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After reading this article you will learn about Organic Soils:- 1. Meaning and Formation of Organic Soils 2. Classification of Organic Soils 3. Characteristics 4. Management.
Meaning and Formation of Organic Soils:
Soil can be classified into groups—mineral and organic, on the basis of organic matter content. In the mineral soils, organic matter content may vary from 0-20 per cent. Organic soils occupy less than 1% of the world’s land area. Generally, in these soils, organic .matter content is more than 20%. Organic soils are highly productive soils particularly for vegetables and flowers, provided properly managed.
Organic deposits accumulate in marshes, bogs and swamps by decaying of water loving plants such as mosses, grasses, pondweeds, shrubs and trees since generations. Microorganisms break down the organic tissues and aid in the synthesis of organic matter and humus.
Classification of Organic Soils:
On the basis of stage of breakdown of original plant materials, organic soils have been classified into following two groups:
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1. Peat soil:
Organic soils, which have slightly decayed or non-decayed plant materials are called peat soils. In peat soils, original plant deposits can be identified, especially in the upper horizons. Peat soils are coarse textured or fine- textured depending on the nature of deposited plant residues.
2. Muck soil:
Organic soils having markedly decomposed original materials are termed as muck soil. Muck soils are usually fine- textured because of well decomposition of original plant deposits.
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In the comprehensive Soil Taxonomy classification system, organic soils are identified as the order Histosols.
Characteristics of Organic (Peat and Muck) Soils:
(A) Physical Characteristics:
(i) Colour:
The colour of cultivated organic soils is dark brown to deep black.
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(ii) Bulk density:
The bulk density of organic soils is quite low in comparison to mineral soils. Bulk density of well composed organic soil is only 0.20-0.30 compared to 1.3-1.5 for mineral soils. Thus, organic soils are light weight when dry.
(iii) Soil structure:
The surface layer of organic soils are granular or crumby. Its cohesion and plasticity are low compared to mineral soils. Organic soils are therefore, porous, open and easy to cultivate.
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(iv) Water-holding capacity:
Compared to mineral soils, organic soils having high water-holding capacity. Therefore, a given layer of organic soil at optimum moisture will supply only slightly more water to plants than a comparable mineral soil.
(B) Chemical Characteristics:
(i) Cation exchange capacity:
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Cation exchange capacity of organic colloids are higher than those for the inorganic colloids (Table 11.1).
(ii) Soil pH:
pH of an organic soil at a given percentage base saturation is generally lower than that of a representative mineral soil. Organic soils are highly acidic with a pH value less than 5.5.
(iii) Buffering capacity:
Histosols have a higher buffering capacity than mineral soils.
(iv) Carbon-Nitrogen ratio:
The representative organic soil possesses a high carbon-nitrogen ratio (20:1) compared to 12:1 for a representative mineral soil. Even so organic soils show vigorous nitrification (nitrate release) in spite of their high C/N ratio. Apparently some of the carbon in peats is very resistant to microbial attack and is not readily usable by general purpose decay organisms. Consequently, these organisms are not excessively encouraged, and they do not tie up the nitrates.
(v) Availability of nutrients in organic soils. Nitrogen. Nitrogen content in organic soils are high in comparison with a mineral soil.
Phosphorus and Potassium:
Both the phosphorus and potassium content of an organic soil are low as compared to a mineral soil. Unlike mineral soils, organic soils do not fix phosphorus and potassium.
Calcium:
Organic soils are comparatively high in calcium. In-spite of this high lime content, the majority of organic soils are distinctly acidic. Owing to high cation adsorption capacity of organic soils that they may be at a low percentage base saturation and carrying large amounts of exchangeable calcium. At the same time, the percentage base saturation is such as to assure a decidedly acid condition (Table 11.2).
Management of Organic Soils:
The productivity of organic soils depends upon proper management. All sorts of crops can be grown on organic soils but it is especially suitable for vegetable, flowers and pasture.
(i) Tillage operation:
Organic soils are porous and open, therefore, generally needs packing rather than loosening. A soil compacting roller is an important implement in management of organic soils. The compacting of soils allows the roots to come into closer contact with the soil and facilitate the capillary movement. Compacting of soil also tends to reduce the blowing of the soil during dry weather.
(ii) Water management:
A reasonably high water table (between 45 and 75 cm from the soil surface) assures a ready water supply for vegetables and other shallow- rooted crops grown on organic soils. It also reduces wind erosion and oxidation of organic matter from the soil surface.
(iii) Use of lime:
Ordinarily, use of lime in organic soils is less in comparison to mineral soils because organic soils are usually adequately supplied with calcium. But acidic muck soils contain high inorganic matter and result in dissolution of iron, aluminium and manganese to the extent of toxicity. Under these conditions, liming is necessary to obtain normal plant growth.
(iv) Use of fertilizers:
Organic soils are very low in phosphorus and potassium elements, therefore, phosphatic and potassic fertilizers should be applied.
Nitrogen is needed in organic soils when succulent vegetables are grown.
(v) Use of micronutrients:
Organic soils need some of the micronutrients such as copper, zinc, manganese and boron.
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