Soil acidity
Soil acidity, also reaction of the soil solution, or soil reaction, is a property of soil caused by the presence of hydrogen ions in the soil solution and exchangeable hydrogen ions and aluminum, iron(III), manganese ions in the soil absorbing complex (SAC).
Determination
Reaction of the soil solution — the ratio of the concentration of hydrogen ions H+ and hydroxide ions OH- in the soil solution, expressed as the hydrogen ion exponent (pH) of a water or salt extract. The pH is determined by the equation:
where
- CH^+
- — concentration of hydrogen ions in solution, mol/dm3.
In pure water, the concentration of hydrogen ions coincides with the concentration of hydroxide ions and is equal to 1⋅10-7 mol/L, that is, pH = 7.
The soil solution contains carbon dioxide dissolved in water (erroneously called “carbonic acid”), which dissociates into hydrogen ions H+ and bicarbonate ions HCO32-:
The acidity created in this case is neutralized by bases absorbed by the soil and by calcium and magnesium carbonates:
The hydrogen ions formed as a result of this reaction have an acidifying effect on the soil solution.
When sodium is present in the soil absorbing complex, sodium bicarbonate and carbonate are formed in the solution:
Besides carbon dioxide, the soil may contain organic acids formed as a result of the activity of soil biota and plants.
Besides dissolved carbon dioxide and organic acids, salts of aluminum, iron(III), and manganese can acidify the soil solution.
Significance
The reaction of the soil solution influences the nutrient regime of soils, the activity of soil microorganisms, the transformation of nutrient elements of fertilizers and soil, and the agrophysical, agrochemical, physicochemical, and biological properties of soils. Excess acidity suppresses the activity of beneficial soil microflora (ammonifiers, nitrifiers, azotobacter, etc.) and favors the development of pathogenic bacteria and fungi. An acidic reaction negatively affects the growth and development of plants. A high concentration of hydrogen ions worsens the physicochemical state of the protoplasm of root system cells, hinders its growth, disrupts the permeability of root membranes and metabolism in roots, thereby worsening the conditions of plant nutrition.
Indirectly, the reaction of the soil environment affects fertility: increased acidity leads to an increase in the mobility of humus compounds and their leaching beyond the arable layer; the effect of hydrogen ions H+ on the mineral part of the soil leads to its depletion in colloidal particles. Soil acidification causes a deficiency of exchangeable calcium and magnesium, which sharply worsens structure, absorption capacity, and buffering capacity. An acidic environment increases the concentration of aluminum and manganese toxic to plants and reduces the mobility of some elements, for example, molybdenum, creating disturbances in the nutrient regime of soils. Increased acidity suppresses soil biota, primarily nitrifying and nitrogen-fixing bacteria, earthworms, mites, and springtails. In general, the biological activity of acidic soils is significantly lower than that of neutral soils.
Soils with excessive acidity are characterized by:
- low content of mobile forms of nitrogen, phosphorus, potassium, and micronutrients;
- unfavorable agrochemical and agrophysical properties;
- increased content of mobile forms of aluminum and manganese;
- low biological activity;
- negative influence of high concentration of hydrogen ions H+ on the physicochemical state of protoplasm, root system growth, and plant metabolism;
- development of pathogenic microflora, for example, fungi of Penicillium, Fusarium, Trichoderma;
- mobilization of heavy metals.
Systematic application of physiologically acidic fertilizers, for example, ammonium nitrate, worsens the agrochemical properties of the soil, which reduces crop yields.
Table. Influence of agrochemical properties of soil on barley yield (after 18 years of ammonium nitrate application).
| Yield, c/ha | Sum of absorbed bases, mmol per 100 g soil | Exchangeable acidity, mmol per 100 g soil | Hydrolytic acidity, mmol per 100 g soil | Degree of base saturation, % |
|---|---|---|---|---|
| 25-30 | 12-15 | 0,5 | 3,5 | 86-90 |
| 20-25 | 8-15 | 1 | 3-4 | 60-90 |
| 15-20 | 6-13 | 1,5-2,0 | 3-8 | 50-60 |
| 10-15 | 4-5 | 2,0 | 7-8 | 50 |
| 5-10 | 4-5 | 2,5 | 7-8 | 50 |
| 5 | 4-5 | 3,0 | 9-10 | 40-50 |
| Correlation coefficient | +0,81 | -0,89 | -0,65 | +0,85 |
Types of soil acidity
Soil acidity is distinguished into actual and potential (exchangeable and hydrolytic).
