PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

7. MINERAL NUTRITION OF PLANTS

The primary products of Photosynthesis serve as the source of the essential set of Organic compounds for plants. These are formed from CO₂ and H₂O during The process of aerial plant Nutrition. Their chemical Structure is based on C, O₂, and H₂. Through metabolic processes, these primary photosynthetic products are converted into Amino Acids, Proteins, Lipids, high-energy compounds, Nucleic Acids, and secondary organic metabolites. Their formation requires a combination of non-metals and metals (N, P, S, K, Ca, Mg, Fe, Cu, and others), essentially most of the elements of the periodic table. Plants obtain these elements from the soil. Therefore, plant growth intensity, and agricultural crop yields in particular, depend primarily on soil nutrition conditions.

This is why such great importance is attached to measures aimed at increasing soil fertility, maintaining proper Fertilization regimes, and adhering to all agrotechnical requirements.

It has long been known that crop yields depend on soil fertility. It was established much later that soil fertility is largely determined by the presence of salts. One of the first to suggest this was the French craftsman B. Palissy in the 16th century. It was not until the mid-19th century that the German chemist and physiologist J. von Liebig formulated the fundamental principles of modern agricultural chemistry.

Development of the Theory of Root Nutrition in Plants

Concepts regarding soil nutrition in plants began to take shape in connection with THE DEVELOPMENT OF crop production. As early as the 6th–5th millennium BC, wheat, barley, rye, corn, flax, hemp, various garden crops, and fruit trees were cultivated, and ash, silt, and manure were used as soil fertility enhancers. The first physiological experiment aimed at studying plant nutrition was conducted by the Dutch naturalist J. B. van Helmont in 1629. He planted a 2.25 kg willow branch in a clay pot containing 91 kg of dry soil and regularly watered it with rainwater. After 5 years, the plant and the soil were weighed separately. It turned out that the willow weighed 77 kg (an increase of about 75 kg), while the mass of the dry soil had decreased by only 56.6 g. Thus, the plant's mass had increased 33-fold, excluding the annually shed leaves. Van Helmont concluded that the entire plant mass was created from the Water supplied during irrigation. This experiment served as the basis for the "water theory" of plant nutrition, which was long held in botany.

Much earlier, Aristotle (384–322 BC) had hypothesized that plants absorb food from the soil in the form of complex substances. At the end of the 18th and beginning of the 19th century, this concept was developed by the German agronomist A. Thaer, who created the "humus theory" of plant nutrition. It had long been known that the darker the soil, the more fertile it is. The dark color of the soil depends on the content of various organic residues, or humus. According to the humus theory, plants are nourished by water and humus.

However, data on The Role of mineral elements in plant nutrition gradually accumulated. One of the founders of Russian agronomy, A. T. Bolotov, outlined the Basic principles of Plant Mineral Nutrition. In 1770, his book "On the Fertilization of Lands" was published—the first Russian monograph on agricultural chemistry. Bolotov developed techniques for applying fertilizers to the soil and listed several dozen types of fertilizers suitable for agricultural use.

The Swiss naturalist N. T. de Saussure systematized the data on plant nutrition known at that time and established that the soil provides plants with nitrogen and mineral elements. When growing plants from seeds in distilled water alone, no ash increment was observed. In his work "Chemical Researches on Vegetation" (1804), he noted that different salts are absorbed by roots from an aqueous solution at different rates. The French agricultural chemist J. B. Boussingault (1837) experimentally proved that plants could be grown in pure sand if mineral salts (ash and saltpeter) were added to it.

The German chemist J. von Liebig, one of the founders of agricultural chemistry, challenged the humus theory and in 1840 published the book "Chemistry in Its Application to Agriculture and Physiology," where he substantiated the theory of Mineral Nutrition of Plants. According to this theory, Mineral Substances in the soil are The basis of fertility. The scientist believed that humus was only necessary for The formation of CO₂, which accelerates the weathering of parent rock and increases the fertile soil layer. He was the first to propose applying pure mineral substances as fertilizers. While correctly assessing The Importance of mineral elements in plant nutrition, J. von Liebig simultaneously believed that plants absorbed nitrogen from the air in the form of ammonia. Only later, in 1856, under the pressure of irrefutable facts, was J. von Liebig forced to admit that nitrates could be a source of nitrogen for plant mineral nutrition. Another error in his theory was the denial of the role of soil organic matter in plant development. As is now known, humus is necessary not so much for the plant itself, but for the soil microflora, which plays a significant role in plant nutrition.

