FRACTIONATION AND CHARACTERIZATION OF ZINC

IN SOILS OF DRY AND INTERMEDIATE ZONES OF

SRI LANKA

K.A. Nandasena and K.P.A.D. Pathirathna

Department of Soil Science, Faculty of Agriculture, University of Peradeniya, Sri Lanka.

ABSTRACT

Zinc (Zn) is one of the essential micro nutrients required for plant growth. Zn appears to be the third most yield limiting element in soil. Main source of Zn in soil is the Zn bearing minerals in the lithosphere. Average Zn content of world soils ranges from 10-300 ppm. Zn in soil exists in complexation and association with various oxides, organic matter, minerals, in an adsorbed form to colloids and in soil solution.

Present study was conducted to fractionate and characterize Zn in soils collected from 0 – 30 cm depth of fourteen locations of Dry and Intermediate zones of Sri Lanka. Fourteen soils were subjected to fractionation described by Murthy (1982) with certain modifications. Available (water soluble plus exchangeable), organic matter complexed, amorphous sesquioxide bound, crystalline sesquioxide bound and residual Zn fractions were separated from total Zn of the soils. Some of the soil parameters were also determined and used in a correlation study with the extracted Zn fractions.

Results showed that total Zn of these soils varies from 60.9 – 391.1 ppm. Available Zn (water soluble plus exchangeable Zn) ranges from 1.8 – 4.18 ppm with the distribution of 0.58 – 4.36% of total Zn. It can be concluded the available Zn in soils is very low. Organic matter complexed Zn, amorphous sesquioxide bound Zn and crystalline sesquioxide bound Zn range from 1.14 – 5.79, 1.34 – 2.62 and 3.53 – 12.46 ppm with the distribution of 0.36 – 5.45%, 0.5 – 3.14% and 1.12 – 11.89% respectively. Residual Zn is the largest fraction obtained. It ranges from 47.78 to 374.67 ppm with the distribution of 78.45 – 97%.

When different Zn fractions correlated with some soil parameters, the total Zn was given a significant correlation with clay and silt fractions of soils (at 0.05 level). However, total Zn was not correlated with organic matter content of soil. This reveals that soil Zn is mainly associated with the mineral fraction of soils. Among different Zn fractions, crystalline sesquioxide bound Zn and residual Zn were significantly correlated with clay fraction of soil. This may be attributed to the close association and complexation of crystalline sesquioxide bound Zn and residual Zn with the clay fraction of soil.

INTRODUCTION

Zinc (Zn) is one of the essential micro nutrients required for plant growth. Zn appears to be the third most yield limiting element in soil and its content varies from soil to soil. Main source of Zn in soil is the Zn bearing minerals in the lithosphere. An average Zn content in the lithosphere is about 80 ppm and the Zn content in world soils ranges from 10 – 300 ppm (Lindsay, 1972). Zn in soil exists in complexation and association with various oxides, organic matter, minerals, in an adsorbed form to colloids and in soil solution (Barber, 1995). Knowledge pertaining to the distribution of different Zn fractions in soil is much useful to understand the availability of Zn for crops and Zn dynamics in soil. Very few studies had been conducted so far to estimate available Zn as well as to fractionate Zn in some soils of Sri Lanka (Pavanasasivam and Kalpage, 1974; Nandasena and Rathnayake, 2001; Nandasena, 2003). However, these studies were mainly confined to up country soils. Information on the distribution and characterization of different Zn fractions is lacking for soils of other regions of Sri Lanka. Therefore, the present study was conducted to fractionate and characterize Zn of soils of Dry and Intermediate Zones of Sri Lanka.

MATERIALS AND METHODS

Representative surface soil samples were collected at 0 – 30 cm depth from fourteen locations covering rice fields and cultivated uplands of Dry and Intermediate Zones of Sri Lanka (Table 1). Soil samples were air-dried and passed through 2 mm sieve prior to the analysis. Some of the physico-chemical characteristics of the fourteen soils were determined using standard analytical techniques and given in Table 1. Total Zn of soils was determined by atomic absorption spectrophotometer after wet digestion of soils with concentrated perchloric - nitric acid mixture. Fractionation of Zn was performed according to the method described by Murthy (1982) with certain modifications. All determinations were replicated twice. In this sequential fractionation procedure, calcium chloride, copper acetate, acidified ammonium oxalate and dithionite citrate bicarbonate extractants were used to extract water soluble and exchangeable, organic matter complexed, amorphous sesquioxides and crystalline sesquioxides bound Zn respectively. The residual Zn was calculated by subtracting all extractable Zn from total Zn of the soil. Simple correlation was performed between different Zn fractions and some selected soil parameters study their relationships.

RESULTS AND DISCUSSION

Soils used in this study, are Low humic glay and Reddish brown earths belongs to Alfisol order in Dry and Intermediate zones of Sri Lanka. Physico-chemical characteristics of soils are widely varied and at present, used for rice and other field crops cultivation. Total Zn of surface horizon of fourteen soils ranges from 60.9 ppm (Mawathagama) to 391.1 ppm (Matale). Studied soils showed a wide variation in total Zn. This may be due to the variation in organic matter content and in the mineral composition of the soils. In the world soils, as an average the total Zn content ranges from 10 to 300 ppm. A study reported that relatively a high amount of total Zn was recorded in a soil of Sitaeliya of which organic matter content is high compared to a soil from Bandarawela (Nandasena, 2003).

