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Soils and Soil Microflora

 The soils of Sri Lanka have been classified into 14 great groups. The soils of the Sinharaja belong to the Red-Yellow Podzolic group, with newly formed alluvial soils along river valleys. The origin of these soils is mostly residual while weathering of parent material into laterites and lateritic soils is increased by high rainfull and temperature. The soils are also impermeble due to the presence of ferrogenous and kaolinitic soil material rich in alumina. Traces of magnesia and lime prevalent in the original rock can be detected wherever leaching has not been excessive. Variation in soil depth is considerable and can range from a few centimeters in very rocks areas to 4 or 5 meters on lateritic soil on slopes.

                       Rain forest soils are usually well developed. Soil profiles of  Sinharaja  studied by  Sri Lankan  scientists are summarized in Table 3.

Table 3. Information on soil profiles

Horizon Texture Comb structure Consistency Presence of roots & mottling

Soil depth and color for two sites

cm Munsell value cm Munsell value


moderately fine



roots abundant 0-2 5YR 3/2 0-1 5YR 3/2


- - - - 2-4


5YR 3/2

7.5YR 5/6



10YR 5/6

10YR 5/8


moderately coarse strong to moderate subangular blocky


roots sbundant at the beginning completely absent at the end 17-57 7.5YR 5/6 20-50 0YR 5/4


- - - maroon mottling of laterites while increases with depth 57-100 5YR 5/6

5YR 4/6

50-100 10YR 5/8

10YR 4/

While the mineral constituents of soil arise largely from parent material and bed rock, organic substances originate from the living matter above ground. Analysis of soil nutrient content in surface soils at the Sinharaja has shown greater concentration of exchangeable cations in the upper layers probably due to increased activity of soil micro-organisms as well as continuous enrichment by litter. Nutrient analysis of litter in the Sinharaja shows that nutrients from decaying plant matter are released in the order of potassium, magnesium, calcium, phosphorus and nitrogen. Some studies on soil nitrogen fixation shows that the activity is highest in surface soils upto 10.5 centimeters, while fixation in leaf and twig litter is considerably less.

                          Not surprisingly rain forest trees have a well-developed surface root system, and are highly efficient in nutrient absorption, and even under high rainfall conditions, there is little or no leaching. The soil itself if poor in nutrients because the nutrients released from decomposing organic matter are immediately utilized by the vegetation. Therefore the nutrients are locked up in the bio-mass by very tight, highly efficient nutrient recycling.

                          The major agents of decay and nutrient recycling are bacteria and fungi. Studies carried out on the micro-fungi at the Sinharaja have revealed the presence of 35 different species, the commonest being Trichoderma harzinum and Penicillium simplicissimum. The more common fungal species are listed in Table 4.  

Table 4. The more common microfungal species occurring in undisturbed forest soils at Sinharaja
Division Phycomycetes Ascomycetes Fungi imperfectil
Species Circinella simplex Gelatinospora Brasiliensis(2) Trichoderma harzianum (3)
  Mucor hiemalis(4) Chetomium seminudum Penicillium simplicissimum (16)
  Mortierella rammaniana Botryphaerla ribis Aspergillus (8)
    Eupenicillium erhlichii Curvularia (5)
      Fusarium (5)
      Pestalatiopsis (4)
      Pithomycetes (2)
      Cladosporum (3)
      Arthrinium arundinis