Soil acidity is a soil property due to the presence of hydrogen ions in the soil solution and exchangeable hydrogen and aluminum ions in the soil absorbing complex.
The pH interval of 5.5-7 corresponds to the most agronomically favorable soil structure, high quality of humus and optimal water regime.
Reaction of the soil solution medium
Reaction of the soil solution medium is the ratio of the concentration of H+ and OH-ions in the soil solution, expressed as the pH of the aqueous or saline extract. Fertilizers tend to change the reaction of the soil solution.
Soil reaction affects the nutrient regime of soils, plant growth, development and yields, the activity of soil microorganisms, the transformation of forms of nutrient elements of fertilizers and soil, agrophysical, agrochemical, physical and biological properties of soils. Fertilizers and ameliorants make it possible to regulate soil reaction in the desired direction for cultivated crops.
The reaction of the soil solution is determined by the concentration of hydrogen ions (H+) and hydroxide ions (OH–). In pure water with a neutral reaction, the concentration of hydrogen ions coincides with the concentration of hydroxide ion and is equal to 1⋅10-7 mol/L. When 1 mmol of hydrochloric acid and nitric acid are added to 1 L of water, which dissociate completely in aqueous solution, the concentration of hydrogen ions will be 1 mmol H+, or 1⋅103 mol/dm3. The concentration of hydrogen ions is expressed in terms of pH, which is equal to:
pH = -lg(CH+),
where CH+ is the concentration of hydrogen ions in the solution, mol/dm3.
In a solution with a neutral reaction, the concentration of hydrogen ions is 0.0000001 = 1 · 10-7 mol/dm3, or pH = 7.
According to the reaction of the medium (pH) soils are divided into:
- very strongly acidic – < 4.0 (pHsalt),
- very acidic – 4.1-4.5;
- moderately acidic – 4.6-5.0;
- slightly acidic – 5.1-6.0;
- neutral – 6.1-7.4;
- slightly alkaline – 7.5-8.5 (pH water);
- strongly alkaline – 8.6-10.0;
- sharply alkaline – >10.0.
The reaction of soil solutions can vary widely from pH = 3-3.5, which is typical for sphagnum peats and forest litter of sphagnum forests, to pH = 10-11 in saline soils.
Soils with neutral or near-neutral reaction are favorable for the majority of cultivated crops, but significant areas of agricultural lands are characterized by unfavorable reaction.
The soil solution contains carbon dioxide (more precisely, hydrogen ion H+ and hydrocarbonate ion HCO3–, since the compound H2CO3 under normal conditions does not exist), which is formed as a result of the activity of soil biota and the dissolution of atmospheric carbon dioxide in water. It has acidifying effect on soil solution:
H2O + CO2 = H+ + HCO3–.
The acidity created in this case is neutralized by absorbed bases and calcium and magnesium carbonates:
(soil)=Ca + 2H2O + CO2 = (soil)=H2 + Ca(HCO3)2,
CaCO3 + H2O + CO2 = Ca(HCO3).
In the presence of sodium in the soil absorbing complex, sodium hydrogen carbonate or sodium carbonate may be formed in solution:
(soil)-Na + 2H2O + CO2 = (soil)-H + NaHCO3,
(soil)=Na2 + 2H2O + CO2 = (soil)=H2 + Na2CO3.
Hydrocarbonates in solution undergo dissociation:
Ca(HCO3)2 + 2H2O = Ca(OH)2 + 4H+ + 2CO32-,
NaHCO3 + H2O = NaOH + 2H+ + CO32-.
Carbonates and hydrocarbonates of calcium, magnesium and sodium in aqueous solutions have an alkaline side.
The reaction of soil solution of different soils depends on the composition of absorbed cations and carbonate content. If sodium content is high in absorbing complex, for example in salts and saline soils, reaction of soil solution is determined by presence of sodium carbonate. In these soils it reaches 8-8.5. If absorbing complex of calcium cations or calcium and magnesium carbonates prevail, for example in carbonate soils and many chernozems, reaction is mainly determined by calcium bicarbonate, and pH of such soils is within 7-8. If soils besides calcium and magnesium contain aluminum and hydrogen, for example, leached and degraded black earths, sod-podzolic soils, reaction of soil solution is conditioned by simultaneous presence of hydrogen ions and calcium hydrocarbonate ions, as well as soluble organic acids and their salts. The lower the concentration of calcium and more hydrogen, the more the reaction of the medium will shift to the acidic side from 5 to 7.
