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Video: Why Lime Soils
Liming is currently considered not only as a means to destroy acidity, but also as a way to mitigate many of the unfavorable properties of soils.
Many people thought that liming was a simple technique: "The soil is acidic - add lime"! It turned out that this is not entirely true. Liming should be carried out depending on the soil's need for lime, on the mechanical composition, the absorption capacity of this soil, on the cultivated crop, technogenic soil pollution, the phytotoxicity of aluminum, manganese and iron, on the introduction of organic and mineral fertilizers.
Liming is also called chemical melioration, a method of radical improvement of all soil properties with an acidic reaction of the environment. In addition, liming is also the introduction of calcium and magnesium to improve plant nutrition with these elements. And in order for gardeners to better understand this, today we will talk in detail about all aspects of liming.
In agriculture, liming began to be used for a very long time. Even the farmers of Gaul and the British Isles during the Roman rule (about 2000 years ago) used marl and chalk in their fields, meadows and pastures. In the XVI-XVIII centuries. liming of soils was widely used in all countries of Western Europe. However, at that time they did not yet know the nature of the action of lime and considered it as a means of replacing manure. Very high doses were often applied and liming was repeated too often, which sometimes led to negative results. The conscious use of lime to eliminate soil acidity began only in the last century.
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The dacha plots of Petersburgers are located mainly on acidic soddy-podzolic or peat soils, where it is impossible to obtain high yields of agricultural crops without the introduction of lime, even with the use of organic and mineral fertilizers.
Acidic soils are characterized by the presence in them in the absorbed state of a large number of ions of hydrogen, aluminum and manganese, which sharply worsen the physical, physicochemical, biological properties and, in general, fertility. Therefore, for the radical improvement of such soils, chemical amelioration is necessary in combination with other agricultural techniques, including the introduction of organic and mineral fertilizers. Liming is based on a change in the composition of absorbed cations, mainly by introducing calcium and magnesium into the soil absorbing complex of these soils.
Most cultivated plants and soil microorganisms develop better with a slightly acidic or neutral reaction of the medium (pH 6-7). Alkaline and overly acidic reactions have a negative effect on them. However, different plants have different attitudes towards the reaction of the environment - they have an unequal pH range, favorable for their growth and development, have different sensitivity to deviation of the reaction from the optimal one.
Five groups of plants can be distinguished:
1. The most sensitive to acidity: beets, cabbage, currants. They grow well only with a neutral or slightly alkaline reaction (pH 7-8) and respond very strongly to the introduction of lime even on weakly acidic soils.
2. Sensitive to acidity: beans, peas, broad beans, carrots, celery, sunflowers, cucumbers, onions, apples, plums, cherries. They grow better with a slightly acidic or neutral reaction (pH 6-7) and respond well to liming.
3. Weakly sensitive to acidity: rye, timothy, tomato, radish, raspberry, strawberry, pear, gooseberry. These cultures can grow satisfactorily in a wide range of pH 4.5-7.5, but the most favorable for their growth is a weakly acidic reaction (pH 5.5-6.0). They respond positively to high doses of lime. The positive effect of liming on the yield of these crops is explained not so much by a decrease in acidity, but by an increase in the mobilization of nutrients and an improvement in plant nutrition with nitrogen and ash elements.
4. Insensitive crops: potatoes. It needs liming only on highly acidic soils. Grows well in slightly acidic soils. When high doses of lime are introduced and the reaction of the medium is brought to neutral, the potato reduces its quality - it is heavily infected with scab. The negative effect of increased doses of lime is explained not so much by the neutralization of acidity as by a decrease in the assimilable boron compounds in the soil, as well as by a violation of the ratio of cations in the soil solution. Excessive concentration of calcium ions makes it difficult for the plant to enter other ions, in particular magnesium, potassium, ammonium, copper, boron, zinc, and phosphorus.
5. Insensitive crops: rhubarb, sorrel, radish, turnip. They grow better on acidic soils (optimal pH 4.5-5.0) and poorly with an alkaline and even neutral reaction. These crops are sensitive to an excess of water-soluble calcium in the soil, especially at the beginning of growth, so they do not need liming. However, when applying low doses of lime fertilizers containing magnesium, the yield of these crops does not decrease.
The influence of an acid reaction on plants is very complex and multifaceted. Hydrogen ions, penetrating in large quantities into plant tissues, acidify the cell sap, change the course of all biochemical processes. The growth and branching of roots, the physicochemical state of the plasma of the root cells, the permeability of the cell walls deteriorate, the use of nutrients from the soil and fertilizers by plants is sharply disrupted. With an acid reaction, the synthesis of protein substances is weakened, the content of protein and total nitrogen decreases, the amount of non-protein forms of nitrogen increases; the process of converting monosaccharides into other, more complex organic compounds is suppressed.
