions just referred to; for plants can not live on water and air alone. They consist, to be sure, in largest proportion of compounds of carbon, hydrogen and oxygen; and they have the marvelous power of producing these compounds under the influence of sunlight from the carbon dioxide of the air and the water of the soil. But they contain also as essential constituents certain other elements, especially nitrogen, phosphorus and potassium, which they can not obtain from the air, which they must therefore extract from the soil. These elements are, however, present only in small quantity even in virgin soil; and they soon become exhausted through the harvesting of successive crops. It is therefore necessary, in the long run, to return to the soil the quantities of nitrogen, phosphorus and potassium that are contained in the vegetable products taken from it.
The sources from which we can obtain these three plant-foods cheaply and abundantly is so large a question that only one of them, nitrogen, will be here considered. Of the three this is by far the most expensive—by far the most difficult to obtain in sufficient quantity at low cost.
Before discussing the present and prospective sources of supply of useful compounds of this element, it should be mentioned that, though the consumption of these compounds in fertilizers exceeds all other uses of them, yet enormous quantities are required in other industries. Thus, the powerful modern explosives which have made practicable great engineering works, like the Panama Canal and the Hudson River tunnels, are all nitrogen compounds—made by the action of nitric acid on glycerin, cotton, or some other material. Most of the so-called coal-tar products, the artificial dyestuffs, drugs and perfumes, are also prepared from the substances distilled out of the tar by first treating these substances with nitric acid. Ammonia, too, a compound of nitrogen with hydrogen, is used in large quantity in refrigerating plants and in various chemical industries.
Up to a few years ago, there were only two important commercial sources of nitrogen-compounds—the great natural deposits of sodium nitrate (the so-called Chili saltpeter) in Chili, Peru and Bolivia; and the crude ammonium sulfate obtained in the manufacture of gas and coke from coal. But the saltpeter deposits will, at the present rate of exploitation, become exhausted within a period variously estimated at from 30 to 100 years; and, in the meantime, owing to increased cost of production, the price of the saltpeter is steadily rising, thus restricting its availability as a fertilizer. The ammonia produced in gas and coke works is only a by-product; and the quantity of it can not of course be increased beyond that corresponding to the demand for the main products, gas and coke. The total quantity of ammonia thus produced is in fact entirely insufficient to furnish the nitrogen used in fertilizers; and by far the larger proportion of commercial nitrogen is still derived from the saltpeter deposits of South America.
The nitrogen from these sources costs to-day in American or European markets not far from 15 cents a pound—a price which is causing a nitrogen famine among the crops of the world; for the cost is too high to admit of spreading it in adequate quantity over the millions of acres of land under cultivation. This condition of things offers a challenge to the scientific investigator. For, though nitrogen is one of the commonest elements, forming as it does, four fifths of our atmosphere, yet we are drawing nearly all our nitrogen from South American mines or from gas works and are paying fifteen cents a pound to get it in a form available for plant life.
It might seem as if the problem of converting the nitrogen of the air into compounds that can be assimilated by plants was essentially a chemical one; but recent discoveries have opened also to the biologist a great field of investigation in this direction. For it has been found that, although the higher plants can not utilize directly the nitrogen of the atmosphere, there are certain common kinds of bacteria, which make their homes on the roots of leguminous plants, such as the pea, bean and clover, which have the power of absorbing nitrogen from the air and of converting it within the roots of the plant into organic nitrogen compounds.
This discovery explains for the first time the fact long known to farmers that the richness of the soil can be increased by rotation of crops—a fact so extraordinary, till its explanation was understood, that one might well have wondered whether it was not one of the fallacious traditions which are so common among farmers and sailors. This increased fertility is now readily accounted for as follows. Suppose that a crop of wheat is first grown on a piece of land, and that thereby the nitrogen compounds in the soil are largely consumed in producing the nitrogen compounds contained in the grain. Suppose now that the next year the same land is planted with clover. As it grows, the bacteria referred to develop upon its roots, absorb nitrogen from the air, and store up in the roots an abundant supply of nitrogenous compounds. After the clover crop is harvested, these roots decay in the soil, yield up to it their nitrogen-content, which becomes available for the nourishment of a new wheat crop during the following year.
