PHYSICAL CHEBTISTRY I N THE SERVICE OF AGRICULTURE'
BY F. K. CAMEROK
There is some difficulty in approaching this subject, for the reason that what constitutes agricultural chemistry can not be clearly defined, and the boundary lines between it and other branches of applied chemistry are not always evident. Much of biological chemistry and much of geological chemistry, lines along which notable achievements have been made by the applications of the principles and methods which in recent years have come to be called physical chemistry, could with propriety be claimed also for agricultural chemistry. One finds agricultural chemists engaged in the examination of drugs, fertilizers, leathers, and tannins, etc., as well as in the examination of foods or soils. Important applications of physical chemistry are to be found along many of these lines, which might be claimed for the agricultural chemist, but disputed as belonging to the field of the industrial chemist, or others. T h e manufacture of nitric acid by electrochernical methods, while a problem of industrial chemistry, is important because of the use of nitrates i n agriculture. But confining oneself strictly to the work professedly done in the immediate interests of agriculture or farm practices, there is to be found much evidence of the increasing influence of physical chemistry. I n common with other branches of applied chemistry, agricultural chemistry has acquired much for its analytical processes from the work of the physical chemists. Applications of the solubility laws, of the principles of mass action or reaction velocities to precipitation processes, color changes, etc., are the common property of analytical chemistry in which the agricultural chemist shares. Viscosity, specific gravity, optical rota~ ~ _ _ _ _ A paper read before the Physical Chemistry Section of the International Congress of Arts and Sciences at St. Louis. -~
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tion, indices of refraction, freezing-points and boiling-points of mixtures and solutions, the distribution of a solute between nonconsolute solvents, are all used more or less commonly by agricultural chemists in operations with which they specially have to deal, and the niechanisnis of the processes actually employed are very largely determined by the researches of those who have developed the science of physical chemistry. But aside from strictly analytical features, physical chemistry has suggested and aided developments in agricultural research. Without attempting to give an exhaustive resum4 of the entire field, brief attention may be called profitably to a few examples in which physical-chemical principles are, or may be, aids to the study of agricultural problems. T h e determination of the heats of combustion in connection with the food work of both man and other animals is a physical-chemical problem and an agricultural one now receiving a great deal of attention. It is believed to be of far-reaching importance in the developtnent of our civilization. T h e very important work of Atwater with hunian beings, and the similar work of Arnisby with farm animals in the respiration calorimeter is very largely governed by physical-chemical considerations. T h e effect of pressure upon clieiiiical processes, either between mineral coinponents or upon processes induced by organisins, is now receiving attention, as witness the work of Hite upon the changes occurring in milk. One of the most important problenis in agricultural cheniistry is that of the adsorptive effect of the surfaces of soil particles upon other substances. It is well known that finely divided particles have the power of concentrating in the atmosphere around them moisture vapor, carbon dioxide, and other gases and that the aiiiount of this effect varies not only with the extent of surface exposed by the particles, or their degrees of fineness, but also with the nature of those surfaces as well as the nature of the gases themselves. This action is a selettive one, Some gases being reiiioved from a mixture more readily than
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others. T h e adsorption or concentration produced by solid quartz in the case of carbon dioxide is quite small, while with powdered charcoal it is large, and with soils it varies within rather wide limits - a matter of considerable importance to some plants which are seemingly much affected by the composition of the soil atmosphere. I n a manner apparently analogous to the adsorption of such constituents from the atmosphere, finely divided particles, such as are found in soils, show an apparent selective adsorption for dissolved substances, the most familiar example being the adsorption of coloring materials from solution by charcoal. It was early shown by Liebig, Way, and others, that soils show this action upon dissolved mineral salts, frequently in a very marked way, and not only do they possess the power of abstracting individual salts from a solution of several salts, but they seem to possess the power of selecting coiistituents from the component salts of the mixtures. This is strikingly shown in the case of ammonium salts when passed through a soil, ammonia being very largely retained, while the acid residue accumulates in the leachings. Potassium and phosphoric acid seem in general to be adsorbed far more readily and to a greater extent than other common mineral constituents. That great differences exist in the adsorptive power of different soils and also in their action upoii different salts and the constituents of these salts is well known, arid this is obviously of very great importance for the practice of fertilizing with mineral salts. T h e study of the phenomena is a rather complicated one, largely owing to the metathetical reactions and mutual replacements between the soil c o m p o ~ ~ e nand t s the other solutes, but it offers interesting mass action a i d electrolytic dissociation problems which should appeal to the physical chemist. T h e selection of one constituent of a salt in preference to another seems to be dependent upon the amount of dissociation of the dissolved salts, an interestiug subject which has not as yet been worked out. Salts which are hydrolyzed readily show this separation of constituents by adsorption most markedly. Flocculation and deflocctilation of clays and colloids and similar material,