Actual acidity
Actual acidity of the soil solution is caused by the presence of hydrogen ions and bicarbonate ions, as well as soluble organic acids and hydrolytically acidic salts. Actual soil acidity is also influenced by nitric acid formed during the activity of nitrifying bacteria and by physiologically acidic fertilizers, for example, NH4Cl, (NH4)2SO4.
It is determined as the pH of the soil solution or a water extract from the soil.
Potential acidity
Potential acidity is caused by exchange-absorbed by the soil absorbing complex (SAC) hydrogen, aluminum, iron(III), and manganese ions. Depending on the ability of these ions to be exchange-displaced by other ions, potential acidity is subdivided into exchangeable and hydrolytic.
Exchangeable acidity
Exchangeable acidity is caused by the presence in the SAC of hydrogen, aluminum, iron, and manganese ions, which can be displaced by cations of neutral salts, including from fertilizers, for example, KCl, KNO3, K2SO4:
In slightly acidic soils, exchangeable acidity is insignificant; in alkaline soils it is absent. Exchangeable acidity of acidic soils passes into actual acidity when the solid phase of the soil interacts with water-soluble fertilizers, ameliorants, and salts dissolved in the soil solution.
The value of exchangeable acidity serves as an indicator of the need for liming of soils. The value of exchangeable acidity is expressed in the pH of a salt extract (pHsalt), that is, a suspension of soil in 1 N KCl solution, or in milligram-equivalents per 100 g soil. When treating soil with a neutral salt solution (KCl), in the soil suspension, along with the existing actual acidity, displaced cations appear from the SAC, causing exchangeable acidity; therefore, the value of exchangeable acidity is always greater than that of actual acidity.
Hydrolytic acidity
Hydrolytic acidity is caused by part of the cations of the SAC of potential acidity that can be displaced when the soil is treated with 1 N solution of an alkaline salt, for example, sodium acetate CH3COONa:
The alkaline reaction of the aqueous solution of this salt allows a more complete displacement from the SAC of hydrogen, aluminum, iron, and manganese ions than neutral KCl:
Hydrolytic acidity (Hg) is defined as acidity including actual, exchangeable, and hydrolytic acidity. It is greater than exchangeable acidity and is expressed in milligram-equivalents per 100 g soil.
In the absence of actual and exchangeable acidity, hydrolytic acidity is not harmful to plants and microorganisms. This is observed in all chernozems except southern chernozems.
Hydrolytic acidity is important in determining the degree of soil saturation with bases (G) and for justifying the replacement of superphosphates with phosphate rock (phosphoritization). For acidic soils, such as bog, podzolic and sod-podzolic, gray forest, red soils, and yellow soils, the value of hydrolytic acidity allows determining the optimal lime rate.
Classification of soils by reaction
The total area of arable lands of CIS countries with increased acidity is about 40-45 million ha; those needing liming — more than 55-60 million ha. These primarily include sod-podzolic, light-gray and gray forest, bog soils, and red soils.
The reaction of soil solutions can vary from pH = 2.5-3.5, characteristic of sphagnum peats and forest litter of sphagnum forests, to pH = 9-11 in solonetz.
By reaction (pH), soils are divided into:
- very strongly acidic — < 4,0 (pHsalt),
- strongly acidic — 4,1-4,5,
- medium acidic — 4,6-5,0,
- slightly acidic — 5,1-6,0,
- neutral — 6,1-7,4,
- slightly alkaline — 7,5-8,5 (pHwater),
- strongly alkaline — 8,6-10,0,
- sharply alkaline — > 10,0.
Properties
The reaction of the soil solution of various soils depends on the composition of absorbed cations and carbonate content. If the absorbing complex has a high sodium content (solonetz, solonchaks), the reaction of the soil solution is determined by the presence of sodium carbonates. In these soils it reaches 8-8.5. When calcium cations or calcium and magnesium carbonates predominate in the absorbing complex (calcareous soils, many chernozems), the reaction is mainly determined by calcium bicarbonate; the pH of such soils is 7-8. If the soil, besides calcium and magnesium, contains aluminum and hydrogen (leached and degraded chernozems, sod-podzolic soils), the reaction of the soil solution is determined by the simultaneous presence of hydrogen ions and calcium bicarbonate, as well as soluble organic acids and their salts. The lower the calcium concentration and the higher the hydrogen concentration, the more the reaction shifts to the acidic side.
