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MOST IMPORTANT SCIENTIFIC DISCOVERIES
The law of the minimum. History and essence of scientific discovery
Directory / The most important scientific discoveries All animals, as well as man, eat food of either plant or animal origin. Therefore, the question of where plants get their nutrition from is one of the questions of the greatest importance. “The best researchers have been thinking about this issue for a long time,” writes Z. Shpausus. “For a long time, attention has been drawn to the fact that a plant during its life grows from an insignificant seed grain to its normal size and at the same time a huge weight gain is found. Aristotle believed that plants absorb from the soil the necessary materials for their construction in their final form, so that there is no need for any transformation of these materials inside their body.In 1600, Van Helmont, by his experience, was able to prove the incorrectness of these assumptions.He weighed 200 pounds of dry earth and stuck into it a branch of willow, the weight of which was equal to 5 pounds.When plentifully watered with water, this branch manifested itself as a whole willow: it took root and over the next five years grew into a decent tree weighing 164 pounds. Particularly surprised Van Helmont is the fact that the earth lost only 60 grams of its original weight. Thus, the earth could by no means be recognized as the sole source of nourishment for the growing tree, for in this case the 159 pounds of weight gain from the willow branch would have to correspond to an equal loss in the weight of the earth. Ingenhaus and de Saussure, at the end of the XNUMXth century, were the scientists who first developed the modern theory of plant nutrition, according to which plants absorb carbon dioxide from the air, which results in a greater increase in the dry matter weight of plants than would be expected based on the quantities actually the carbon dioxide they take in. Therefore, we have to assume that new organic matter is formed from carbon dioxide and water. These scientists already at that time believed that the presence of certain salts in the soil was also necessary. However timely and correct in many respects these conclusions may have been, they were nevertheless forgotten at the beginning of the XNUMXth century and were replaced by the humus theory, which mainly goes back to Thayer, who was its most zealous defender. The point of view of Thayer, the founder of the doctrine of crop rotation, was that the fertility of the soil depends solely on humus. That is the only source that supplies plants with nutrients. Humus - loose dark earth - contains a lot of carbon - the main component of all plants. According to the defenders of the humus theory, it contains all the substances necessary for plant life in an already prepared form. Salts are not, in their opinion, particularly important, so that their origin and significance should not be thought much about. Humus and water are the sources of plant nutrition. This teaching was so clear and convincing that for a long time no one doubted its validity. One of those who nevertheless doubted him was the young chemistry professor Justus Liebig (1803-1873). Based on the facts collected earlier and at the same time on the results of his work, Liebig initiated a new era in agriculture. In his book "Agricultural Chemistry", published in 1840, Liebig first of all investigated what constituent parts a plant builds its organism from and where it obtains these substances from. “Based on numerous analyses,” writes Z. Shpausus, “he was able to establish that each plant contains ten elements that are all of the greatest importance for its normal growth. These are the following elements: carbon, hydrogen, oxygen, nitrogen, calcium, potassium , phosphorus, sulfur, magnesium and iron. Let us add that at present a number of elements are known that are present in plants only in the form of traces, but, nevertheless, play an important role in their life activity. Naturally, all these substances are contained in plants, not in the form in which they are known as chemical elements, but they are constituents of the compounds from which the plant is built.Where do plants get these substances? We have already seen that the carbon taken up by the leaves in the form of carbon dioxide comes from the atmosphere, while the water supplies the plant with hydrogen and oxygen. But what about nitrogen, which is an integral part of the proteins necessary for life? True, nitrogen is contained in the atmosphere in enormous quantities, because it is 78 percent of this element, but only a few plants are able to absorb and use nitrogen from the air. Such plants include the so-called leguminous plants, including beans, peas and lupins. It is easy to make sure that nodules hiding bacteria inside can be found in their roots. Nodule bacteria have the ability to convert nitrogen from the air into organic nitrogenous compounds, which can then be absorbed by the corresponding plants. The plant makes it possible for bacteria to live, and for this they prepare nitrogen available for assimilation for their hosts. This process of mutual assistance is referred to in biology as symbiosis. However, this process is only an exception. The vast majority of plants must draw nitrogen compounds directly from the soil, because they cannot directly absorb nitrogen from the air. Liebig was of the opinion that gaseous ammonia, which is formed during the decay of organic compounds and therefore always present in an insignificant amount in the atmosphere, is quite sufficient to cover the nitrogen needs of plants. Ammonia dissolves in raindrops, reacts with carbon dioxide to form ammonium carbonate, and in the form of the named salt enters the soil, from which it can be absorbed by plant roots. The remaining six elements are found as salts in the soil. Being dissolved in water, they can penetrate plants through their roots. True, they are present in the soil in a limited amount, but animals and plants, when their remains decay, return to the soil those salts that they received from it during their growth. After that, salts can again serve as nutrients for plants. This is the end of the cycle