which is now receiving much attention by physical chemists, is of the first importance to agriculture, and in fact much of the progress which has been made in this line of investigation has resulted from work done primarily for agriculture, as in the case of the magnificent researches of van Benimelen. I t is now generally recognized that the immediate source of the food of plants (excepting of course the carbon dioxide absorbed from the atmosphere by leaves) is the soil moisture or soil solution and that before any mineral or other plant food can be absorbed or taken up by the plant it must be first dissolved. I t has been aptly said that the soil is the “stomach” of the plant where its food is prepared and put in solution ready for absorption. From this point of view the problems of plant nutrition, and, to a large extent, fertilizer practices, become solution studies, and, therefore, physical-chemical ones. I t has long been known that the addition of one fertilizer to the soil will frequently cause an increased absorption of other fertilizer constituents by the crop. It has been frequently found, for instance, that the addition of a potash salt to a soil will cause an increased absorption of phosphorus. One of two explanations is generally offered ; first, that the potassium stimulates and strengthens the plant so that it can take up and use‘ more phosphorus and can, by the development of a more vigorous root growth, ( ( g oafter” and obtain the phosphorus which it requires ; second, that the addition of the potassium salt increases the solubility of the slightly soluble phosphates in the soil and thus makes them more available, on the principle that the addition of a salt which does not yield a common ion will increase the solubility. Both explanations may be right, so far as they go (although it is interesting to note here that potassium chloride, for instance, appears to decrease the solubility of the phosphates of iron, aluminum or calcium), but neither one nor both is entirely satisfactory in view of such observations as those of Wolff, made as early as 1865, that the absorption by a plant of one salt from a solution may be very materially affected by the presence of another salt, though both be entirely dissolved at the begin-
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niiig of the experiment. We must for the present admit that there are factors here with which we are poorly equipped to deal. We do not understand why or how the plant can exercise a selective power in absorbing solutes from the nutrient solution in the soil or artificial water cultures, yet there may be very important quantitative relations involved here which should be studied by, or in association with, a physical chemist. This selective power is one of the difficulties encountered in trying to explain absorption as an osmotic phenomenon, assuming the integument of the root to be a semipermeable membrane. I t is conceivable that the plant could thus obtain its needed water from the soil, but not cause the transmission of the solutes, unless our usual conceptions as to the nature and functions of semipermeable membranes are to be materially altered. T h e conception that we have to deal here with an imperfect ' 7 semipermeable membrane, something which shows a tendency to act as a semipermeable membrane, but leaks, explains anything or everything, and therefore nothing. T h a t the membrane may have two distinct functions, acting as a semiperheable membrane, but also simultaneously exercising some physiological activity toward the solute or that it is only semipernieable to one class of substances (such as organic compounds) and permeable to others (such as mineral salts), may be a better hypothesis, but brings in a concept with which the physical chemist is not prepared to deal at present. T h e concentration of the root sap is much higher than that of the soil solution, and it is difficult to see how7 rupture of the root covering could be prevented, should it act as a semipermeable membrane, although a more thorough knowledge of the phenomena of transpiration and guttation, which we know to be frequently large in amount, might offer a satisfactory explanation. Furthermore, we are troubled as yet by having no clear concept as to what constitutes a plant food, although of course i t is but little more than a matter of analysis to determine what elements are involved, and we have some knowledge of their functions, as for instance that of potassium in the development
of starch. But we are as yet utterly in the dark as to whether potassium is taken up as such, perhaps as an ion or in combination as a salt ; whether the plant has the power to absorb ions from the solutions, or whether it absorbs salts alone and then excretes a part of them as acids or bases. We know that the plants can absorb organic solutes and use them in metabolic processes, but whether they ever do normally in order to avail themselves of mineral constituents thus combined, or for other reasons, is uncertain. These questions are fundamental ones for plant physiology and for a rational scheme of specialized fertilizer practice, and it does not seem too much to hope that physical chemistry may here furnish suggestions atid methods which will enable the physiologist to advance materially a subject of enormous practical as well as theoretical importance. T h e soil may be considered as a complex system containing components in three