Optimal soil acidity
For the majority of cultivated crops, soils with a neutral or near-neutral reaction are favorable. The interval pH 6,0-7,0 corresponds to the most agronomically favorable soil structure, high humus quality, optimal water regime, and conditions for soil biota activity.
Groups of crops by relation to soil acidity and liming
For some crops, the optimal reaction may shift to the more acidic side or be in a wide range of pH. All plants during the first 2-3 weeks after germination are sensitive to an unfavorable reaction. For example, alfalfa, sugar beet and cotton do not tolerate acidic soils; others, on the contrary, prefer a slightly acidic reaction: lupine, potato, buckwheat, flax.
By relation to soil acidity and responsiveness to lime application, agricultural crops are conventionally divided into five groups.
- Crops of the first group are characterized by the greatest sensitivity to the reaction; the optimal environment is slightly alkaline with pHwater = 7,0-8,0; pHKCl = 6,8-7,5. These include: sugar, fodder and table beet, white cabbage, mustard, alfalfa, sainfoin, rapeseed, onion, garlic, celery, spinach, pepper, parsnip, soybean, hemp, currant, cotton. On very acidic soils, these crops reduce yield by 2-3 times; plants are severely affected by diseases. Soils intended for cultivation of first group crops are subject to liming first.
- Crops of the second group are characterized by an optimal soil reaction close to neutral, with optimal pHKCl = 6,0-6,5. A decrease in pH to 4,5 leads to a reduction in yield of this group by 1,5-2 times and increases disease incidence. Crops of the second group include: wheat, barley, maize, clover, pea, vetch, bean, chickpea, grass pea, lentil, cauliflower and fodder cabbage, kohlrabi, turnip, rutabaga, lettuce, leek, cucumber, brome grass, foxtail. They respond well to liming.
- Crops of the third group tolerate moderately acidic and alkaline soils and do not have a pronounced optimal reaction value. Their growth is influenced by accompanying factors. With a favorable nutrient regime, they can produce good yields in the range pHKCl from 5 to 7,5. A slightly acidic environment with pH 5,5-6,0 is considered optimal. Plants of the third group respond positively to liming of strongly and medium acidic soils. Crops of the third group include winter rye, oats, buckwheat, tomato, sunflower, carrot, pumpkin, zucchini, parsley, radish, turnip, rhubarb, Jerusalem artichoke, timothy.
- Crops of the fourth group include crops that tolerate a moderately acidic soil reaction well; optimal pHKCl = 5,1-5,6. For flax, optimal values are in the range pH 5,5-6,0; for potato and berry crops — pH 4,5-6,5. These crops respond positively to liming while maintaining an optimal ratio between calcium, potassium, magnesium, boron, and other nutrients. Excess calcium causes a decrease in yield and product quality; in potato, the incidence of scab increases; in flax — bacteriosis. Neutralization of excess acidity reduces the availability of boron, copper, and zinc for these crops, and excess calcium hinders potassium and magnesium uptake. Liming for fourth group crops is effective under a strongly acidic reaction. Crops of this group include potato, fiber flax, millet, sorghum, raspberry, strawberry, and gooseberry.
- Crops of the fifth group are characterized by an optimal soil reaction at pHKCl = 4,5-4,8. They are little sensitive to excess acidity; liming is carried out only on very strongly acidic soils with pHKCl < 4,0, since calcium can negatively affect the growth of these crops, especially during germination and in the initial growth phases. Crops of the fifth group include: sorrel, tea, coffee, cocoa, yellow and blue lupine, goat's rue, serradella.
For most agricultural crops during germination and in the initial growth phases, an environment close to neutral is required — pHKCl 5,8-6,2 or pHwater — 6,4-7,0.
Optimal pH values for the growth and development of the same crop may differ depending on soil type. On soils with increased organic matter content and light texture, the optimal pH interval shifts to the acidic side.
The physiological (biological) optimum of the reaction for plants may differ from the ecological (technological) optimum, which is associated with the mobility of nutrients and conditions of disease development. For example, for potato and flax plants, if the soil is not infected with diseases, the biological optimum is pHKCl 6,0-6,2; however, under conditions of disease pressure (potato at neutral and slightly alkaline reaction is affected by scab caused by actinomycetes, flax — by fusarium), under field conditions the yield and quality of these crops are higher at pHKCl 5,2-5,6, that is, at the ecological optimum. The mismatch between the biological and ecological optimum of the reaction for many agricultural crops is due to changes in the availability of nutrients when soil pH changes.