Organic colloids hold Zn through adsorption and possibly by precipitation mechanisms. It has been reported that the excess organic matter appears to inactivate soil Zn and retard its uptake by the plants (Barber, 1995). When Zn containing minerals are present in soil that directly proportional to the result of the total Zn analysis Udo et al. (1970) reported that the high amounts of Zn occurred with the increased levels of organic matter and fine clay fraction in ten Arizona soils.

In this sequential fractionation procedure, calcium chloride, copper acetate, acidified ammonium oxalate and dithionite citrate bicarbonate extractants were used. Calcium chloride extraction was inserted to the sequential procedure to extract water soluble and exchangeable Zn. Copper acetate solvates organic matter complexed Zn (Juo and Kamprath, 1979). Acidified ammonium oxalate and dithionite citrate bicarbonate solvate amorphous sesquioxides bound and crystalline oxide bound Zn respectively (Mckeague and Day, 1966). The residual Zn was calculated by subtracting all extractable Zn from total Zn of the soil. All the Zn fractions are given in the Tables 2 and 3.

The water soluble and exchangeable Zn were ranged from 1.8 (Kurunegala) to 4.18 (Weuda) ppm. In all soils, the percentage distribution of water soluble and exchangeable Zn ranged from 0.58 (Matale) to 4.36% (Madatugama – LHG). In relation to the total Zn a high amount of water soluble and exchangeable Zn is present in Madatugama LHG soil (Table 3).

Copper acetate appears to remove mainly an organic matter bound Zn which is considered as organic matter complexed Zn. In Sri Lankan soils, complexed Zn ranged from 0.36 (Mahailuppallama – LHG) to 5.45% (Mawathagama) of total Zn. It has been reported that the exchangeable and complexed Zn of two Philippines soils ranged from 0.6 – 7.4% of total Zn (Murthy, 1982).

Generally, high amounts of amorphous and crystalline sesquioxides such as aluminium and iron oxides are found in highly weathered tropical soils. The acidified ammonium oxalate solubilizes amorphous sesquioxides bound Zn in the soil. The distribution of amorphous sesquioxides bound Zn was also showed a variation in studied soils and ranged from 0.50 (Mahailuppallama – LHG) to 3.14% (Mawathagama) of total Zn (Table 3). Other studies conducted on Zn fractionation showed somewhat variations in results. Murthy (1982) showed that the amorphous sesquioxides bound Zn in wetland rice soils in Philippines ranged from 1.8 – 6.8% of total Zn. It was also reported that the amorphous sesquioxides bound Zn in up country soils of Sri Lanka ranged from 4.47 to 15.14% of total Zn (Nandasena, 2003). The variation in amorphous sesquioxides bound Zn could be attributed to the soil type having different mineralogical composition.

Crystalline aluminium and iron oxides bound Zn fraction of studied soils ranged from 1.12 (Mahailuppallama – LHG) to 11.89% (Dambulla) of total Zn (Table 3). The high crystalline oxides bound Zn in soils may be attributed to the presence of high amounts of aluminium and iron oxides. These oxides precipitate with Zn firmly and transform into crystalline forms having definite chemical structure. Negative correlation between the Zn bound to crystalline sesquioxides and plant uptake suggests that the crystalline sesquioxides bound Zn is not available for plant uptake (Murthy, 1982).

Residual Zn is the fraction remained after the extraction of water soluble, exchangeable, organic matter complexed and oxides bound Zn from the total Zn of soil. This fraction gave the highest amount of Zn compared to other Zn fractions (Table 3). Most of the residual Zn may be associated with soil minerals other than sesquioxides and humus fraction of soil. Residual Zn of soils ranged from 78.45 (Mawathagama) to 97% (Mahailuppallama – LHG) of total Zn of soil.

A simple correlation was performed to study the interrelationships between different Zn fractions separated and some selected soil parameters. The results of the correlation study with the correlation coefficient (r) values are given in the Table 4. In this study, total Zn of soils of Dry and Intermediate zones and organic matter did not give any significant correlation. This is contradictory with the previous study conducted using up country soils where a significant positive correlation occurred (Nandasena, 2003). Total Zn of studied soils was significantly correlated with both clay% (r = 0.6010) and silt% (r = 0.5760) of soil. This may attribute to the fact that the soil Zn is mainly associated with the finer mineral fraction (clay and silt) of soil. Clay and silt fractions of soils include Zn adsorbed silicate clay minerals, Zn bearing minerals, Zn bound sesquioxides etc. Out of all Zn fractions, crystalline sesquioxides bound Zn and residual Zn only showed significant positive correlation with clay fraction of soils (Table 4). This also indicates that the crystalline sesquioxide bound Zn and residual Zn have a close association with finer fraction of soil. These two Zn fractions are deriving from Zn bearing minerals of soil. The crystalline oxides bound and residual Zn therefore perhaps the major forms of Zn in soils that are not available for plant uptake. None of the fractions other than crystalline sesquioxide bound Zn and residual Zn showed a significant correlation with soil parameters such as pH, organic matter, CEC, clay and silt contents (Table 4).

CONCLUSIONS

Residual Zn fraction was the largest fraction and ranged from 78.45 – 97% of total Zn in Sri Lankan soils. Crystalline sesquioxide bound Zn fraction was the second largest fraction which ranged from 1.12 – 11.89%. Relatively low amounts of Zn were recorded for water soluble and exchangeable Zn, organic matter complexed Zn and amorphous sesquioxide bound Zn. Total Zn is closely associated with clay and silt fractions of soils. Similarly crystalline sesquioxide bound and residual Zn are also associated mainly with the clay fraction of soil.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the assistance given by Mr. M.M.C.B. Ekanayake, Technical officer, Department of Soil Science, Faculty of Agriculture during the laboratory analyses.