An interesting illustration of these considerations has been furnished within recent years by the vegetation of the island of Krakatoa. It will be remembered that this island was overwhelmed in the year 1883 by an eruption of its volcano, which destroyed all vegetation and buried the original soil beneath a thick layer of volcanic ashes. It might have been expected that this new soil of ashes, which was of course free from all nitrogenous organic matter, would not be able to support plant life; yet the island soon became covered with an abundant growth. This vegetation was found, however, to be of an unusual character, in that it consisted very largely of leguminous plants—that is, of those plants which, with the aid of bacteria, can take their nitrogen directly from the air.
These facts suggest that the problem of supplying plants with the nitrogen needed by them may ultimately be solved most simply and directly by the biologist. For through further study of the conditions determining the activities of different species of nitrogen-absorbing bacteria, considered in relation to the kind of crop, the character of the soil and other agricultural conditions, it may prove practicable, by inoculating the soil with the proper kind of bacteria and by treating it in such ways as will best regulate bacterial growth, to secure all the needed nitrogen from the air. Already, government agricultural stations are furnishing pure cultures of nitrogen-absorbing bacteria which have a limited value in the case of certain soils.
Until such a perfect solution of the problem can be worked out by the biologist, we shall, however, be dependent on nitrogenous fertilizers; and one of the great tasks of the chemist is to cheapen such fertilizers by obtaining the nitrogen contained in them directly from the air. During the last ten years great progress has been made in this direction; and it remains to describe briefly, without entering into technical details, the general lines along which this problem has been successfully attacked.
Two kinds of processes have been developed. One of these has the object of producing nitric acid, a compound of water with one of the oxides of nitrogen. The other kind of process has for its object the production of ammonia, a compound of nitrogen and hydrogen. For use in a fertilizer the nitric acid, which is a liquid, or the ammonia, which is a gas, must of course be converted into a solid salt. This is most cheaply done by neutralizing the nitric acid with lime or the ammonia with sulfuric acid, yielding calcium nitrate or ammonium sulfate, respectively. Whether the nitrate or the ammonium salt is made the constituent of the fertilizer makes little difference; for, though plants directly assimilate the nitrogen only in the form of nitrate, yet there are always present in soils the so-called nitrifying bacteria, whose function it is to convert ammonium compounds into nitrates.
Nitric acid is a compound whose constituents, nitrogen, oxygen and water, are present in unlimited quantities in the air. The raw materials are available free of cost. The problem is therefore only to make them combine under economic conditions. The difficulty arises from the fact that nitrogen is an extremely stable substance; so that, instead of tending to form compounds with oxygen, the nitrogen oxides tend rather to break down into their elements, nitrogen and oxygen. Thus, scientific investigations have shown that if a mixture of these two gases in the best proportions is exposed to a temperature of 1500° centigrade, that is, to a white heat, only one third of one per cent, unites to form nitric oxide, however long the mixture be heated. But these investigations have also shown that while most compounds decompose with rise of temperature, this one, nitric oxide, becomes more stable, the higher the temperature. Thus at 3000° five per cent, of the mixture of nitrogen and oxygen will unite to form nitric oxide. To get a fair yield of our product we must therefore expose air to an enormously high temperature. But this isn't all; for we must cool off the gas without causing the nitric oxide which has been formed to break up again into nitrogen and oxygen. To do this, we must call to our aid another chemical principle, which is this: although the quantity of a product finally formed in a chemical process sometimes increases and sometimes (as in this case) decreases with falling temperature, yet the rate at which that product forms or decomposes always decreases very rapidly as the temperature is lowered. We must, therefore, expose the air to a very high temperature and then very suddenly cool it to a temperature so. low that the nitrogen oxide already formed does not decompose at an appreciable rate.
These conditions have been practically realized in only one way—by causing an electric discharge, similar to that in an ordinary arc lamp, to take place in air. The temperature of the arc is enormously high, but the air just outside of it is comparatively cool; so that any nitrogen oxide formed at the boundaries of the arc mixes at once with the colder air and thus escapes decomposition. The excess of air containing the oxides of nitrogen is then passed into towers filled with quartz over which water is trickling, whereby nitric acid is formed.
