Acid Soils: Origin, Classification, Effects and Reclamation

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Soils with pH values below 7 are acid soils.

In the regions of high rainfall, soils are acidic in their reaction because of the facts that soluble basic salts such as those of Ca, Mg, K, Na, are leached away by drainage water and insoluble acidic residues composed chiefly of oxides and silicates of iron, silicon, aluminium are left which accumulate in pretty high amount. These salts are acidic in reaction, hence the soils are acidic. Besides that reason, there may be other causes also which produce acidity in the soil.

Important factors which produce acidity in soil are as follows:

(1) Continuous removal of lime and other base elements by crops and accumulation of acids contained in the manures.

(2) Application of acid forming fertilizers in the soil.

(3) Microbial action.

(4) Formation of soil on the acidic rocks.

In India acid soils occur in the high rainfall areas covering about 25 million hectares of land with a pH below 5.5 and 23 million hectares of land with a pH between 5.6 and 6.5. These estimates are calculated by Bhaumik, H.D. and Donahue, Roy, L., 1964 (Reference: Soil acidity and the use of lime in India. Farm Information unit. Directorate of Extension, Ministry of Food and Agriculture, Government of India). In India, acid soils occur in Assam, Meghalaya, Arunachal Pradesh, Mizoram, Nagaland, NEFA, Manipur, Tripura, West Bengal, Bihar Uttar Pradesh, Himachal Pradesh, Jammu and Kashmir, M.P., Maharashtra, Kerala, Karnataka, Tamil Nadu and Andhra Pradesh. Punjab, Haryana, Rajasthan and Gujarat are the only states in India where acid soils do not occur.

Very few plants can grow well in strong acid soils. Soil acidity below pH value 5.5 is generally injurious to plants. Plant roots are badly affected if the pH value exceed limits of tolerance for particular crops. High degree of soil acidity (pH 5 to 6.5) decreases the availability of plant nutrients particularly phosphorus, calcium, magnesium, molybdenum, iron, manganese, potassium sulphur nitrogen, boron, copper and zinc. It also affects adversely the important microbiological processes, such as nitrogen fixation by Azotobacter, Clostridium and nodule inhabiting bacteria (Rhizobia) of leguminous plants.

Origin of Acid Soils:

Several factors are responsible for the origin of acid soils. Generally climate, hydrologic cycle vegetation, parent rocks and human interference play important roles in the origin and development of acid soils. Acid soils occur generally in humid regions where the rainfall is regular and very heavy. Dry regions are devoid of acid soils.

Climate:

In humid regions where evaporation is less than precipitation, chances for the development of acid soils are good. For the development of acid soils it is also necessary that water percolating down the soil profiles must reach the water table. In India, it is believed that the regions with acid soils must receive more than 750 mm annual rainfall. The regions with annual rainfall 1350 mm may have acid soils with pH value 5.0 or even less than that.

In temperate regions the acid soils can develop even if the rainfall is scanty. In hilly regions where the loss of water through evaporation is very slow due to very low temperature the conditions for the development of acid soils are very favourable, although the rainfall is scanty there.

Vegetation cover:

In temperate regions or hilly areas covered with conifers the acid soils can develop easily. According to Bloomfield (1953), the foliage leaves of conifers lack alkali elements and their mineralization process is very slow When the leaf-litter on the ground is degraded organic acids are released which gradually make the soils acidic. Plants found in the coastal regions and marshy places after death and decay produce acids which render the soils acidic.

Parental rocks:

Though the development of acid soils is possible on all types of rocks and parental rock materials in presence of favourable climate and vegetation, yet the development of acid soils on alkaline rocks take longer time as compared to the acid soils developing on the acidic parental rocks. Acid soils develop more quickly from parental rock materials with simple composition than from the parental rock materials of complex composition. It is so on account of presence of less adsorbed cations, poor buffering capacity and quick percolation of water through them.

Topography:

Sloppy places with good drainage conditions are supposed to be good for the development of acid soils. On hill slopes, the development of acid soils is easy. Acid soils do not develop generally in river basins. The plains with good drainage may also develop acid soils in due course of time.