J. von Liebig formulated the "law of the minimum," according to which the application of any amount of mineral substances will not result in a yield increase until the deficiency of the substance present in the minimum amount is eliminated, as well as the "law of restitution," which points to the necessity of returning to the soil the nutrients absorbed by plants. He argued that failure to follow THE PRINCIPLE OF complete restitution would lead to soil depletion and a decrease in fertility. In general, these principles are correct, but some of J. von Liebig's followers derived from them the "law of diminishing soil fertility." The practice of modern agriculture has proven the complete invalidity of this pseudo-law. Crop yields under proper agrotechnical management and mineral fertilization are constantly increasing.

The "humus theory" was finally refuted by the experiments of W. Knop and J. Sachs (1859). They confirmed that it is entirely possible to grow a normal plant in water until full maturity by providing it with only seven elements: nitrogen, phosphorus, sulfur, potassium, calcium, magnesium, and iron. These experiments solidified The Theory of mineral nutrition and created the basis for using the vegetation method, including water and sand cultures. The nutrient solution developed by Knop is still used today.

J. B. Boussingault, using the vegetation pot method and precise quantitative measurements, proved that higher plants, as a rule, cannot fix atmospheric nitrogen. Leguminous plants possess this ability. Thus, it was first established that legumes, unlike most other plants, contribute to the accumulation of nitrogen in the soil. The German botanist and microbiologist H. Hellriegel proved in 1880 that leguminous plants perform Nitrogen Fixation in Symbiosis with nodule Bacteria. The bacteria themselves in the nodules of legumes were first discovered by the Russian botanist M. S. Woronin in 1866.

Significant research on the biological processes occurring in the soil was conducted by S. N. Winogradsky, who is rightfully considered the founder of soil microbiology. It is currently known that A wide variety of microorganisms live in the soil and transform nitrogen-containing compounds:

1) ammonifiers, which decompose organic nitrogenous compounds (proteins, nucleic acids, urea, etc.) with the release of ammonia;

2) nitrogen fixers — microorganisms that fix molecular nitrogen;

3) nitrifiers, which use oxygen to oxidize ammonia to nitrates;

4) denitrifiers, which convert nitrates into molecular nitrogen. In the absence of O₂, denitrifiers use the oxygen from nitrates, thereby depleting the soil and returning nitrogen to the atmosphere.

In addition to bacteria involved in the transformation of Various Forms of nitrogen, the soil contains bacteria that decompose Cellulose, process various sulfur and phosphorus compounds, and silicate bacteria that participate in the release of potassium from soil silicates, among others. Some microorganisms provide plants with Vitamins and amino acids, so the role of soil microorganisms is much greater and more complex than simply participating in mineral substance METABOLISM.

Russian scientists P. A. Kostychev and V. V. Dokuchaev developed the foundations of scientific soil science. The Soviet agricultural chemist K. K. Gedroyc substantiated the theory of the soil adsorption complex. Substances, including mineral ones, are held in the soil in various ways: mechanically, through physical interactions, and through chemical and biological binding. Gedroyc attached particular importance to physicochemical, or exchange, adsorption, which is carried out by inorganic (zeolitic) and organic (humic) Components of the soil. He established that acidic groups of both organic and inorganic (silicate groups) PARTS OF THE soil play a major role in exchange adsorption. Soils are primarily cation exchangers, although they also contain anion-binding groups.

All these studies led to a clear understanding that soil fertility is linked both to the Specific characteristics of the parent rock (mineral composition and structural state of the soil) and to The activity of soil microorganisms that mineralize organic residues.