linking the dead and living nature. The plant takes inorganic substances from the soil and from the air and builds its own organism from them, consisting of organic compounds. This plant matter is the food of animals and humans, and in physiological secretions, and after death in the form of corpses of these creatures, it enters the soil and turns into inorganic starting materials. And in this cycle, plants play the main role, because only they are able to use inorganic building materials. Thus, the ten elements are essential for plant life. The absence of one is enough for the plant to die. Soil fertility always depends on the element that is in the soil in the minimum amount. This is a law which is of the utmost importance for practical agriculture. Liebig called this law "the law of the minimum". Of course, we must not forget that, along with nutrient salts, there are also a number of other factors, such as the water regime of the soil, temperature, etc., which also affect soil fertility. But how to explain the ever-decreasing fertility of arable land? Liebig explains in detail. If the farmer returns to the soil in the form of manure all the nutrients that have been extracted from the soil by plants, then the content of nutrient salts in the soil will remain the same and the fertility of his plot will not decrease. However, if he sells part of his products to the city, then the nutrient salts will be lost for his site and next year they will no longer be at the disposal of the plants growing on this site. If this process is repeated year after year, the yields will have to worsen every year. Liebig argued: “The basic principle of agriculture should be considered the requirement that the soil be fully returned to everything that was taken from it. In what form this return will be made, whether in the form of animal excrement or in the form of ash or bones, is more or less indifferent. The time is coming when the arable land and each plant will be provided with the necessary fertilizer for it, which will be produced at chemical plants. These words of Liebig were justified a thousand times over the past time, but in his era they repeatedly served as a pretext for mockery and witticisms. "I'll tell you what, colleague: I am again convinced that before me lies the most shameless book of all that has ever fallen into my hands. Are you, in fact, familiar with its contents?" von Mol, a professor at the University of Tübingen, assessed with great irritation the book of Liebig that lay before him. “It turns out that the plant world no longer owes its nutrition to the earth, no, plants feed on air, water and the so-called nutrient salts that they look for in the soil! It is amazing how he still finds at least some explanation for the need to cultivate the land. But maybe, he will even come to the conclusion that the farmer does not need the land at all and that the peasant will be able to grow his grain in glass vessels. Look, in this newspaper he can read the only correct answer to his nonsense! Fritz Reuter openly mocks Liebig in his essay “My Way of Life”: “And this era was marked by a significant development of agriculture. Professor Liebig published a completely meaningless book for the peasants ... One could really go crazy with these terms. who was ready to be left penniless, following all the advice contained in this book, and who at the same time wanted to stick his nose into science, he acquired this book for himself and sat over it until his head gradually became fooled by its content, and when he reached such a state, he began to wonder whether gypsum was an irritating or nourishing substance (for clover, not for man!) it is, by its very nature, a smelly substance." If there is not enough natural fertilizers, it is necessary to apply mineral fertilizers to the soil to cover the costs of nutrient salts. So Liebig reasoned about the production of his "patent fertilizer". The plant produces carbon, hydrogen and oxygen in sufficient quantities naturally. Liebig considered it possible to say the same about nitrogen. Magnesium, iron and sulfur are needed only to a small extent by plants, and they are present in the soil in a very significant amount. The introduction of calcium fertilizers is not very difficult, because calcareous marls are found in exceptional abundance. The situation is different with potassium and phosphorus. In this regard, the nutrient reserves of the soil must be replenished with fertilizer salts. Both of these elements are also contained in Liebig's "patent fertilizer". One English firm undertook the production of this fertilizer on a large scale. However, in the fields fertilized with these salts, no significant increase in yield was noted. Is it possible that mineral salts do not affect the growth of plants in any way, is his teaching erroneous? These were hard times for Liebig and his supporters to endure. Many years passed before Liebig understood the reason for the failure of his fertilizer. In the production of "patent fertilizer" he sought to convert his potash and phosphorus fertilizers into the form of water-insoluble compounds. Thus, Liebig wanted to avoid that his fertilizer salts were washed out of the soil into its deeper layers already at the first rain. But by converting these salts into water-insoluble compounds, he only made them indigestible for plants, since plants can only absorb dissolved salts. Thanks to this, all fertilizers were introduced in vain. Having understood the reason for the negative results of applying such fertilizers, the scientist corrected the mistake. Liebig also had to admit that he was wrong in assuming that the amount of gaseous ammonia in the air was sufficient for plant growth. Potassium, phosphorus, nitrogen and lime - this is what the formula should now say, on which the increase in soil fertility depends. Even during his lifetime, Liebig had the opportunity to establish with satisfaction that his doctrine of fertilizer salts was universally recognized. More and more asserted the belief in the need to apply artificial fertilizers to arable land. Experiments have undeniably shown that fertilized arable land brings much better yields. Author: Samin D.K.
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