Lecture
The most accurate view of soil as a source of the mineral substances necessary for plants was expressed by the French naturalist B. Palissy. As early as 1563 he wrote that «salt is the basis of the life and growth of all crops». If a field is sown for several years in succession without being manured, the crops will draw from the soil the salt they need for their own growth; the soil is thus depleted of salts and ceases to yield harvests. His ideas about the causes of soil exhaustion, and about the need to return ash substances to the soil in the form of fertilizers, were confirmed by precise experiments only 300 years later. In 1656 J.R. Glauber put forward the hypothesis that saltpeter was the main factor in crop yield, and that applying it to the soil caused a significant increase in yield. He attributed the fertilizing effect of manure to the formation of saltpeter. But the views of B. Palissy and J.R. Glauber were not properly appreciated in their time and had no significant influence on agriculture. In 1789 Rückert created a theory of soil exhaustion fairly close to Liebig's. He believed that every plant requires a particular soil composition best suited to its development. Some plants, under prolonged cultivation, deplete the soil and yields fall, while at the same time other crops thrive quite well on that same soil. Soil exhaustion can be remedied by applying a fertilizer containing the missing substance, and therefore different plants require different fertilizers. However, this correct scientific proposition was not convincing to his contemporaries owing to the insufficient sophistication of soil and plant analysis methods at the time.
In fact, no definite understanding of the role of mineral salts in plant nutrition, or of the soil as their source, had taken shape during the eighteenth century. Only in a manuscript by the French scientist Lavoisier, published after his tragic death in 1794, are there remarkable lines showing that the basic tenets of the mineral theory of plant nutrition had been formulated fifty years before Liebig. Lavoisier wrote that plants draw the materials they need for their organization from the air surrounding them, from water, and from the mineral kingdom in general. However, while the theory of root nutrition in plants had not yet been formed in the eighteenth century, the role of the atmosphere as a source of carbon for plants had been fully proven. The brilliant ideas of M.V. Lomonosov (1753) on the air (aerial) nutrition of plants (photosynthesis) were soon confirmed by the work of Priestley (1775), Ingenhousz (1779), and Senebier (1782). It took a long time to uncover the mechanism of this process and to develop the theory of air nutrition in plants. But the theory of root nutrition in plants developed along a more complicated path. Despite the fairly definite statements and well-known works of Palissy, Glauber and others on the role of mineral salts in plant nutrition, the Swedish chemist Wallerius suggested in 1761 that plants feed on humus. He based this on the practical observation of the effect of manure and any kind of humus on soil fertility, and mistakenly believed that plants absorb humus directly through their roots, that only the organic matter of the soil is nutritive for plants, and that the other constituent parts are merely auxiliary and, in his opinion, may help dissolve the fatty substances of humus (chalk, for example). This incorrect humus theory of plant nutrition was most actively and widely promoted by the eminent German agronomist A. Thaer (1752 - 1828). He believed that soil fertility depends entirely on its humus content and that, apart from water, humus is the only soil substance that can serve as food for plants. In this theory, mineral substances were assigned a secondary role, that is, as substances that accelerate the process of decomposition in the soil and convert humus into an assimilable form. Since Thaer enjoyed great popularity and authority at the time, his mistaken views on the humus theory of plant nutrition became widespread. The views of the German scientist Sprengel, the closest forerunner of Liebig, deserve undoubted attention. His scientific positions came very close to Liebig's theory of mineral nutrition. In his book «The Doctrine of Fertilizers», Sprengel wrote that plants form organic bodies from the inorganic substances they obtain from the soil and air, with the help of light, heat, electricity and water. He explained the causes of falling yields under continuous cultivation. Sprengel paid particular attention to the need to replenish the mineral substances lost from the soil, since plants obtain carbon, hydrogen and oxygen from the air, whose composition, unlike that of the soil, always remains constant. He considered it essential to apply bound (combined) forms of nitrogen to the soil, since most plants are unable to assimilate atmospheric nitrogen. In effect, Sprengel created the doctrine of the importance of mineral substances for plant nutrition and of the need to return them to the soil in order to prevent its exhaustion. In only one respect did Sprengel differ from Liebig: he regarded atmospheric carbon dioxide as the main source of carbon for plant nutrition, but he did not deny the possible use of soil humus by the roots. In this respect, Sprengel's views coincided with the humus theory of plant nutrition. He did not have a single experimental fact that would have allowed him to rule out this possibility. The final collapse of the humus theory of plant nutrition came after the publication in 1840 of Justus Liebig's (1803 - 1873) book «Chemistry in Its Application to Agriculture and Physiology», which made an enormous impression on scientists and practitioners, drew universal attention to the question of the mineral nutrition of plants, and was a great success. The central tenet of Liebig's doctrine of plant nutrition is that only inorganic nature supplies plants with their original food. Humus, in his view, serves as a source of carbon dioxide in the soil, which accelerates the weathering of silicates and prepares mineral food for plants. A logical continuation of Liebig's mineral theory of plant nutrition is his theory of fertilization and soil exhaustion, and his justification of the need for crop rotation. Liebig believed that all plants only exhaust the soil, but that different crops exhaust it in different directions. Therefore, alternating crops in a rotation only slows the process of exhaustion, but it will occur sooner or later if the soil is not replenished with everything that has been removed by the growing crops. Since most farms sell and export grain more than anything else, a deficiency of phosphoric acid will be the first to appear, of which