Human interference:

Continuous efforts by man for developing permanently submerged areas into cultivable land, or for improving drainage in submerged or saline lands, regular use of nitrogen fertilizers like ammonium sulphate which cause acidity in the soils are responsible for decrease of soil pH. In urban areas, industrial wastes containing sulphur or sulphur dioxide also contribute much in the development of acid soils.

Classification of Acid Soils:

According to the intensity of acidity, the acid soils are of the following five types:

(1) Slight acidic (pH range 6.6 to 6.1)

(2) Medium acidic (pH 6.0 to 5.6)

(3) Strong acidic (pH 5.5 to 5.1)

(4) Very strong acidic (pH 5.0 to 4.6)

(5) Extremely strong acidic (pH 4.5 or lower)

Acid soils occurring in different climatic regions are classified as follows:

(i) Acid soils of temperate climate including podzol, brown podzol, grey brown podzol, brown forest soils, and grey forest soils.

(ii) Acid soil of tropical and subtropical climates including yellow podzolic soil, lateritic soil and latosols.

(iii) Acid soils of other great soil groups including wet soils (hydromorphic soils), washed peaty soils, mucky, cat-clay (acid sulphate) soils. Cat-clay or acid sulphate soils with pH 3.5 or lower are the soils that abound in organic material as well as H₂SO₄.

According to soil classification system (1970) developed by U.S. Soil Scientists, soils of the world have been classified into 10 soil orders. Among these Aridisols, Vertisols and Mollisols are nonacid soils and the remaining 7 orders contain acid soils. But acid soils occur mainly in three orders, Oxisols, Alfisols and Histosols. In modem soil classification soil orders have been divided into suborders. Suborders Humox, Humod, Aqualf and Udalf include acid soils.

In modem system of soil classification, acid sulphate soils have been assigned separate position and these soils have been placed in a group called sulpha-aquepts. This group includes soils in which the top horizon contains sulphuric horizon at some level or the other in top 25 cm thick layer. This is mineral or organic sublayer with yellow colouration due to xarocite.

Acid sulphate soils of tropics possibly belong to Typic Sulpha-aquepts and those of temperate regions are mainly Typic Sulpha-aquepts and Hapla aquepts. Mineral and organic soils rich in sulphur which remain regularly submerged are referred to as sulphidic soils. Since such soils develop under the influence of saline water which is rich in sulphur, they are placed in Halic subgroup.

Mandal (1974) has classified acid soils of India into the following seven groups:

1. Laterite soils

2. Lateritic and laterite red soils

3. Mixed yellow red soils

4. Ferruginous red soils

5. Podzolic soils

6. Terai soils

7. Peaty soils.

In this classification, acid sulphate soils and degraded alkali soils have not been assigned proper places. Nevertheless, it would be appropriate if they are classed with acid soils as the pH levels of such soils indicate that they are acidic in nature.

Recently Mishra, S.G. (1976) has suggested that the acid soils should be classified into the following two categories on the basis of organic contents in them:

(1) Acid mineral soils (organic matter less than 20%)

(2) Acid organic soils (organic matter 20% or more)

1. Acid mineral soils:

Such soils are further classified into the following three subgroups:

(i) Acid mineral soils rich in organic matter in upper layer:

Such soils are commonly found in temperate and sub-temperate regions and develop by podsolization process. Since these regions are covered with thick forest vegetation, the surfaces of such soils are covered with decomposing organic matter. The degradation of organic matter results in the organic acids, such as citric acid, acetic acid, oxalic acid and so on. Microbial degradation of organic matter also produces CO₂, which combines with water to form carbonic acid (H₂CO₃).

These acids along with rain water percolate down through soil profiles. Along with these acids and rain water sesqui oxides are also leached out from the upper horizons and become deposited in lower horizons. This depletion of sesqui oxides from the top layer and their accumulation in lower sub layers is referred to as Podsolization and such soils are called Podzols.

(ii) Acid mineral soils devoid of organic layer:

Acid mineral soils found in plains usually do not possess organic layer. Such soils originate mainly as a result of laterisation and part due to podzolisation process. Laterite soils, Red soils, and hydromorphic acid soils found in India belong to this category.

