Soil acidity, types, indicators and influence on plants

silicates and humates, the change in its reaction is the first thing that catches the eye.

Conditions for plant growth largely depend on the soil reaction. The intensity of microbiological processes, mineralization of organic substances, dissolution of poorly soluble compounds, and various physicochemical processes are also largely determined by the soil reaction. It affects the effectiveness of fertilizers, which, in turn, can acidify or alkalize the soil solution.

The soil reaction manifests itself during the interaction of cations in the soil solution with exchange-adsorbed cations of the solid phase of the soil. Depending on the reaction of the soil solution, soils can be acidic or alkaline.

Soil acidity is a soil property caused by the presence of hydrogen ions in the soil solution and exchangeable hydrogen and aluminum ions in the soil adsorption complex. There are two types: active and potential acidity. The latter is subdivided into two forms – exchangeable and hydrolytic.

Active acidity refers to the acidity of the soil solution. It has a direct effect on plant life and can be favorable or unfavorable for them. Active acidity is caused by the presence of carbonic acid, organic acids, and hydrolytically acidic salts in the solution. It is usually determined by measuring the pH of a water suspension or a water extract from the soil.

Depending on the pH value, the reaction of the soil solution is classified:

The highest acidity of the soil solution occurs in sphagnum peats – pH 3.5. Sod-podzolic and some peat soils have an acidic or strongly acidic reaction (pH 4–5); in leached chernozems and gray forest soils, the reaction is slightly acidic (pH 5.5–6.5). The reaction of the solution in ordinary and thick chernozems is close to neutral (pH 6.5–7).

Potential acidity. This is the acidity of the solid phase of the soil, caused by the presence of exchange-adsorbed hydrogen and aluminum ions. It is believed that potential acidity is also caused by iron and manganese cations. There are two forms of potential acidity – exchangeable and hydrolytic.

Exchangeable acidity is the acidity of the solid phase of the soil caused by exchange-adsorbed cations of hydrogen, iron, aluminum, and manganese, which manifests itself under the action of neutral salt solutions.

It does not have a direct effect on plant life, but it influences active acidity when interacting with cations of the soil solution. For example:

SAC  4 KCl ⇄  SAC 3.

4K

As a result of the displacement of hydrogen ions from the soil adsorption complex, the soil solution becomes acidified, that is, the exchangeable acidity decreases, while the active acidity increases and affects plant growth.

Aluminum chloride in the solution undergoes hydrolytic dissociation with the formation of a weak base and a strong acid, which also acidifies the soil solution:

AlCl 3  3Н 2О 3  3HCl.

Exchangeable acidity is determined by shaking a soil sample with a 1N KCl solution. The pH is determined in the suspension, or the extract is titrated and the acidity is calculated in mmol-eq/kg.

By pH value, soils are divided into:

– very strongly acidic <4.0

– close to neutral 5.6–6.0

Exchangeable acidity is significant (strongly acidic and acidic) in sod-podzolic soils and red soils, while it is practically absent in chernozems. With a slightly acidic reaction of the soil solution (low active acidity), exchangeable acidity is low or absent.

It should be remembered that the hydrogen exponent, or pH, is the negative decimal logarithm of the activity of hydrogen ions in the solution. Arithmetic operations cannot be performed with it. Converting pH to H+ concentration is done using the mantissa of logarithms.

Hydrolytic acidity. During tillage with a neutral salt solution, not all adsorbed hydrogen and aluminum ions are displaced. Potential acidity is revealed more fully when treating the soil with a solution of a hydrolytically alkaline salt (hence the name of this form of potential acidity). Usually, sodium acetate is used for this:

   

 SAC 3 SAC 3COOH

   

Hydrolytic acidity is the acidity of the solid phase of the soil caused by firmly fixed hydrogen ions, detected by the action of a hydrolytically alkaline salt solution on the soil.

The acidity value is determined by titrating the 1N sodium acetate extract with an alkali and is expressed in mmol-eq/kg of soil, and in some cases, potentiometrically, expressed as a pH value.

Potential acidity determined in this way includes both H+ and Al3+ ions displaced by the neutral salt solution KCl, as well as H+ ions of the soil solution. Therefore, it is greater than the exchangeable acidity, which is usually not subtracted from the total acidity obtained in the sodium acetate extract.

Sod-podzolic soils are characterized by significant hydrolytic and strongly expressed exchangeable acidity. Leached chernozems have low hydrolytic acidity and small exchangeable acidity.

Degree of soil base saturation. For soil properties, not only the absolute value of acidity is important, but also its share of the exchange capacity, which is called the degree of base saturation.