phases, the solid, gaseous, and liquid, the liquid phase as forming a nutrient solution for crop production being the one of immediate interest to agriculture, and the solid and gaseous phases having but little importance except as they affect or determine the liquid phase. Perhaps a distinction should be made as to a fourth kind of matter, the living organisms, in which would be included bacteria, molds, ferments, and enzymes. T h e difficulty of determining what constitutes the variables in any given soil is generally unsurmquntable, and the variance of the system is therefore indeterminable so that no practical applications of the phase rule to a soil system has yet been made satisfactorily. Yet this way of looking at the subject has a real value since it gives a clear, orderly perspective of the problem. When one attempts to simplify the problem by considering only certain components, other difficulties arise. We know as yet but little of the phenomena exhibited towards a solvent by a mixture of three or more electrolytes; but, even when but two are considered, those with which we are most interested in soil investigations seem to be the ones whose dissociation products do not obey the mass law and give unexpected osmotic pres-
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sures. We need a vast deal more information in this field than we now possess, and for solutions of moderately high concentrations as well as for very dilute ones. T h e greater number of the rock-forming minerals which comprise the inorganic coniponents of the soil are properly to be regarded as electrolytes, but electrolytes which showa slow rate of solution and in theprocess are more or less completely hydrolyzed. In fact this slow solution and hydrolysis is the main factor in the weathering " process, the action of carbon dioxide and other solutes being more or less secondary. Here lies a field in which our knowledge is very meager and in which much profitable research could be done. There are almost no data of value concerning the solubility of minerals in water or salt solutions ; in the greater part of the work in the past the importance of the time element has not been recognized. We do not even know whether or not the hydrolysis of these substances be a reversible process to which the mass law would be applicable. Certainly the ~2 priori indications of the dissociation hypothesis as to the probable effect of other solutes upon the solubility of these hydrolyzing substances are frequently at variance with the observed phenomena, and a most fruitful field for experimental work by the physical chemist is here indicated. Physical chemistry has opened u p another promising field by the work which has been done upon the effect of solutions of electrolytes upon seedling plants, as in the studies of Coupin, Kahlenberg, and True, and othersof probably no less importance. Some of this work, as that of Kahlenberg and True, was done primarily as dissociation studies. From this point of view the method has proved unsatisfactory for several reasons. T h e criteria used as the death point of the plant or the inhibition of the growth of a radical, are hard to determine with exactness. Seedlings of the same plant, of the same age, and apparently the same past history, show idiosyncrasies. T h e effect of both cation and anion, as well as the undissociated electrolyte, must always be considered, and finally it seems impossible to eliminate entirely consideration of some physiological factor or factors ((
exhibited by the seedling and probably connected with its selective power in absorbing solutes as food. From the points of view of physiological research or agricultural study, however, this method gives great promise. Using the dissociation hypothesis as a guiding principle, but with the proper reserve as to the validity of its indications until phenomena have actually been observed, its vaiue has already been shown, and much more can be and probably will be done along this line of investigation. Not only has the death limit of plants in solutions of ordinary salts, or mixtures of such salts as are found in the soils of arid regions, been determined, but results of economic importance for the growth or introduction of crops in those areas have been reached and the germination of seeds under these conditions has received attention with reference to the possibility of starting a crop through the temporary alleviation by irrigation of the soil, and dilution of the salts it contains. This work further offers methods which will probably be of great value in controlling the breeding of salt or alkali ” resisting plants. It has generally been observed with these mineral solutes, which prove toxic a t some more or less definite concentration, that they are usually stimulating a t m u c h higher dilutions, and this fact has promise of having economic importance as to growth and development of field crops ; as to the treatment of malt in breweries to save time in the germination period ; and in the preparation of seeds before sowing in the field to insure more certain and rapid sprouting. With the employment of some criteria more satisfactory than the death limit it seems probable that very valuable studies could be made on the concentrations which are most conducive to growth, development of fruit, etc., in plants, and economic results obtained for the greenhouse and orchard if not for the field. Conductivity measurements have proven most useful in soil investigations, as in the method first suggested by Whitney and developed by Briggs, in which the resistance of a soil saturated with water and placed between parallel electrodes was determined. Knowing the amount of water present in the saturated ((