there is more in grain than in straw. Straw and hay go for livestock feed and bedding. Therefore, when only manure is applied as fertilizer, the soil fails to receive back the elements that were removed from the farm together with its produce. Liebig's indication of the need to apply phosphates first and foremost, as the factor limiting yield, was later confirmed by the priority given to phosphate application compared with other types of mineral fertilizers. To obtain soluble monocalcium phosphates, he recommended treating bones with sulfuric acid. The superphosphate industry developed even more intensively when phosphorites began to be treated with sulfuric acid in the same way. This technology was first introduced in England by Lawes in 1843. Liebig persistently recommended returning to the soil those mineral substances of which the soil was especially depleted. If these deficits were not remedied, other substances would prove useless. This proposition later came to be called «Liebig's law of the minimum», although Liebig himself did not use this term and considered this scientific proposition to be relative. In his book «55 Theses» (1855) he noted: «An element that is completely absent, or not present in the required amount, prevents the other nutritive compounds from producing their effect, or at least diminishes their nutritive action». By this he emphasized the irreplaceability of the elements of plant nutrition. 10. Liebig was the first to clearly express the idea of the conscious regulation of the exchange of matter between man and nature. «The doctrine of the necessity of return, — wrote K.A. Timiryazev, - represents, however one may try to limit its significance, one of the most important achievements of science». The breadth and depth of Liebig's views on plant nutrition are astonishing even today. He wrote: «...Any soil can be considered fully fertile for a given type of plant, say for wheat, only if every one of its particles that comes into contact with the roots contains all the necessary nutrients, and moreover in a form that allows the roots to assimilate these substances at any stage of the plant's development, at the proper time and in the proper mutual proportion» (Liebig J., 1840). Liebig also attached great importance to the whole range of factors affecting yield, for example the geographic latitude of the given locality, its elevation above sea level, the annual amount of precipitation, and its distribution across the individual
the seasons of the year, the average temperature of spring, summer and autumn, the highest and lowest temperatures for each season, and finally the physical, chemical and geological properties of soils — that is, he approached the assessment of fertility taking into account a whole complex of factors. He was not aware of the role of the biological properties of the soil, since the significance of soil bacteria in the formation of nitrates was established only 35-40 years after Liebig's death, and the biological nature of other soil processes was revealed even later. Liebig did not conduct experiments with plants; in substantiating his scientific propositions he relied on general considerations about the cycling of matter in agriculture and on the chemical analysis of plants. His fondness for polemics and his haste in moving from general propositions to practical recommendations led him to a number of incorrect formulations and mistaken conclusions. Thus, J. Liebig believed that the ammonia and oxidized forms of nitrogen contained in the air were sufficient to supply the soil with nitrogen and, consequently, to satisfy the plant's need for it. He therefore also judged the value of manure by its content of ash substances — potassium, phosphorus, and others. A major role in studying the question of the sources of nitrogen for plant nutrition was played by J.B. Boussingault (1802 - 1887). From 1836, as a professor at the University of Lyon, he studied the balance of income and expenditure of nutrients over a crop rotation and established the important role of legumes in enriching the soil with nitrogen. A number of Boussingault's principal works on the study of the cycling of matter in agriculture became the foundation for the creation of a new branch of knowledge — agrochemistry. The synthesis of Liebig's proposition on the role of ash elements and Boussingault's thesis on the importance of nitrogen in fertilizers played a major role in the further development of the theory of root nutrition in plants. Boussingault developed the nitrogen theory of fertilization, setting it against Thaer's humus theory. He linked the depletion of soil fertility to the removal of nitrogen with the harvest. At the same time, he established that certain crops, for example clover and alfalfa, do not deplete but rather enrich the soil with nitrogen. Moreover, Boussingault proved this with precise agrochemical studies in field experiments under crop rotation. The fact that the nitrogen deficit in a crop rotation is covered by clover and alfalfa was established by him during 1836 - 1838. Boussingault was not only a thinker. He carried out precise agrochemical and physiological experiments, stressing that to test the opinion of scientists «one must ask the opinion of the plant itself». Boussingault is rightly considered the founder of agrochemistry.
In addition to his field experiments, he conducted numerous studies on the nitrogen nutrition of plants in special vessels, thereby laying the foundation for the vegetation (pot-culture) method. He carried out a number of studies on the assimilation of carbon by plants, established that atmospheric carbon dioxide is the source of carbon nutrition for plants, and studied the influence of external conditions on the assimilation of carbon by leaves. His work on nitrogen exchange laid the foundation for the biochemical direction in agrochemical research. In 1886, Hellriegel's work on the assimilation of nitrogen by legumes was published. Nodules on the roots of legumes had been discovered by M.S. Woronin as early as 1865, but the connection between them and the assimilation of nitrogen by these crops had not been established. After a series of experiments, Hellriegel concluded that legumes, when growing in soil containing the corresponding bacteria, become infected by them and form nodules on their roots, after which they acquire the capacity for nitrogen fixation. In doing so, leguminous crops increase the amount of organic matter in the soil while at the same time acting as nitrogen accumulators. After a good stand of clover, the soil receives no less nitrogen and organic matter than it would from a dose of 30-35 t/ha of manure. Thus, Hellriegel's discovery of the peculiarities of nitrogen nutrition in leguminous plants completes an important stage in the development of knowledge in the theory of agrochemistry, forming the basis of the modern doctrine of plant nutrition.
Comments