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JUSTUS VON LIEBIG AND TODAY'S AGRICULTURAL CHEMISTRY' KARL SCHARRER Agricultural-Chemical Institute of the Justus von Liebig College, Giessen, Germany (Translated by I. F. Wischhusen)
TEEUniversity of Giessen Tvas founded in 1607 and closed its doors in 1945. During these 338 years many noted scientists mere active on all its faculties. Without doubt, the most famous researchers on the faculty of natural science were Justus von Liebig and Roentgen. Roentgen's most productive years were spent later in Wuerzburg, but Liebig performed his greatest work and issued his most vital publications here in Giessen. Significant as the discoveries of Roentgen were because of their wide applicability, they were nevertheless excelled by the tremendous results that flowed from the work of Justus von Liebig. Rarely, if ever, has there been a scientist whose researches were of such pronounced and far-reaching influence, not alone in respect to our theoretical knowledge, but also in revolutionizing all our practical and material ways of life, because they helped t o ban hunger, want, and suffering from this world. For a hundred years they were to guide our material and spiritual culture upon a well-ordered plane, through the elimination of worries concerning the necessities of life. To promote a better understanding of what follows, some brief biographical data may be given. Justns von Liebig was horn in 1803, the son of a. druggist and mint dealer in Darmstadt. and earlv in life worked in his father's &nt shop, where he laid'the foun&tion of his chemical knowledge. After graduation from high school in his birthplace, he persuaded his father to let him study chemistry, a subject which in those days was regarded as unusual. He attended the Universities of Bonn and Erlangen, and later went to Paris where at the Sorbonne he attended the lectures of the famous scientists of that time in chemistry and physics, and justified the familiar European saying: "La chimie est un science francaise!" Liebig worked for some time in the lahoratory of the distinguished physicist and chemist Gay-Lussac. In Paris he also became acquainted with a number of German scientists, especially Alexander vou Humholt, who helped him in many ways and intraduced him ta the great research workers in the French ertpitol. Liebig returned to Germany in 1824. In the same year, upon the recommendation of Alexander von Humholt, the grand duke of Hesse appointed him professor extraordinary of chemistry a t the University of Giessen. He was then twenty-one years old. On the death of Professor Zimmerman in 1825, Liehig succeeded him as full professor of chemistry. Liehig remained a t Giessen until 1852 and those twenty-eight yertrs wen the most productive of his life, during which he performed n e d y all his great experiments and wrote his Literary works. Toward the end of his activities a t Giessen, Liobig receivcd calls to the universities of Vienna and Heidelberg, which he drelincd. Aftcr careful consideration, ha aecdpted a czll to th. University of Munich in 1852, Official address a t the Seven Hundredth Anniversary Celcbration, City of Giessen, Germany, July 19, 1948.
and moved to the Bavarian capital where he was active until his death in 1873.
Liebig's importance to pure chemistry will not be discussed a t this time, except to mention that his greatest contribution in this field consisted of the reformation of the methods of teaching chemistry. The kind of practical participation of young chemists introduced by Liebig is the main policy in the training of chemists t o this day. Liebig required his students to conduct actual experiments in the laboratory from the first day of their studies, to engage in practical trials of all the various qualitative and quantitative analyses, and actually to prepare inorganic and organic compounds. This sort of study is essentially maintained to this day, but in the meantime has been extended and intensified to embrace other branches of chemistry. Liebig's main accomplishments are, however, to be found in the field of applied chemistry. In particular, two books which he wrote in 1810 and 1842, respectively, may be looked upon as the birth of two new sciences. I n 1840 he published his work: "Organic Chemistry in its Application to Agriculture and Wysiology," a book that is known also in short as " Agricultural Chemistry." I n 1842 there appeared from him a further uublication: "Oreanic Chemistrv in its ADD^^oation t o ~ h ~ s i o l and o g ~~athology,"which, for &Art, is known as "Animal Chemistry." The first book, "Agricultural Chemistry," has wrought far-reaching changes in agricultural practices, and exercised a revolutionizing effect upon the food supply of the people of Europe, so that during the last 100 years, except for wartimes, hunger and want were unknown, in contrast to former times when famines occurred regularly. This effect is to be credited essentially t o the teachings of Liebig as described in this book. To understand the new views presented by Liebig in his "Agricultural Chemistry" it is necessary t o remember that in 1809 there appeared a book by another distinguished scientist, Albrecht Thaer, entitled "Principles of a Rational Agriculture." In this book which was in many ways of considerable importance to practical agriculture, Thaer also dealt with the science of plant nutrition and formulated his "humus theory" which essentially stated that the nutrients for cultivated crop plants consist solely of water and organic matter. Many prominent scientists disputed this conception, particularly de Saussure (in Switzerland) in his book "Recherches Chimiques sur le Vbgbtation," and Rucker
