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After reading this article you will learn about:- 1. Distribution of Acid Soil 2. Classification of Acid Soils 3. Pedogenic Processes.
Distribution of Acid Soil:
Out of 157 million hectares of cultivable land in India, 49 million hectares of land are acidic, of which 26 million hectares of land having soil pH less than 5.6 (pH < 5.6) and the rest 23 million hectares of land having soil pH range 5.6 to 6.5. A tentative soil map in India showing approximate pH range is given in Fig. 14.5.
Out of total cultivable land in West Bengal (7.58 million hectares), about 2 million hectares of land are acidic of different intensity because of varying climatic conditions of different districts. Acid soils of West Bengal with approximate soil pH range in given in Fig. 14.4.
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In West Bengal, laterite, red gravelly soils and alluvial and terai soils are the most dominant type of acid soils. Laterite and red gravelly type of acid soils are mostly found in some district like, Medinipur, Bankura, Birbhum, Burdwan and Purulia whereas alluvial and terai acid soils are found in Jalpaiguri and Cooch Behar districts.
Recently it has been reported that acid sulphate soils are found in the district of South 24 Parganas.
Classification of Acid Soils:
Due to wide variation in Indian climatic conditions, there is occurrence of acid soils of widely divergent nature.
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Broadly acid soils of India can be classified into seven distinct groups viz:
(1) Laterite,
(2) Laterite and Lateritic red,
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(3) Mixed red, black and yellow
The classification of acid soils state-wise with most important pedogenic characters, area and approximate pH range are given in Table 14.1.
Pedogenic Processes for the Development of Acid Soils:
A natural consequence of the vast area and the climatic variations and a wide range of parent materials involved in soil of widely divergent nature. The factors which have been particularly dominant in the development of acid soils are rainfall, temperature, hydrological conditions and vegetation.
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The major and important processes involved in the development of acid soils are:
(i) Laterisation of varying degrees.
(ii) Podsolization in areas with sub-temperate to temperate climate.
(iii) Intense leaching in light alluvial soils in high rainfall of partly decomposed organic matter.
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(iv) Marshy conditions with significant amounts of under decomposed or partly decomposed organic matter.
(i) Laterisation:
The term laterite is derived from the word ‘later’ meaning brick or tile and was originally applied to a group of high clay Indian soils found in Malabar Hills of Kerala, Tamil Nadu, Karnataka, Maharashtra and Madhya Pradesh.
It refers specifically to a particular cemented horizon in certain soils which when dried, become very hard like brick. Such soils (in tropics) when massively impregnated with sesquioxides (iron and aluminium oxides) to the extent of 70-80% of the total mass are called laterites or latosols (oxisols).
The soil forming process is called “Laterisation”. Laterisation is the process that removes silica, instead of sesquioxides to concentrate in the solum. The process operates under the following conditions.
1. Climate:
The climate zones in which laterites are extensively found are:
(i) High rainfall zone with strongly expressed dry season,
(ii) High rainfall zone with weakly expressed dry reasons and
(iii) Sub-humid zone with pronounced wet and dry seasons.
Besides these, the specific conditions under which laterites develop are:
(i) A minimum amount of water necessary for weathering and leaching of bases and combined silica and
(ii) Intervals of dry season and conditions leaching to segregation of secondary iron and aluminium oxides and hydroxides and their crystallized products such as bauxite. Alternate wet and dry seasons favour the cementation of the vesicular lateritic mass.
The process of laterisation operates most favorably in warm and humid (tropical) climate with 2000 to 2500 mm rainfall and continuous high temperature (about 25°C) throughout the year. Such conditions cause intense leaching and favour rapid decomposition of parent rock. Then the soluble products of weathering are continuously being dissolved and leached to deeper ground water stream by the gravitational force.
2. Natural Vegetation:
The organic remains of deciduous forest plants are incorporated into the soil very slowly, because much of the vegetative matter is continuously being oxidised due to the prevailing high temperature.
The other portion of the vegetation is eaten up by ants and other organisms and consequently, the soils remain low in humus. Further because of the pronounced leaching environments, the organic matter and soluble minerals are removed rapidly.
3. Parent Materials:
Basic parent materials having sufficient iron bearing ferromagnesian minerals (pyroxene, amphiboles, biotite etc.) which on weathering release iron are congenial for the development of laterites. The iron released during weathering combines with oxygen (O2) to form oxides.
2Fe + O2→ 2FeO
4Fe + 3O2→ 2Fe2O3
Such iron oxides (Fe2O3 or FeO) coat the clay, silt and sand particles and impart red colour to the soils.
(ii) Podzolisation:
It is a process of soil formation resulting in the formation of podzols and podzolic soils. Podzolisation is the negative of calcification, whereas calcification, tends to concentrate calcium in the lower part of the ‘B’ horizon. Podzolisation leaches the entire solum. Apart from calcium (Ca2+), the other bases are also removed and the whole soil becomes distinctly acidic.
