The emergence of agricultural chemistry as a science from Carl Sprengel to Justus von Liebig

The first person to correctly establish the laws of nutrition and fertilizer application in agriculture was the German agricultural chemist Carl Sprengel (1787–1849). C. Sprengel published his views on plant nutrition in the works: "Die Boden Kunde" (1837), "Die Lehre fon den Urbarmachungen" (1838), and "Die Lehre von Dünger" (1839). His views can be judged by the following examples. "Plants form organic bodies from inorganic substances obtained from the soil and air with the help of light, heat, electricity, and moisture," he writes in his book "Studies on Fertilizer." He was also aware that certain mineral substances are necessary for the formation of protein bodies in plants; thus, he speaks of the constant presence of phosphorus in gluten, drawing a parallel with animals, whose brains constantly contain phosphorus. Considering salts necessary for plant life and knowing their origin from the soil, C. Sprengel came to explain the decline in yields under continuous cropping and the necessity of returning mineral substances to the soil. "The air remains always the same in its composition, but the same cannot be said of the soil; therefore, it is necessary to compensate for what it has lost, and more attention should be paid to the so-called mineral substances than to oxygen, carbon, and hydrogen, since plants find the latter in the air; as for nitrogen, it must also be applied in a bound form, since most plants do not have the ability to attract sufficient nitrogen through their leaves from the air." Speaking of individual fertilizers, C. Sprengel is always guided by their chemical composition, with special attention paid to those nutrients that are scarce in the soil. Thus, we see in C. Sprengel the very doctrine of the importance of mineral substances and the necessity of returning them. In only one point does he agree with the humus theory and differ from J. Liebig: C. Sprengel, while considering atmospheric carbon dioxide the main source of carbon for plants, does not deny the simultaneous use of soil humus by the roots. Despite the originality of his views on plant nutrition and numerous publications on this subject, C. Sprengel did not have a significant influence on practice.

The development of a new doctrine of plant nutrition and its practical application, which took place in parallel with the introduction of mineral fertilizers, is the greatest contribution of the prominent German chemist and foreign corresponding member of the St. Petersburg Academy of Sciences (1830), Justus von Liebig (1803–1873), who is rightfully considered one of the founders of agricultural chemistry as an independent science. He showed that chemical elements enter plants in two ways: some, like carbon, from the air, and others in the form of aqueous solutions from the soil. J. Liebig conducted extensive research, consistently determining the composition of soils and the content of mineral substances in various plant organs. In essence, he was the first to experimentally prove that plants selectively absorb chemical elements from the soil. Based on this discovery, J. Liebig developed the widely known theory of mineral nutrition of plants, which facilitated the adoption of mineral fertilizers in agriculture. He was the first to correctly point out the advisability of returning mineral substances, particularly phosphorus, to the soil and its depletion under monoculture. The theory of mineral nutrition was finalized and presented in his famous book "Chemistry in its Application to Agriculture and Physiology." E. Russell (1955) reports on the resonance this book received in the scientific world. He writes that in the 1830s and 1840s, agricultural chemistry did not attract much attention. However, in 1840, J. Liebig's famous report on the state of organic chemistry, titled "Chemistry in its Application to Agriculture and Physiology," made an impression on the scientific world like a thunderbolt from a clear sky. In his work, J. Liebig refuted the humus theory of plant nutrition. He argued that plants have an inexhaustible supply of carbon dioxide in the air. If carbon dioxide is present in the soil, then at the early stage of plant growth, time is saved, as it enters the plant roots and serves only as an additional source to what is absorbed by the not yet fully formed photosynthetic apparatus. J. Liebig, the first to properly appreciate the role of phosphorus and potassium in plant life, is credited with the truly iconic words: "Without these two elements, our fields cannot be fertile." It was then that a fantastic idea for those times occurred to him: that fields needed to be fertilized, that various potassium, nitrogen, and phosphorus salts had to be artificially applied, having calculated how much is needed for the plant to be able to use them. This idea, as A.E. Fersman notes, was met with distrust by agricultural specialists of the 1840s–1850s. His attempt to include Chilean saltpeter in agrotechnology was unsuccessful, and the cargo of this salt, brought by sailing ships from South America, did not find buyers due to its high cost and was thrown into the sea. Sources of phosphorus were not known at the time, and the bone meal proposed by J. Liebig also resulted in overly expensive fertilizers. At that time, it was not known how to use potassium either, and only occasionally, by collecting plant ash, was it scattered over the fields.

J. von Liebig believed that the capacity of the arable soil layer to extract ammonia, potassium, phosphoric and silicic acids from corresponding solutions has its limit, depending on the properties inherent in each type of soil. The fluctuations of the adsorbed amounts, J. von Liebig stated, are as great as the differences between individual types of soils. In this thesis, we have a brilliant anticipation of what K.K. Gedroits' theory of the soil adsorption complex gave to agricultural chemistry 60 years later. The analytical mastery, breadth, and depth of J. von Liebig's views on plant nutrition captivate even the modern reader of his work. He wrote: "Any soil can be considered fully fertile for a particular plant species, say wheat, only if each of its particles, in contact with the roots, contains all the necessary nutrients and, moreover, in a form that allows the roots to absorb these substances at any stage of plant development, at the proper time, and in their proper mutual ratio." In these words of J. von Liebig, we see a premonition of the idea about different compounds of nutrients in the soil, about the importance of nutritional conditions and the ratio between nutrients during the phases of the growing season of plants, which was proven only many years later. If his works are viewed from the perspective of modern science, they contain fairly complete and reliable information about the role of the assimilation apparatus of a plant, green manuring, the ratio of nutrients, "intensive crop production," and the definition of the concept of "fertilizer." J. von Liebig stood at the origins of the discovery of the fundamental laws of agriculture, primarily the "Law of the Minimum" and the "Law of Return of substances to the soil."