In addition to dissolved carbon dioxide and organic acids, aluminum salts can acidify the soil solution:
АlСl3 + 3Н2O = Аl(ОН)3 + 3НСl = Аl(ОН)3 + 3Н+ + 3Cl–.
Actual acidity of the soil solution is due to the presence of hydrogen ions and hydrocarbonate monomers and partially soluble organic acids and hydrolytic acid salts. Appears when determining the pH of the soil solution or aqueous extract from the soil.
Actual acidity arises from the formation of organic and amino acids from decomposition of soil organic matter and organic fertilizers, as well as the presence of carbon dioxide and water. In addition, organic acids and amino acids are the products of root excretions of plants and soil microorganisms and carbon dioxide from the respiration of living organisms.
Also actual acidity of soils is created by nitric acid formed in the process of life activity of nitrifying bacteria and physiologically acidic ammonium fertilizers (NH4Cl; (NH4)2SO4).
Potential acidity is due to the exchange-absorbed by the soil absorbing complex ions of hydrogen, aluminum, iron and manganese. Depending on the ability to exchange displacement of these ions by other ions, potential acidity is divided into exchangeable and hydrolytic.
Exchangeable acidity is due to the presence of ions of hydrogen, aluminum, iron and manganese in the soil absorbing complex (SAC), which can be displaced by cations of neutral salts, including those from fertilizers, such as KCl, KNO3, K2SO4:
[SAC](H2, Al2, Fe2) + nKCl → [SAC](HK, AlK3, FeK3) + HCl + AlCl3 + FeCl3 + (n-7)KCl;
AlCl3 + 3H2O → Al(OH)3 + 3HCl;
FeCl3 + 3H2O → Fe(OH)3 + 3HCl.
In weakly acid soils exchange acidity is insignificant, in alkaline – is absent. Exchangeable acidity of acidic soils turns into actual acidity in the interaction of the solid phase of the soil with water-soluble fertilizers, ameliorants and salts of the liquid phase.
Exchangeable acidity (рНsalt) is an indicator of the need to liming of soils.
The value of exchange acidity is expressed in pH of salt extract, i.e. soil suspension in 1 n. solution of KCl, or in milligram equivalents per 100 grams of soil. When treating the soil with neutral salt solution (KCl) in the soil suspension along with the existing actual acidity, cations displaced from the SAC appear, causing exchange acidity, so the value of exchange acidity is always greater than the actual acidity.
Hydrolytic acidity is due to a portion of the potential acidity of the SAC cations, which can be displaced when treating the soil with a 1 n. alkaline salt solution, such as sodium acetate CH3COOHNa:
CH3COONa + H2O ⇔ CH3COOH + Na+ + OH–.
The alkaline reaction of the water solution of this salt allows for a more complete displacement of hydrogen, aluminum, iron and manganese ions from the SAC than the neutral KCl:
[SAC](H2, Al2, Fe2) + 14CH3COONa + 12H2O → [SAC](Na2, Na6, Na6) + 14CH3COOH + 2Al(OH)3 + 2Fe(OH)3.
Hydrolytic acidity (Hg) is defined as total acidity including actual, exchangeable and hydrolytic acidity. It is greater than exchange acidity and is expressed in milligram-equivalents per 100 g of soil.
In the absence of actual and metabolic acidity hydrolytic acidity is not harmful to plants and microorganisms. It is noted on all black earths, except for southern ones.
Hydrolytic acidity is important in determining the degree of saturation of soils with bases (G) and to justify the replacement of superphosphate with phosphate flour (phosphoritization). For acidic soils such as bog, podzolic and sod-podzolic, gray forest, red soils, yellow soils the value of hydrolytic acidity allows to determine the optimal rate of lime application.
In alkaline soils, such as southern black earths, chestnut soils and solonetz soils, we distinguish between actual and potential alkalinity.
Actual alkalinity is due to the presence in the soil solution of hydrolytic alkaline salts such as Nа2СO3, К2СO3, NaНСO3, КНСO3, Мg(НСO3)2, Са(НСO3)2, МgСO3.
Actual alkalinity is manifested by soil treatment with water and is expressed in mg-eq/100 g of soil or pH-water. The values obtained show the degree to which soils need to neutralize excess alkalinity by gypsum or acidification.
Potential alkalinity is manifested in soils whose SAC in the exchange-absorbed state contains sodium, which can increase the alkalinity of the soil solution when it is displaced into solution:
[SAC](Ca, Na2) + Ca(HCO3)2 ⇔ [SAC]Ca2 + 2NaHCO3.
According to the sodium content in the SAC the degree of the soil’s need for neutralization and the rate of consumption of gypsum-containing materials or technical acids are determined.
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.
Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.