In fact, the process is essentially one of acid leaching. The characteristic process in podzolisation is the dissolution and removal of sesquioxides (iron and aluminium oxides) from the A’ horizon and their deposition in the ‘B’ horizon in association with the movement and deposition in the ‘B’ horizon of organic matter.
Mobilisation of iron and aluminium is brought about by some soluble organic compounds which the rain washes off either living or recently dead leaves of the vegetation growing in the soil.
These compounds include polyphenols which under acid conditions, will reduce any ferric iron present, either in weatherable minerals or as films of ferric hydroxides on sand grains and will form water soluble complexes with both ferrous and aluminium ions.
Podzolisation process operates under the following conditions:
1. Climate:
A humid temperate climate is most favourable for podzolisation.
2. Parent Material:
Siliceous (Sandy) materials, having poor reserves of weatherable minerals, favour the operation of podzolisation as it helps in easy percolation of water.
3. Vegetation:
Vegetation like coniferous pines, hemlock etc. is very much appropriate for the process. During decomposition of such plant residues acidic reaction results and pH values may fall below 5.0. The activity of many beneficial micro-organisms will be reduced under such strong acid conditions.
4. Leaching and Translocation of Sesquioxides:
Due to very limited activity of microorganisms, the polysaccharide production declines drastically. At the same time polymerization of the soluble organic products of decay does not take place properly and they remain in soluble form.
The organic acids thus formed react with sesquioxides and the remaining clay minerals forming organic-sesquioxides and organic-clay complexes which are soluble and as such move along with percolating water to the lower ‘B’ horizon. Aluminium (Al3+) ions in a water solution hydrolyze and make the soil solution very acidic.
2Al + 6H2OD2Al(OH)3 + 6H+
As the iron and aluminium move out, the ‘A’ horizon gives a bleached grey or ashy appearance.
5. Re-Precipitation of Sesquioxides and Humification of Organic Material:
The secondary accumulation of organic materials is by polymerization. The re-precipitation of illuviated sesquioxides in the ‘B’ horizon may be mechanical, chemical or biological. The soluble ferrous iron (Fe2+) formed in the ‘A’ horizon (zone of washing out) is oxidised to insoluble ferric iron compounds (Fe3+) formed in the B’ horizon (zone of accumulation).
The percolating water may not be sufficient to leach further downward and force precipitation of these materials in the ‘B’ horizon. The negative charges on the clay may immobilize the positively charged ions of iron and aluminium. Bacteria also sometimes destroy the chelating and complexing compounds which mobilized iron and aluminium from the ‘A’ horizon.
Problem 1:
Calculate the efficiency and neutralizing index of the following liming material
Per cent calcium carbonate equivalent: 90%
50% of the material passing through a 60 mesh
25% of the material passing through a 20 mesh
25% of the material passing through an 8 mesh.
Solution:
Efficiency rating will be,
50% × 100= 50/100 × 100 = 50
25% × 60= 25/100 × 60=15
25% × 20 = 25/100× 20 = 5
Total efficiency rating = 70
So total efficiency rating or the finesses factor of the liming material is 70 per cent calcium carbonate equivalent (CCE) = 90%.
Therefore,
Neutralizing index (N.I.) of the liming material = CCE × fineness factor
= 90% × 70
= 90/100 × 70 = 63.
Problem 2:
If sandy loam soil having cation exchange capacity (CEC), 20 C mol (p+) kg-1and 30 per cent base saturation at pH 4.5. Calculate the theoretical amount of lime (CaCO3) required per hectare of land (0-15 cm depth) for raising base saturation to 60 per cent [weight of soil per hectare furrow slice (0-15 cm depth) = 2.2 ×106kg]
Solution:
We know,
Per cent base saturation (BS) = me of basic cations (S)/Total exchangeable cations (T) × 100
Base saturation of the soil before lime application (initial) = 30%
Base saturation to be raised through liming (final) = 60%
Therefore, me of base cations initially present in the soil (S)
= T × BS (percentage)/100
= 20 × 30/100 = 6
Again, me of basic cations at 60% base saturation of soil (S)
= T × BS/100
= 20 × 60/100 = 12
So me of basic cations will be required to raise the base saturation at 60%
= (12 – 6) = 6.
1 me of CaCO3 weighs 50 mgs
(Since equivalent weight of CaCO3 = = 50)
6 me of CaCO3 weighs 50 × 6 mgs = 300 mgs
= 300/106 kg
0.1 kg soil requires 300/106 kg CaCO3
1 kg soil requires 300/106 × 0.1 kg of CaCO3
2.2 × 106 requires 300 × 2.2 × 106/106 × 0.1 kg of CaCO3
= 6600 kg of CaCO3 = 66 quintals
Therefore, 66 quintals of lime (CaCO3) will be required per hectare of land to raise the base saturation at 60%.
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