Such soils originate in the following ways:

(a) CO₂ of atmosphere as well as of soil dissolves in water to form carbonic acid (H₂CO₃) which, when percolates down the soil profiles, degrades carbonates and primary minerals present in the soils and make the soil acidic.

(b) In tropics, at high temperatures maximum degradation of silica takes place and in top soil layer the quantity of sesqui oxides increases. This process is referred to as laterisation. Laterite and Red loam soils found in India have probably originated through this process. At low temperatures Yellow red Podzolic and Grey Podzolic soils originate which are less acidic.

(iii) Degraded alkali soils:

The top layer of some alkali soils shows a pH value less than 7 due to desalinisation or dealkalization. Such soils are referred to as degraded alkali soils. In this process. The alkali salts are washed by irrigation or rain water and exchangeable Na ions of soils are displaced by H⁺ ions of water.

2. Acid organic soils:

According to the amount of organic matter, acid organic soils can be classified into the following two types:

(i) Peaty Soils

(ii) Mucky Soils

(i) Peaty soils:

Peaty soil are characterised by presence of poorly degraded organic matter In India, peaty soils occur in Kashmir, Himachal Pradesh, Assam and other Hill states.

(ii) Mucky soils:

Such soils contain highly degraded organic matter. They have relatively higher pH values than the peaty soils. Thus they are less acidic. Mucky soils are also found in Kashmir, Himachal Pradesh, Assam, and some other states.

Effects of Soil Acidity on Plants:

Soil acidity affects the plants both directly and indirectly.

These effects are briefly mentioned below:

1. Direct influences.

These are as follows:

(a) Toxic effects of low H⁺ ion concentrations on root tissues.

(b) Influence of soil acidity on the permeability of the plasma membrane for cations.

(c) Disturbance in the balance between basic and acid constituents through roots.

(d) Affects enzymatic processes since enzymes are particularly sensitive to pH changes Different crop plants have their specific optimum pH requirement. Rice, oat and linseed can endure a fairly acidic reaction (pH = 5.0) while barley, sugar-beet, lucerne etc. can tolerate a fairly alkaline reaction (pH = 8.0)

2. Indirect effects.

These are listed below:

(a) Availability of various nutrients, e.g., phosphorous, copper, and zinc.

(b) High solubility and availability of elements like aluminium, manganese and iron in toxic amount due to high acidity in the soil.

(c) Deficiency of some nutrients such as calcium and potassium due to soil acidity.

(d) Prevalence of plant diseases.

(e) Beneficial activities of soil microbes are adversely affected.

Reclamation of Acid Soils or Correction of Soil Acidity:

Acidity of soil is due to predominance of H⁺ ions over OH^– ions, the bulk of H⁺ ions being held in close association with clay-organic colloid complex. Strong acid soils are not much productive. The soils which are less productive owing to high degree of acidity can be made more productive by liming (application lime).

When lime is added to moist soil, the soil solution becomes charged with cations and the exchangeable hydrogen and aluminium ions on clay-organic colloid complex as well as the H⁺ ionsmsoil solution are displaced by calcium ions. Hydrogen combines with OH^– to form neutral water or with CO₃ or HCO₃^– to form unstable H₂CO₃, which readily dissociates to form CO₂ and water.

Acidity of soil can also be corrected by adding exchangeable Mg⁺⁺ to exchange complex But addition of or Mg⁺⁺ or both to the soil will not necessarily solve the problem of soil acidity.

The important points to be considered in liming are:

(i) The salts of these elements which are going to supply these ions (Ca⁺⁺ or Mg⁺⁺) and

(ii) The overall reactions of salts in the soils Salts of strong acids as gypsum (CaSO₄) or calcium chloride (CaCl₂) can be applied to supply calcium 10ns to the soils but it is worth considering what will be the effects of these salts on soil acidity. The application of these salts will indeed increase the acidity in the soil, instead of decreasing it. Therefore, it is suggested that calcium salts of strong acids must not be applied for correcting the acidity of soils.