It is not necessary to enter further into details; for these are the essential features of the commercial process for the manufacture of nitric acid which is now being carried out on a large scale at Notodden in Norway. Aside from the cost of installing and maintaining the electrical and absorbing apparatus, the only large expense involved in the process is the cost of power used in producing the electric discharge. The works must therefore be located where water-power is obtainable at the lowest possible cost; and Norway was naturally chosen as the seat of the industry in Europe. The saltpeter factories there are already utilizing 200,000 horse-power; and thousands of tons of their product have been shipped to this country, for use in fertilizing the fruit orchards of California and the sugar plantations of Hawaii.
Almost simultaneously with this process for the manufacture of nitrate there is being developed a process for the artificial production of ammonia, its competitor in the fertilizer field. The aim is to produce this compound also from its elementary constitutents, nitrogen and hydrogen. Nearly pure nitrogen can now be obtained cheaply from the air by a commercial process which up to twenty years ago had been carried out only on the smallest laboratory scale; namely, by liquefying air with the aid of a liquid-air machine, and then distilling the mixture of nitrogen and oxygen, much as a mixture of alcohol and water is distilled in the rectification of spirit. The nitrogen, having a much lower boiling-point, passes off first, yielding a gas containing less than half a per cent, of oxygen, which can readily be removed from it by chemical means. Pure hydrogen can be obtained cheaply by the decomposition of water in two or three different ways. The raw materials needed for the production of ammonia, although notlike the air and water used in making nitric acid, are therefore obtainable at low cost; and the main problem again consists in finding a practical way of causing them to combine.
It is a curious fact that difficulties are met with here which are just the reverse of those encountered in the synthesis of nitric acid. Ammonia is a compound on which temperature has the opposite effect: instead of forming in larger proportion as the temperature is raised, it forms in smaller proportion; thus, if a mixture of nitrogen and hydrogen be heated for a long time to 800° centigrade, only one hundredth of one per cent, of ammonia forms, while it can be calculated that at 400° one half of one per cent, of ammonia must finally result. We ought therefore to work at as low a temperature as possible; but we then meet the difficulty that the rate of combination becomes extremely slow. Thus, owing to the extreme inertness of nitrogen, no detectable quantity of ammonia is produced, even when nitrogen and hydrogen are heated together for several hours at 400°. When, however, it is known that a chemical change tends to take place in a certain direction and when the only difficulty is that it is going on too slowly, there is always a reasonable hope of overcoming this difficulty; for we know that chemical changes are often greatly accelerated by mere contact with suitable solid substances. Such substances are called catalyzers, and Professor Wilhelm Ostwald, one of Germany's distinguished scientists, predicted a dozen years ago that the great advances in the chemical industries within the next few decades would be made through the more extensive employment of catalytic processes. This prediction has found one of its many fulfilments in the commercial development of the method for the production of ammonia here under consideration. For after many years' investigation, certain metals have been found which cause a rapid combination of nitrogen and hydrogen even at comparatively low temperatures. The first metal that was found to have this power in a marked degree was osmium, a metal similar to platinum. As the total quantity of this element in our possession is estimated to be 200 pounds, and as it is valued at about $1,000 a pound, this discovery was hardly a practical one. Later it was found, however, that under special conditions some of the commoner metals, such as uranium, manganese, and even iron, when extremely pure, can be made to serve the purpose. Without entering into further details, it may be stated that a satisfactory yield of ammonia can be attained by carefully purifying the hydrogen and nitrogen gases, by highly compressing them (up to 50 or 100 atmospheres) and then passing the compressed gases slowly over one of these metals at 500-600°; and that a large factory for the manufacture of ammonia by this process is now being erected in Germany.
Certain other chemical processes for the fixation of atmospheric nitrogen, less direct than those already described, but nevertheless commercially practicable, have also been developed and put into operation within the past ten years. There is therefore little doubt that from these sources a large additional supply of nitrogen-compounds will soon be available and that their cost will be gradually lowered. To the vital problem of feeding the human race the chemist is therefore making an important contribution.