Optimal acidity levels in crop rotations
The change in the availability of macro- and micronutrients during liming should be taken into account. Liming of soil with pH above 6,6 is ineffective, since the removal and leaching of calcium from the soil increase and the mobility of micronutrients decreases, except for molybdenum. Lime fertilizers have a long-lasting action, therefore the relation to it of all (or leading) crops of the rotation is taken into account, that is, the specialization of the crop rotation and the soil texture.
The table presents approximate optimal pHKCl levels of soils for rotations with different specialization. Optimal on sod-podzolic and gray forest soils are lime rates close to full by hydrolytic acidity, ensuring a reaction of pHsalt 5,4-5,8.
Table. Approximate optimal levels of soil reaction (@abbr[pH value|pH]KCl) for crop rotations of various types.
| Soil texture | Types of crop rotations | Cultivated pastures and hayfields | |||||
|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | grass | legume-grass | |
| Sandy and sandy loam | 5,0-5,3 | 5,3-5,5 | 5,8-6,0 | 5,5-6,0 | 5,8-6,0 | 5,2-5,4 | 5,4-5,6 |
| Light and medium loamy | 5,5-5,6 | 5,5-6,0 | 6,0-6,2 | 5,8-6,0 | 6,0-6,2 | 5,4-5,6 | 5,6-5,9 |
| Heavy loamy and clayey | 5,5-5,8 | 5,8-6,2 | 6,2-6,5 | 6,0-6,2 | 6,2-6,5 | 5,6-5,8 | 6,0-6,2 |
| Peaty | 4,6-4,8 | 4,8-5,2 | 5,2-5,8 | 5,0-5,4 | 5,2-5,6 | 4,6-4,8 | 5,0-5,2 |
Factors increasing soil acidity
Soil acidity may be caused by the application of physiologically acidic mineral fertilizers, which, as a result of plant uptake, accumulate hydrogen ions in the soil.
The main factors of soil acidification include removal with the harvest and leaching of calcium and magnesium. The removal of calcium and magnesium can reach from tens to hundreds of kilograms per 1 ha. The highest removal is characteristic of cabbage, alfalfa, and clover, which are distinguished by high sensitivity to soil acidity.<1>
The loss of calcium and magnesium by leaching is influenced by the leaching water regime of the soil, which depends on the abundance of atmospheric precipitation. For example, on acidic soils, leaching can amount to 200-300 kg/ha; on calcareous soils — significantly more. Light-textured soils under a leaching water regime lose significantly more calcium and magnesium.<1>
Management
The main agrotechnical method of regulating soil acidity is liming — the application of calcium carbonate CaCO3.
For alkaline solonetz soils, gypsuming is used — the application of calcium sulfate CaSO4 (gypsum).
Alkalinity
In alkaline soils, such as southern chernozems, chestnut and solonetz soils, actual and potential alkalinity are distinguished.
Actual alkalinity
Actual alkalinity is caused by the presence in the soil solution of hydrolytically alkaline salts, for example, Na2CO3, K2CO3, NaHCO3, KHCO3, Mg(HCO3)2, Ca(HCO3)2, MgCO3.
Actual alkalinity is determined in a water extract of the soil. It is expressed in mg-eq/100 g soil or pHwater.
Actual alkalinity serves as an indicator of the need of soils for neutralization of excess alkalinity by gypsuming or acidification.
Potential alkalinity
Potential alkalinity manifests in soils whose SAC contains sodium in an exchange-absorbed state, capable, when displaced into solution, of increasing the alkalinity of the soil solution:
By the sodium content in the SAC, the need of the soil for neutralization of excess alkalinity and the application rates of gypsum-containing materials or technical acids are determined.
References
- Bazdyrev G. I., Loshakov V. G., Puponin A. I. et al. Agriculture. Textbook for universities. M.: Kolos Publishing House. 2000.
- Mineev V. G., Sychev V. G., Gamzikov G. P. et al. Agrochemistry. Textbook. / Ed. Mineev V. G. M.: Publishing House of VNIIA named after D. N. Pryanishnikov. 2017.
- Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. Agrochemistry. / Ed. B. A. Yagodin. M.: Kolos. 2002.