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and Sprengel in Germany, too, opposed the viewpoint, but their opinions failed to prevail. Liebig successfully attacked the humus theory in his book in 1840, and advanced his "mineral theory." It is the essential content of this mineral theory that the main nutrients of green plants, besides carbon, hydrogen, oxygen, and nitrogen are six essential mineral nutrients, viz.: sulfur, phosphorus, iron, calcium, magnesium, and potash. Liebig further indicated that certain plants would respond to a supply of sodium and chlorine, and that all plants required silicon. In comparison with today's conception of nutrition of crop plants, it will be seen that essentially Liebig held a body of knowledge similar to ours. In contrast with Thaer, who still believed that the organic substances of plants were formed from organic materials only, Liebig expressly stated his doctrine of carbon dioxide assimilation. It was his conception that the higher plants, through their chlorophyll and the sun's rays, could, from carbon dioxide in the air, in the presence of water and heat, form organic substances by a reduction process; thus performing a photochemical synthesis. In regard to nitrogen, Liebig a t first held views different from ours. He thought that atmospheric nitrogen combinations, especially the constantly present ammonium nitrite, when carried into soils through precipitation, would fully suffice to supply plants with their nitrogen requirements. He therefore regarded additional nitrogen fertilization as superfluous. Strongly opposed to such a conception a t that time were two English agricultural chemists of the Agricultural Experiment Station a t Rothamsted, Lawes and Gilbert. They were convinced that the atmospheric nitrogen combinations were insufficient to supply plants with their nitrogen requirements, so that fertilizer applications of nitrogen to the soil in the form of suitable mineral fertilizers were absolutely necessary. These disagreements between Liehig and Lawes and Gilbert for a time assumed violent form and only in later years was Liebig gradually convinced that nitrogen fertilizer application was necessary for plant growth. As far as silicon as a plant nutrient is concerned, today's knowledge classes it as a trace element rather than a major element. In regard to sodium and chlorine, today's knowledge corresponds with Liebig's opinion that these two elements are desirable for all the socalled halophilic and chlorophyllic plants, but are superfluous, even injurious, for all other plants. Our present knowledge of the science of plant nutrition shows that besides the major plant nutrient elements, certain accessary elements or microelemeuts, also known as trace elements, are absolutely necessary for plant growth. In this category besides silicon already mentioned, belong boron, copper, manganese, zinc, and according to some American investigations, probably also molybdenum. Liebig's "mineral theory" not only placed the theoretical principles of plant nutrition upon a new basis, but it gave tremendous stimulus to practical agriculture. Liebig succeeded where previous research workers failed
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in convincing practical agriculturists of the significance of mineral fertilizers in the growth of crops. This was the result of his thorough presentation of the subject in his brilliant publications, and of his authority and reputation that were already well established throughout the world by his researches in the field of pure chemistry. Thus it was Liehig who in the final analysis made possible the tremendous development of the commercial fertilizer industry that caused such phenomenal increases in crop yields of our principal cultivated plants through the application of inorganic fertilizers. The production of superphosphates developed gradually, as did the use of phosphoric acid generally, in the form of phosphates in commercial fertilizers. Later on the importance of potash salts from the Stassfurt deposits was recognized; up until then they had been looked upon as a waste product nuisance in salt mining. Nitrogen was imported into Europe more and more as natural saltpeter from Chile and Peru, until in the early years of the twentieth century ammonia salts and nitrates were synthesized from air nitrogen according to the processes of Birkeland-Eyde and Schoenherr, as well as Haber-Bosch and Ostwald. The synthesis of calcium nitrogen compounds was also developed, thus providing another method of converting inert air nitrogen into valuable nitrogen combinations. Even though Liebig's chapter on his "mineral theory" which resulted in the use of commercial fertilizers was one of the most influencial in his book "Organic Chemistry in Its Application in Agriculture and Physiology," we are nevertheless indebted to this book for other stimulating principles, first of all for his "Law of the Minimum." This law states essentially that crop yields are determined by the nutrient available to the plant in the least amount, and that the absence or deficiency of any one nutrient cannot be compensated for by the excessive presence of other nutrients. This important law of the minimum has had far-reaching consequences, and remains not limited to plant nutrition but is applicable also to animal nutrition. In later times i t was further developed and mathematically perfected, particularly through the studies of E. A. Mitscherlich. This scientist formulated his so-called "Law of Activity of Growth Factors" which holds that plant yields may be increased in proportion to the supply of nutrients absent below the need for maximum yields. Mathematically this may be expressed by the following formula:
In this equation x represents the amount of plant nutrient, y the amount by which plant nutrient x has produced yields, A the maximum yield obtainable from this growth factor, and c the constant efficiency value (so-called) of the particular growth factor. In conformity with this law, it may be seen readily that the better a plant is supplied with a certain nutrient, the less may be expected in the way of additional
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yield increases from further supplies of such nutrient. Vice versa, the more poorly a plant is supplied with a nutrient to begin with, the more it will respond in the way of yield increases to a supply of that nutrient. It is interesting to note that in recent times it has been found that similar principles play a role in animal nutrition. Feed quantities and feed results are not directly proportional, but feed results depend upon the plane of nutrition of the particular animal. Expressed in other words, the growth increase in an animal as a result of feeding may be expected to be lowest when the plane of nutrition of the animal is highest, and vice versa. We know today that Liebig went too far in rejection of Thaer's humus doctrine, and also that there is a certain justification for that theory. Not in the sense, as Thaer assumed, that organic matter as such is a nutrient for plants; but that fertile soils require the presence of organic matter. Under agricultural practices crop production is a matter of growth and decay within the top layer of disintegration of the earth's crust. The supply of inorganic nutrients is in itself sufficient to sustain the l i e of plants, even for crop production. This has been proved, for instance, in water cultures as best developed with "hydroponics" in recent times by the Americans. But under normal conditions in farming and gardening, the soil source of nutrients continues to be the basis of agriculture. I t therefore is not sufficient that the soil contain the essential plant nutrients in adequate quantities, and that those nutrients removed by crops and precipitation be replenished, but soil fertility levels must be maintained and improved. Therefore, policies must be adopted looking toward the creation of optimum temperature and moisture conditions, a healthy bacterial life and a correct soil structure, or what in short is known in our farm practice as "the soil mold" and in soil science is defined as "permanence of crumb structure." All this is possible only when soils are supplied with sufficient quantities of organic matter in the form of properly conserved domestic manures, and when the calcium status is satisfactory. Heavy soils are loosened by carbon dioxide generated from the decomposition of organic matter, due to the activities of microorganisms; the temperature and water conditions of such soils are thereby improved, and favorable conditions are created for bacterial flora of the soil. Light soils are enriched in organic matter and their consistency improved by the application of domestic fertilizer materials. Lime supplied to soils not only neutralizes humic acid, forming a so-called mild humus, but calcium-ions take care of the flocculation of soil colloids that aremostly negatively charged, and thus create the conditions prerequisite for a correct crumb structure of soils. The object of our modem science of fertilization is not only to supply nutrients for an increase in crop yields, but also to maintain, and where possible, to improve soil fertility, a problem which Liebig had already emphasized in his writings. The later editions of his book "Agricultural
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Chemistry" recognized more and more the importance of humus for soil fertility. Modern plant nutrition deliberately avoids both the unilateral humus theory of Thaer, and an exclusively mineral theory. We know that the humic substances contained in stable manures and other domestic organic fertilizers are indispensable for the maintenance and increase of soil fertility, and also that the nutrients they contain are insufficient to supply the enormous needs of our farms and gardens, so that maximum production cannot be achieved unless we apply the needed nutrients to our soils as mineral fertilizers. Liebig also investigated the important problem of how to ascertain the nutrient requirements of soils and plants. He proposed to determine the content of essential plant nutrients in the soil by making soil extracts with concentrated hydrochloric acid or "aqua regia" and analyzing the filtrate. This method is suitable to disclose