Soil pH often affects plant growth and fertilization. It is an indicator of soil fertility.
Acid refers to the ease of releasing protons to other substances, ie hydrogen ions (H
) The material. Base refers to a substance that is prone to protons. When the acid is mixed with water, hydrogen ions and ionized anions (also called acid radicals) are dissociated or ionized. Assuming acid HA, its accompanying anion is A-, which dissociates in aqueous solution as:
Where H is the active acidity, H
The greater the activity, the stronger the acid. Nitric acid (HNO3), sulfuric acid (H2SO4), and hydrochloric acid (HCl) are all strong acids, and carbonic acid (H2CO3), boric acid (H3BO3), various organic acids such as acetic acid (CH3COOH, or expressed as HOAc) and the like are weak acids. Alkaline substances can neutralize acids. Neutralization refers to the reaction of acid and base to form salt and water:
HA BOH -> BA H2O (B is a salt cation, OH is a hydroxide ion, also called hydroxyl)
Strong acids can dissociate almost completely. While the weak acid can only partially dissociate in aqueous solution, a limited amount of H is released. When these Hs are neutralized with alkaline substances, the dissociation reaction proceeds in the right direction, dissociating the number of neutralized Hs.
, Re-neutralize, and dissociate until the weak acid is completely neutralized. Therefore, weak acid not only has active acidity, but also potential acidity. The sum of the two acidities is called total acidity. H
The activity (ie, the concentration of active acid) is in moles per liter (mol/L). However, it is usually expressed in terms of pH. The pH is defined as the logarithm of the reciprocal of the H activity (AH):
Assuming an H activity of 0.0001 mol/l, calculated according to the above equation, the pH is 4.0. The number of pH changes by 10 times for every unit of pH change. To neutralize these H
The amount of basic substances required has increased dramatically.
The weak acid and the weak acid salt can form an acid-base buffer system, which means that the small amount of acid or alkali added to the system can keep the pH of the solution within a narrow range, that is, the ability to resist the change of the pH value. For example, a mixture of acetic acid and sodium acetate, sodium acetate highly dissociated, producing a certain concentration of acetate ions, the ion inhibits the dissociation of acetic acid molecules. If a small amount of acid is added to the system, the acetate ions react with the hydrogen ions to form acetic acid molecules; if a small amount of sodium hydroxide (alkali) is added to the system, the hydroxide ions and the hydrogen ions in the solution generate water, and the acetic acid solution is solved. From the hydrogen ion added to neutralize, the activity acidity of the system is almost constant.
Soil pH is represented by soil pH, the degree to which the soil is saturated with hydrogen ions. The soil acidity mainly comes from humus or organic matter in the soil, aluminosilicate clay, iron-aluminum hydrous oxides, exchangeable aluminum, soluble salts and carbon dioxide. The acidic substances in the soil are almost all weak acids and therefore have a certain buffer capacity. In addition to soil pH and soil parent material and organic matter, but also related to rainfall, the soil through the flow of the alkaline elements of calcium, magnesium, etc. after leaching out, the soil acidity increased. Harvesting crops will also take away a certain amount of calcium and magnesium. Leguminous plants have higher calcium and magnesium content than non-legumes. The nitrogen fixation of leguminous rhizobia can also acidify the soil. Fertilization, especially nitrogen fertilizers, will make the soil acidified, which is harmful in acidic soils, and this may be beneficial in calcareous soils or soils that are deficient in iron, manganese or other trace elements. Reducing soil pH values ​​can make these Elemental effectiveness increases.
If the soil acidity is too high, ie when the pH of the soil is too low, one or more harmful effects may affect the growth of the crop: (1) the levels of toxic aluminum and manganese elements; (2) organisms that decompose organic matter and convert nitrogen, phosphorous, and sulfur. It may be low in quantity and low in activity; (3) Mg may also be lacking when the cation exchange capacity of the soil is extremely low; (4) Symbiotic symbiosis in leguminous plants is greatly reduced; (5) The viscous soil with strong acidity is poorly agglomerated; (6) Phosphorus and molybdenum The effectiveness of such nutrients is low. Many crops grow best in the pH range of 6.0 to 7.0, and some crops require acidic soil conditions.
The decomposition of organic matter or the application of ammonium nitrogen fertilizer will increase the acidity of the soil. When ammonium ions are converted into nitrate, H is released:
Soils with high acidity are generally treated with lime to achieve soil acidity suitable for crop growth. The effect of lime on the soil far exceeds the neutrality of the soil. It also improves soil physical properties, stimulates soil microbial activity, increases the availability of minerals to plants, provides plants with calcium and magnesium, and increases symbiotic nitrogen fixation in legumes.
When determining the amount of lime used, consider both the active acidity and the potential acidity. The method of measuring soil pH is simple, but only the activity acidity can be measured, and the amount of lime applied cannot be determined. Lime requirements relate not only to soil pH, but also to soil buffer capacity or cation exchange capacity. The cation exchange capacity (CEC) means that the cations held by the soil colloids can be replaced by other cations, or they can be exchanged. Exchanged by potassium or aluminum or hydrogen, and vice versa. The method usually used to determine the amount of lime used is to add the soil sample to a certain amount of buffer solution to change the pH of the solution. This amount of change is proportional to the original soil pH and buffering capacity. From this calculation, the lime requirement is calculated when the soil pH is increased to a certain level.
Commonly used lime materials are calcium carbonate, calcite limestone (crystalline calcium carbonate), dolomitic limestone (crystalline calcium carbonate magnesium), raw oyster shell, burnt oyster shell, marl, quick lime, slaked lime, steel slag phosphate, gypsum Wait. There are also some less commonly used lime materials.
In the rotation system with legume crops, lime should be applied 3 to 6 months before the leguminous planting. It is even more necessary to use lime for planting leguminous crops on acidic soils. If winter wheat is followed by leguminous crops (such as clover), it is best to apply lime when growing wheat. Lime should be thoroughly mixed with the soil. Water is an essential condition for the reaction of lime material and soil. Lime in dry soil has little effect on pH. Sandy soil should be applied in small quantities several times, and the amount of clay soil should be applied in large quantities and few times, and mix in 20 cm soil layer.
references:
1. Lime is a good management measure in acidic soils (Proceedings of International Symposium on Fertilizer and Agricultural Development)
2. Soil Acidity and Application of Lime (Soil Fertility and Fertilizer)
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