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soil, which can be readily approximated from its texture, the resistance of the soil can be correlated quite closely with the amount of soluble salts present. A compact field apparatus consisting of a hard rubber cell with parallel electrodes, a circular slide wire bridge with dry battery, resistance coils, and other necessary appendages has been devised. This can be taken into the field and a number of samples over a large area can be examined in a day, the results being correlated with and checked by a few laboratory examinations. I n this wa17 the alkali maps of the U. S.Department of Agriculture have been prepared. In humid areas where the movement and translocation of soluble salts in the soil is quite small this same bridge arrangement with permanent parallel electrodes gives a ready means of determining and controlling the moisture content in the soil, and this has found important application in greenhouse culture. This same bridge attached to a thermocouple has been successfully used to give approximate temperatures in fermenting, as in tobacco curing. This electrical thermometer is also being used in certain specialized agricultural industries, as in the controlling of the temperature in greenhouse culture and promises to be of value in measuring temperature in silo beds, hay mows, and similar situations where elevated temperatures may deyelop. Attention has also been given by several investigators, notably by Elfking Plowman and Stone, to the effect of electrical currents passed through the soil or ionization of the soil as it has been termed, in stimulating or otherwise affecting plant growth, suggesting possible applications to greenhouse culture. Recently Briggs has devised a very accurate and ingenious laboratory method for determining the amount of moisture present in a soil sample by determining its dielectric constant, and in this way the study of the movement of soil inoistures, involving some of the most important problems in soil physics and chemistry, promises to be facilitated enorniously. T o the cases which have just been cited many others could undoubtedly be added, and it is clear that physical chemistry in the past has had, and in the future must have, an important in((
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646 fluence upon agricultural chemistry. But nevertheless the retrospect leaves ninch to be desired and is not altogether inspiring. Physical chemistry has not yet directly produced any new methods or important modifications in agricultural practice. T h e number of agricultural investigators whose work shows an appreciation of, or consideration for, the principles or methods of physical chemistry is small, perhaps less than in any other field of applied chemistry. Some of the reasons for this are not far to seek. Agriculture is a very old art. I t has the conservatisni of old age and long practiced methods. Those engaged in its service, through the field of applied science, are more or less affected by this conservatism, and it must be admitted that the agricultural chemist as a class is in harmony with his environment. Conservatism may often be a good thing and perhaps it never does really stop progress, but it may certainly delay it. Agricultural chemistry can furnish as proud a list of names of masters from Liebig to the present day, as any field of science. But the work of the present generation is almost entirely along conservative lines, already well marked out, and those who are attempting to apply the principles of modern chemistry to the problems which confront them are comparatively few. This condition is well exemplified in the instruction given to our agricultural students. So far as the speaker is aware, Hall’s recent book on the “Cheniistry of the Soil” is the only modern agricultural text-book in which physical chemistry as such has received any recognition. But on the other hand the physical chemist has not responded to the needs of the agricultural chemist by studying the class of problems with which the latter has to deal. A theory which applies only to very dilute solutions, of few components, and does not consider what part the solvent itself may have in determining the character of the solution products, is of too limited application to be very helpful to the man confronted with practical problems. A more thorough knowledge of irreversible reactions and of thernio-chemistry is as much needed as
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a conlprehensive solution theory. It is not so important to accumulate evidence in support of our hypotheses as it is to meet the difficulties for which the hypotheses can not account, or for which we have no working hypotheses. Though it may be obviously wrong to expect the student of pure chemistry to devote his attention to problems of applied chemistry, it is not too much to expect him to furnish the methods for attacking these problems, arid physical chemistry has yet much to furnish in this direction before it can claim a large support from agriculture on utilitarian giounds. I t is true that the problems presented by agricultural chemistry do not commend theiiiselves to the investigator who is interested in chemistry for its own sake. They are generally complex and not well suited to the elucidation or illustration., of hypotheses in pure chemistry. T h e pecuniary rewards which agricultural chemistry offers are not sufficient in comparison with other fields to tempt the man trained in physical chemistry who wishes to use his equipment to this end. But to the man who has the training arid who cares not so much whether his problems be pure science so long as they be attacked in a scientific spirit and with scientific methods. the application of physical chemistry to agriculture offers many opportunities. H e can have the satisfaction not only of doing good scientific work, but of helping directly an industry of ultimate importance to all his race and of immediate importance to the numerically largest class of the race.
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