Liming materials:

More than 90 per cent of the lime used in agriculture for reclamation of acid soils IS generally in the form of calcium carbonate, some in calcium and magnesium carbonates, and much smaller quantity in the form of calcium oxide or calcium hydroxide. To a chemist lime is calcium oxide but to a farmer, agronomist or soil scientist lime usually means calcium carbonate or calcium carbonate equivalents.

The common liming materials used for reclamation of acid soils are as follows:

(1) Calcic limestone (CaCO₃) which is ground limestone.

(2) Dolomite (CaCO₃. MgCO₃).

(3) Quick lime (CaO) which is burnt limestone.

(4) Hydrated (slaked) lime [Ca (OH) ₂].

(5) Coral shell lime.

(6) Marl or chalk (CaCO₃).

(7) Slags Obtained as by-products from iron and steel plants, slags are used in agriculture for reclaiming acid soils.

The slags are of three types:

(i) Blast furnace slags,

(ii) Basic slag and

(iii) Electric furnace slag.

These slags are rich in phosphorus and mixture of CaO and CaCOH)₂. Besides, Ca, Mg, Al, silicates are also present in them.

(8) Press-mud. It is obtained from carbonation plants of sugar mills. Press mud and some other matters containing calcium are used to decrease acidityinthe soils.

(9) Miscellaneous sources of lime, such as, wood ash, ground oyster shells, by-product lime resulting from paper mills, tanneries, water softening plants, and by product CaCO₃ form fertilizer factories using gypsum process (such as Sindri Fertilizer Factory, Bihar, India).

The rate of lime application should always be determined after soil testing. When excessive amount of lime is applied to sandy soils low in humus, injury to plants may be caused which may be attributed to one or more of reasons listed below:

(1) Boron deficiency.

(2) Iron, manganese and zinc deficiency.

(3) Reduced availability of phosphorus to a critically low level.

(4) Reduced potassium uptake.

Such injurious effects may be reduced by application of large amount of compost manure, green manure crops, phosphorus fertilizers, boron or a mixture of minor elements.

While applying liming agents of acid soils, the following points must be taken into consideration:

(i) The liming agents should be used in highly powdered state. The smaller the particles of liming agents the greater will be their effectiveness in correcting the soil acidity.

(ii) Liming materials should be in direct contact with clay Organic exchange complex so that H+ ions of exchange complex may be easily displaced by Ca⁺⁺ ions.

(iii) Liming agents should be applied to soils at least one month before sowing the crops or they should be applied thoroughly mixed with soils just after harvesting the crops.

Important Roles of Liming Agents in Soils:

(1) Liming agents reduce soil acidity and stabilize pH of the soils.

Acid-clay + Ca(OH)₂ → Ca-clay + H₂O

(2) Lime makes phosphorus easily available. This is true mainly because in acid soils phosphorus is fixed by soluble iron and aluminium. Liming reduces the solubility of iron and aluminium and therefore less phosphorus is held in these insoluble and unavailable forms.

(3) Lime makes potassium more efficient in plant nutrition. When K is in sufficient amount in soil plants absorb more potassium than is actually needed but at the same time when lime is available in plenty, plants take up more calcium and less potassium. Economically liming is more desirable because plants absorb more cheap Ca. than expensive potassium.

(4) Lime enhances the decomposition of organic matter, thereby increases the availability of nitrogen and other nutrients locked up in complex forms to plants.

(5) Lime promotes beneficial activities of soil bacteria.

(6) Liming programme extended over a period of years improves the physical conditions of the soil by causing granulation of soil particles, decreasing its bulk density, and increasing its infiltration rate.

(7) Ca and Mg found in liming agents, particularly in Dolomite act as essential elements in the nutrition of plants.

(8) Lime converts toxic elements such as aluminium, Mn, Fe of the soil in insoluble and harmless compounds.

Acid Tolerance in Crops:

Some plants are adversely affected and they suffer injuries when grown in acid soils. Familiar crops which can endure fairly acidic soil conditions are oat, rice and linseed and those which are not adapted to acid soils are wheat, barley, cabbage, sorghum (Jowar), tobacco, lettuce, spinach, onion, eggplant or brinjal.