The acidity and alkalinity of fertilizer

Many farmers purchase and apply chemical fertilizers, basically ignoring the acidity and alkalinity of the cultivated soil and the acidity and alkalinity of chemical fertilizers. They only consider the nutrient content of chemical fertilizers and the ratio of nitrogen, phosphorus and potassium nutrients, resulting in poor fertilization effects. In fact, mastering the acidity and alkalinity of fertilizers is of great help to rational fertilization.

Acid-base reaction characteristics of fertilizers

Commonly used fertilizers are salts composed of bases and acid radicals, which basically have specific chemical properties. On the other hand, after fertilization, it is affected by soil environmental factors, such as the selective absorption of nutrients by roots and microorganisms, the decomposition and transformation of molecular structures of enzymes secreted by microorganisms, the adsorption of cations and repulsion of anions by soil particles, and the dissolution and desalination of rainwater. etc., will have a significant influence on the vegetative growth of plants and the change of soil chemistry.

Chemical acid-base reaction of fertilizers

After the fertilizer is dissolved in water, due to the relative strength of its base and acid radical, the pH value of the fertilizer aqueous solution will be different in acidic or alkaline reactions. According to the level of pH value, fertilizers can be divided into acidic fertilizers, neutral fertilizers and alkaline fertilizers. From the strength of the base and acid radicals of the fertilizer components, it can also be judged whether the chemical properties of the fertilizer are neutral, acidic or alkaline. For example, ammonium sulfate is composed of strong acid and weak base, so its aqueous solution is slightly acidic; potassium sulfate is composed of strong acid and strong base, So its aqueous solution is neutral; Potassium carbonate is composed of weak acid and strong base, so its aqueous solution is slightly alkaline. Some fertilizers have some free acid left in the manufacturing process. For example, superphosphate often contains free sulfuric acid, and its aqueous solution is also slightly acidic.

Fertilizers have the characteristics of chemical acid-base reactions, so when mixing fertilizers, attention should be paid to the heat generation caused by the mixture of acid and alkali, and the deamination caused by the mixture of alkali and alkali.

Acid-base reaction of fertilizers on plant physiology

After the fertilizer is applied to the soil, the acid-base reaction in the soil solution is slightly different from the acid-base reaction of the simple fertilizer aqueous solution. The chemical acid-base reaction of fertilizers is affected by the buffering capacity of soil colloidal particles, and the impact on the acid-base changes of the soil solution in a short period of time will not be great. Instead, it is due to the unbalanced absorption of fertilizer salt ions by plant roots In the future, the pH of the soil solution will change significantly.

Generally speaking, after the cations in the fertilizer components are absorbed by the root system, the remaining nitrate (NO3-), hydrochloride (Cl-), sulfate (SO42-), etc. have the same load as the soil colloid, so it is easy to With the soil gravity water is discharged out of the soil, but at the same time, the basic cations adsorbed by the soil, such as calcium ions (Ca2+), magnesium ions (Mg2+), potassium ions (K+), etc. will also be dragged along with the loss, eventually to soil acidification.

In addition, after ammonium nitrogen fertilizers, such as ammonium sulfate, are applied to the soil, part of the ammonium nitrogen will be absorbed by plants, and the rest will be converted into nitric acid by nitrifying bacteria, so from then on, its process in the soil is like Common nitrate fertilizers can also cause soil acidification.

The acid radicals of fertilizers are lost with water in the soil, resulting in the loss of bases together and the severity of acidification of the soil. Take hydrochloride and sulfate as 1, nitrate as 1/2, and phosphate as 1/3. According to the above, after the fertilizer is applied, it can be classified into physiologically acidic fertilizers, physiologically neutral fertilizers and physiologically alkaline fertilizers in Table 2 according to the changes in soil acidity and alkalinity.

Note 1. Calcium superphosphate has little effect on the pH value when it is applied in acidic soil, but in alkaline soil, it behaves like a physiologically acidic fertilizer due to the side component of gypsum.

Note 2. The performance of urea in dry fields is an acidic fertilizer, the main reason is because of nitrification. The performance of urea in paddy fields is due to the base brought in by irrigation water and the dilution of a large amount of water, so the response to soil acidity and alkalinity is classified as a neutral fertilizer.

Nitrogen fertilizer and potassium fertilizer are two of the three-element fertilizers with the largest amount of fertilizer demand and usage, especially ammonium sulfate, ammonium chloride and potassium chloride, which are physiologically acidic fertilizers. The continuous application of these fertilizers will cause soil acidification main reason.