The degree of soil base saturation is the ratio of the sum of exchangeable bases to the sum of hydrolytic acidity and the sum of exchangeable bases. It is calculated using the following formula:

V  100, or V   100,

Т S  Hг where: V – degree of base saturation, % of cation exchange capacity;

S – sum of exchangeable bases, mmol-eq/kg of soil;

Т – exchange capacity, mmol-eq/kg of soil;

Hг – hydrolytic acidity, mmol-eq/kg of soil.

• Base saturation range — from 5 to 100%
• Strongly alkaline soil reaction — pH > 8.5
• Amelioration of acidic soils — liming
• Amelioration of alkaline soils — gypsuming

Base saturation and alkalinity: how they affect the soil

The degree of base saturation determines what share of the soil exchange capacity is occupied by exchangeable cations. This indicator is critical when planning chemical amelioration. With the same amount of absorbed hydrogen, soils behave differently depending on the total exchange capacity. The higher the hydrolytic acidity, the more lime will be required for neutralization, even with the same degree of base saturation.

An alkaline reaction of the soil solution is just as harmful to crops as increased acidity. It inhibits plant growth and reduces the activity of beneficial soil microflora. In practice, it is customary to distinguish between actual and potential alkalinity. Actual alkalinity is determined by the composition of the soil solution, changes rapidly, and directly affects the crop.

It is caused by dissolved hydrolytically alkaline salts — sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃), and others. Upon dissociation, they create a high concentration of hydroxide ions. Actual alkalinity is expressed via the pH of an aqueous suspension or in mmol-eq/kg of soil based on titration results.

Na₂CO₃ + 2H₂O = H₂CO₃ + 2Na⁺ + 2OH^-

Potential alkalinity is what is caused by exchangeable adsorbed sodium fixed in the soil adsorption complex (SAC). It manifests during the interaction of the solid phase of the soil with the carbonic acid of the soil solution. As a result of the exchange reaction, soda is formed, which sharply alkalizes the environment:

[SAC]₂Na + H₂CO₃ = [SAC]₂H + Na₂CO₃

Acid-base buffering: how soil resists pH changes

The reaction of the soil solution in the field is unstable and constantly changes under the influence of external factors. It is influenced by the release of carbon dioxide by plant roots, the formation of nitric acid during nitrification, and the application of mineral fertilizers. However, the soil is able to resist these changes due to its buffering capacity. Buffering depends on the properties of both liquid and solid phases of the soil.

The buffering action of the soil solution is provided by weak organic and mineral acids, as well as their salts. For example, weak carbonic acid partially dissociates into hydrogen ions and bicarbonate ions. When alkali enters the solution, free hydrogen ions bind it, keeping the pH level at the initial point. A mixture of carbonic acid and its salts, such as calcium bicarbonate, similarly protects the soil from acidification:

Ca(HCO₃)₂ + 2HNO₃ = Ca(NO₃)₂ + 2H₂CO₃

The main role in protecting against changes in the environmental reaction is played by the solid phase of the soil. In this process, the quantity and composition of exchangeable cations are critical. In chernozems, rich in absorbed calcium and magnesium, these elements actively exchange for hydrogen ions of the soil solution. Thanks to this, incoming acids are neutralized and a stable pH level is maintained:

[SAC]Ca + 2H₂CO₃ ⇄ [SAC]₂H + Ca(HCO₃)₂

Soils not saturated with bases (for example, sod-podzolic soils and red soils) possess buffering against alkalization. When alkaline components are applied, alkali cations exchange for hydrogen ions from the SAC, which also prevents a sharp shift in environmental pH. The process proceeds according to the following scheme:

[SAC]₂H + Ca(OH)₂ ⇄ [SAC]Ca + 2H₂O

When working with such soils, it is important for an agronomist to remember two patterns:

• The higher the hydrolytic acidity of the soil, the stronger its buffering action against alkalization.
• Buffering properties determine the permissible doses and frequency of fertilizer application to prevent a sharp change in pH.

Soil buffering capacity directly determines the safety of mineral fertilizer application. Ignoring this property when planning nutrition can lead to sharp pH fluctuations in the root zone and crop suppression. The risk of negative fertilizer impact depends directly on the particle-size distribution and the field's organic matter content.

Heavy soils rich in humus possess a high exchange capacity and, consequently, high buffering capacity. In such fields, the soil solution reaction shifts only slightly even when high rates of mineral fertilizers are applied.

How to regulate soil resistance to pH changes

The direction of soil resistance depends on its base saturation. Soils with a high degree of base saturation successfully resist acidification. If the degree of base saturation is low, the soil complex will be more resistant to alkalization.

In practical agriculture, an agronomist can purposefully increase the buffering capacity of fields, protecting plants from sharp chemical stresses. The following technological methods are used for this purpose:

• regular application of organic fertilizers;
• liming of acidic soils.