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Vertical Distribution of Trace Elements and Their Significance in Some Forest Soils of Bangladesh

M. M. Hasan
Director (Retd.)
Soil Science Division, Bangladesh Forest Research Institute, Chittagong.

Abstract/ Executive Summary:

Soil samples were collected from the genetic horizons of three soil profiles representing three different land types. Trace elements in soil and in leaf samples were determined spectrometric ally. It was observed that the vertical distribution of trace elements was related to the amount of clay rather than to the genetic soil horizons. The clay content in these profiles was the attribute of the alluvial parent material. Therefore, the redistribution of trace elements was not due to pedogenetic effects. The clay fraction contains higher amounts of trace elements in all the profiles. It was, therefore, assumed that the topsoil might be depleted of trace elements with the loss of clay by illuviation or by run-off. Trace element/iron oxide ratios of several soil samples show that the vertical distribution of trace elements in the profiles studied were not significant. It was also observed that in the studied profiles Mn and Cd content was higher but that of other elements was lower than the FAO’s prescribed highest levels of total content of trace elements in soil. Trace element contents of several leaf samples of tree species were examined and found not related to the trace element contents of soil.

Keywords: Clay fraction, Forest soils, Profile, Trace elements

Location: The tidal swamps, consisting of mangrove forests; the high terraces, consisting of deciduous forests and the tertiary hills, consisting of mixed evergreen forests soils of Bangladesh.

Start Date: 01-07-1980

End Date: 30-06-1982

Program Area: Crop-Soil-Water Management

Commodity: Soil fertility

Objectives:

To provide a preliminary guideline on the total trace element content of some soil profiles.

Methodology:

Soil samples representing the genetic horizons were collected from the vertical cross-sections. Samples were dried, ground and passed through a 2 mm sieve. Organic matter was removed by H₂O₂ treatment. Organic matter-free soil samples ground to a uniform size were passed through a 300 mesh sieve. Aqua regia extracts of the soil samples were used for trace element determination. Phillip’s inductively coupled plasma atomic emission spectrometer; a multi-element analyzer for liquids and dissolved solids was used. The soil samples were then dispersed by 2% Na₂CO₃ solution and the clay fraction was collected by syphon method. Silt and sand fractions were separated by sieving. The collected sand, silt and clay fractions were washed repeatedly with alcohol and finally with acetone with a view to making them free of chloride. Oven dry ground leaf samples were burnt to ashes in a furnace at 450⁰C. HNO₃ and trace elements were determined spectrometric ally.

Source of Information: Bano Biggyan Patrika (a journal of forest science), Vol. 12, No. 1 & 2, January & July, 1983

Article Link (Online Journal):

Funding Source:

1. Government

Budget: 1,00,000.00

Total Budget (Tk.) : 1,00,000.00

Achievement/Findings:

Toxicity and deficiency of various trace elements in soils are questions of interpretation based on the crops to be grown. Determination of the plant available fraction depends on the ability of the extract ant employed to simulate plant root with respect to uptake of trace elements. It was not the intention of this discussion to consider this aspect; so, the extractable forms of trace elements were not determined. Total tract element content which is of geochemical interest can seldom be used as an appropriate criterion for plant availability. The elemental fraction incorporated in unweathered minerals may not have direct biological significance. Nevertheless, the total content may be used to fix a limiting level not to be exceeded without a real danger of noxious effects. The analysis data show that in the pedons studied, Mo and Cd contents are higher and the contents of other trace elements are lower than those of other trace elements are lower than those of the FAO’s level. However, for the forest species found in Bangladesh neither the deficiency nor the toxicity of trace elements have yet been studied critically.Leaf analysis shows that some plants absorb some particular trace element at a higher dose than the others. Trace element content in the leaves of various tree species cannot be correlated with the total content of these elements in soil.

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