the nutrient reserves contained in soils, but does not provide a yardstick to determine the amounts actually assimilable by plants, because plants do not possess such solvents as strong mineral acids but only weak carbonic acid originating from root respirations. In recent decades much agricultural chemical research has been devoted to working out methods to determine what soil nutrients are assimilable by plants and which are root soluble. Many such methods have been developed, including purely chemical methods using mild acids or salt solutions, physico-c6emical methods, and plant physiological or microbiological methods. For practical purposes i t is particularly important to ascertain the phosphorus and potash requirements of our soils. Under our climatic conditions, nearly every mineral soil requires nitrogen so that there is less interest in methods for its determination. The Neubauer method of soil testing has been adopted as standard. It employs the plant itself as the solvent of soil nutrients by utilizing the soil-to-plant relationship under controlled conditions to determine the uptake by rye seedlings of root-soluble phosphoric acid and root-soluble potash from a definite quantity of soil. The nutrients contained in the rye seed and in the sterilized sand used to dilute the soil are determined in comparison with a check plot. This process has been improved in recent years through photocolorimetric and spectraanalytical devices into a rapid soil testing method. Liebig's "Animal Chemistry" appeared in 18-12, as stated earlier. While it did not instigate anything like the transformation brought about by his "Agricultural Chemistry" that appeared in 1840, nevertheless it stimulated the science of animal nutrition in many respects. Albrecht Thaer evaluated feedstuffs according to their "hay value" which showed the way to a chemicophysiological evaluation of feedstuffs and foodstuffs according to an indicated method of analysis of their chemical composition. Consequently, this scientist was the first to give an impetus to the evaluation of feed- and foodstuffs from modern scientific viewpoints.
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Liehig divided nutrients into two kinds: plastic and respiratory. He designated as plastic nutrients those which first of all played a role in the structural metaholism of the animal body, the proteins for instance. He called respiratory nutrients those which mainly function in basal metabolism, such as carbohydrates and fats. Our modern science of nutrition divides nutrients into energy or combustion nutrients, and structural or protective nutrients. The energy nutrients include principally substances which produce energy through oxidation, such as fats, carbohydrates, and to a lesser degree also some proteins. The structural nutrients include substances not concerned with caloric effects, hut which function in physiological-chemical activities, such as mineral matter, vitamins, proteins, and to a certain degree also the fats. Liehig clearly recognized that carbohydrates in an abundant nutrition can be converted in the animal organism to fats. Up to his time the conception prevailed that body fats were formed solely from the fats in the diet. It is true that Liehig based his view solely on hypothesis, and that the facts and experiments which he quoted are not valid evidence. He thought that fat formation was related to respiratory processes, i. e., proportional to the oxygen intake of animals. According t o his assumption, animals remained low in fat when in the open on pasture, whereas, when stabled, they gained fat through abundant feed and lack of oxygen due t o lack of exercise. Fat formation, according to him, was taking place in the animal body due to
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&faultyratio of quantity feed intake in comparison with oxygen respiration. It is significant that Liebig clearly pointed out the importance of mineral matter, especially calcium and phosphoric acid, to the nutrition of animals and man. It is particularly timely a t the present juncture to remember the teaching of Liehig because due to the absence of commercial fertilizers our domestic animals receive feed poor in calcium and phosphorus, so that hogs and daily cattle particularly suffer from deficiency diseases caused by a lack of minerals. Gericke has recently shown that the adult as well as the young population of Berlin on today's rations, suffer from decided undernourishment, particularly in calcium and phosphorus, and pointed out what devastating consequences this creates with respect to the incidence of certain diseases, not the least being tuberculosis. Seen from today's perspective, Liebig's "Animal Chemistry" contains many faults, mainly because he overwhelmingly dealt with questions as chemical problems without subjecting them t o physiological tests. But this was done soon after him in an exhaustive manner by his students, Pettenkofer and Voit. The inheritance which Liehig left us in the field of agricultural chemistry proves that this science is not only one of the most important sciences of agricultural husbandry, hut also furnishes an essential foundation for human nutrition because it covers both plant and animal science. It is thus seen that human nutrition cannot he separated from plant and animal nutrition.