Plants are most sensitive to soil acidity during the first period of growth, immediately after germination. At a later date, they tolerate it relatively easily. The acid reaction in the first period of growth causes severe disturbances in carbohydrate and protein metabolism, negatively affects the laying of the generative organs, which is reflected in the subsequent process of fertilization, while the yield drops sharply.
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In addition to the direct negative effect of the increased concentration of hydrogen ions on plants, soil acidity has a multifaceted indirect effect. Hydrogen, displacing calcium from the soil humus, increases the dispersion of the latter and mobility, and the saturation of mineral colloidal particles with hydrogen leads to their destruction. This explains the low content of colloidal fraction in acidic soils, unfavorable physical and physicochemical properties, poor structure, low absorption capacity and weak buffering. Microbiological processes useful for plants in acidic soils are suppressed, therefore, the formation of forms of nutrients available to plants is weak.
Different soil microorganisms also differ in their attitude to soil acidity. Molds thrive best at pH 3-6 and can grow even at higher acidity. Among the fungi, there are many parasites and pathogens of various plant diseases. Their development in acidic soils is enhanced. At the same time, many beneficial soil microorganisms develop better with a neutral and slightly alkaline reaction. The most favorable pH value for nitrifiers, nitrogen-fixing bacteria freely living in the soil (azotobacter, clostridium) and nodule bacteria of alfalfa, peas and other legumes is 6.5-7.5. At a higher acidity, the vital activity of nitrogen-fixing microorganisms is suppressed, and at a pH below 4-4.5, many of them cannot develop at all.
Therefore, in acidic soils, the fixation of nitrogen in the air is greatly weakened or completely stops, the mineralization of organic matter slows down, the nitrification process is suppressed, as a result of which the conditions for nitrogen nutrition of plants are sharply deteriorated. In acidic soils, mobile forms of phosphorus are bound by sesquioxides to form insoluble and inaccessible to plants phosphates of aluminum and iron. As a result, phosphorus nutrition of plants deteriorates. With increased acidity, molybdenum passes into poorly soluble forms, and its availability to plants decreases. On strongly acidic sandy and sandy loam soils, plants may lack the assimilable compounds of boron, molybdenum, calcium and magnesium.
The negative effect of aluminum on many plants is noted when its content in the solution is more than 2 mg per 1 liter. At a higher concentration of aluminum, the yield sharply decreases and even plant death is observed. First of all, the root system suffers from an excess of this element. The roots become shortened, coarse, darken, slick and rot, the number of root hairs decreases. The aluminum supplied to the plant is mainly fixed in the root system, while manganese is evenly distributed throughout all plant organs.
Excessive intake of aluminum and manganese disrupts carbohydrate, nitrogen and phosphate metabolism in plants, negatively affects the laying of reproductive organs. Therefore, the negative effect of an excess of these elements is more pronounced on the generative than on the vegetative organs. Plants are especially sensitive to mobile forms of aluminum and manganese during the first period of growth and during overwintering. With an increased content of them in the soil, the winter hardiness of perennial crops decreases sharply, most of the plants die. Only a few plants tolerate increased concentrations of mobile aluminum without harm.
In relation to aluminum, four groups of plants are distinguished: highly resistant - oats and timothy; medium-hardy - lupine, potatoes, corn; moderately sensitive - flax, peas, beans, buckwheat, barley, spring wheat, vegetables; highly sensitive to excess aluminum - beets, clover, alfalfa, winter wheat and rye. Inhibition of clover is observed even when the content of mobile aluminum in the soil is more than 2 mg per 100 g of soil, and at 6-8 mg, for example, clover falls out strongly.
A strict parallelism is not always observed between the sensitivity of plants to an acidic reaction of the environment and to mobile forms of aluminum. Some plants do not tolerate soil acidity (corn, millet), but are relatively resistant to aluminum, while others grow satisfactorily with an acid reaction (flax), but are very sensitive to aluminum. The different sensitivity of plants to mobile forms of aluminum is associated with their unequal ability to bind this element in the roots. Plants are more resistant to aluminum, capable of fixing it in the root system, as a result of which it does not enter the growth points and fruits.
Under soil conditions, it is often impossible to distinguish between the negative effect of the mobile forms of aluminum and manganese on plants, or the negative effect of the increased concentration of hydrogen ions in solution. You just need to remember that with a high content of aluminum and manganese compounds in the soil, the negative effect of acidity on plants is much stronger.