Despite its progressiveness and originality, J. von Liebig's theory of mineral nutrition also had its drawbacks. It needed supplementation, clarification, and concretization of scientific facts. An example of this is the transformation of his views regarding nitrogen fertilizers. The increase in the amount of nitrogen in meadow soils that were fertilized only with alkalis and phosphates, as well as the constant soil fertility of some fields in Virginia and Hungary and meadows in Holland, led him to increasingly consider the atmosphere as a source of nitrogen for plants. Therefore, some sections of the first and second editions, which emphasized the need for the application of ammonia fertilizers, were excluded by him from later editions. A.N. Engelhardt (1863), in his review of J. von Liebig's book "Chemistry in its Application to Agriculture and Physiology," wrote: "This book should be a handbook for every farmer who wishes to engage in their noble work rationally and not by routine; ... it will be read with interest and benefit by anyone who wishes to become acquainted with the conditions of plant nutrition and with the fundamental laws of agriculture. It is written in an easily understandable way, so that any educated person can read it." Evaluating J. von Liebig's scientific contribution to agronomic science, D.N. Pryanishnikov pointed to the enormous significance of his idea of mineral plant nutrition but emphasized that it took a long time to put it into practice.

In a heated debate with J.B. Lawes, as a representative of the empirical trend in the development of English agriculture, J. von Liebig pointed out that farmers need not only facts but also their scientific understanding. Today, it can be said with great confidence that the visionary words of J. von Liebig, spoken as early as 1840, have been fully justified: "The time will come when every field, in accordance with the plant intended to be grown on it, will be fertilized with a characteristic fertilizer prepared in chemical factories; then the fertilizer will consist only of those substances that are necessary for the nutrition of the plant."

A major role in the development of the theory of plant nitrogen nutrition was played by the French agricultural chemist Jean Baptiste Boussingault (1802–1887), who experimentally proved that plants cannot feed solely on atmospheric nitrogen; they need soil nitrogen. To this end, in 1851, he grew plants under a glass bell jar and in a glass cabinet with an inflow of air purified from nitrogen. It turned out that plants develop normally without atmospheric nitrogen and that free nitrogen is not absorbed by them. He concluded that nitrogen is as necessary to plants as other nutrients and that the plant's need for this element is met by the soil. In another experiment conducted by J.B. Boussingault, nitrogen applied to the soil in the form of nitrates produced yields that increased in proportion to the amount of nitrates applied. Using data from laboratory and field experiments on plants, the scientist proved that manure and ash from manure are not the same in their nutritional properties, and that nitrogen escapes when manure is burned. J.B. Boussingault also established that nitrogen in the soil can exist in both assimilable and non-assimilable forms. This does not matter to a chemist, but it is not a matter of indifference to plants.

J.B. Boussingault is credited with the priority in discovering the effect of saltpeter on harvest. While in Peru, he observed that in the barren sandy soils of that country, the application of a small amount of guano (a product of the decomposition of poultry manure in a dry climate) makes it possible to obtain a fairly high yield of corn. Chemical analysis revealed that guano consists predominantly of ammonia salts. A conjecture arose that it is precisely these salts that impart soil fertility to sandy soil. When analyses and experiments confirmed this, the scientist made the following important conclusion: "The only reagents capable of acting directly on a plant by delivering nitrogen to the organism are nitrates and ammonia salts, either existing in the soil previously or formed during the growing season of the crop."

The difference in nitrogen uptake by grain legumes and cereals determines the strategy for fertilizer application. All crops, with the exception of grain legumes, take nitrogen directly from the soil. In practice, the most effective fertilizers are those that are richest in nitrogen. The higher the removal of this element by the crop, the more the field is depleted.

When drawing up a nutrition scheme, it is important to consider the interaction of elements. Calcium phosphate, alkaline and alkaline-earth salts help plant development only when applied together with a source of available nitrogen. A complete fertilizer is a mixture of saltpeter with calcium phosphate and alkaline salts, whereas atmospheric nitrogen is assimilated in too small an amount.

Balance of elements and nutrient cycling in agriculture

The vegetation method of research became the basis for modern practical agronomy. It allowed for a detailed study of nutrient cycling in agriculture. The main task of agrochemistry became the management of chemical processes in the soil and plants to increase yield and change its composition. With the help of precise methods of chemical analysis, scientists for the first time obtained a tool for calculating the balance of nutrients.

In plant physiology, the proof of aerial nutrition of autotrophic crops became a fundamental discovery. The source of carbon for them is atmospheric carbon dioxide, not soil humus. The study of plant gas exchange allowed the general equation of photosynthesis to be derived in 1840, which is still used today. The result of these many years of research was two monumental works that combined knowledge in chemistry, physics, meteorology, and plant physiology.

6CO₂ + 6H₂O —^hν→ C₆H₁₂O₆ + 6O₂

• Derivation of the photosynthesis equation — 1840.
• Work "Field management in the light of chemistry, physics and meteorology" — 2 volumes (1851).
• Work "Agronomy, agricultural chemistry and physiology" — 7 volumes (1860–1864).