SPECTROSCOPIC DETERMINATION OF METAL IMPURITIES IN COMMERCIAL RAW MATERIAL FERTILISER OF SRI LANKA

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1 Ceylon Journal of Science (Physical Sciences) 18 (2014) Chemistry SPECTROSCOPIC DETERMINATION OF METAL IMPURITIES IN COMMERCIAL RAW MATERIAL FERTILISER OF SRI LANKA A.R.M.S.P. Ratnayake and A.N Navaratna Department of Chemistry, University of Peradeniya, Peradeniya, Sri Lanka. (*Corresponding author s ayanthin@pdn.ac.lk) (Received: 23 September 2013 / Accepted after revision: 04 March 2014) ABSTRACT Uncontrolled application of fertiliser on agricultural fields may have a long-lasting impact on agricultural ecosystems of Sri Lanka. Erroneous and excessive use of fertiliser of local and foreign origin may cause accumulation of metals in agricultural soils, leading to an imbalanced nutrient structure and toxicity towards living organisms. This study focused on determination of the availability of metallic impurities in commonly used raw material fertiliser of Sri Lanka. Seven locally available types of fertiliser were selected for the study, using X-ray fluorescence (XRF) and Atomic Absorption Spectrometry (AAS) as analytical techniques. Urea, muriate of potash, sulfate of ammonia, Eppawala rock phosphate, high grade rock phosphate, imported rock phosphate and triple super phosphate represented 90% of raw material fertiliser types used in Sri Lanka. Special attention was given towards quantitative determination of two heavy metals: Cd and Cr. Sampling was conducted at warehouses of three market leader companies involved in fertiliser import and distribution in Sri Lanka, covering more than 70% of overall fertiliser import in to the country. XRF analysis revealed a group of non-lethal metal species present as impurities. Maximum percentages were respectively: Fe %, Mn % and Cu % in imported direct rock phosphates and their derivatives. Cd was detected in only two fertiliser types: Triple super phosphate and imported rock phosphate. Average Cd concentrations in triple super phosphate and imported rock phosphate were respectively 0.6 ppm and 1.0 ppm. Cr, which was detected in all potassium and phosphate fertiliser, was in the range 1 12 ppm, with a high degree of variability among sample sets belonging to different companies. Sri Lankan rock phosphates seemed to contain fewer impurities compared to imported ones and some imported fertiliser batches turned out to be sub-standard in terms on nutrient content, with high burdens of impurity elements. A recently published report from the World Health Organization has indicated a possible link between heavy metals in fertiliser/agrochemicals and the Chronic Kidney Disease of Uncertain aetiology (CKDu) prevalent in the dry zone of Sri Lanka. Therefore, further studies on rare earth and heavy metal impurities in fertiliser are a timely need. University of Peradeniya 2014 INTRODUCTION In many developing countries including Sri Lanka, inorganic and organic fertilisers are being vastly used in agricultural fields, to meet the demands of an increasing food requirement, for a rapidly growing population (Granhall et al., 1987). During the recent years in Sri Lanka, there has been a significant increase in the usage of inorganic fertiliser in the agricultural sector (World Resource Institute, 2003). The total paddy cul tivated lands in Sri Lanka were 1065 hectares in 2010 and the year round application of subsidized fertiliser was 490 tons (Central Bank of Sri Lanka, 2012). Because of the necessity of fertiliser in enhancing the agricultural output, government intervention in the fertiliser market started in 1962 with the introduction of a fertiliser subsidy scheme. The main objective

2 of the subsidy scheme was to make fertiliser available as cheaply as possible, in order to encourage its wider use (Ekanayake, 2006). Though classified as an essential commodity in the agricultural sector, soil modification caused by fertiliser may have detrimental consequences (Ekanayake, 2006). Erroneous as well as excessive usage on agricultural soils, to promote enhanced growth, may cause abnormalities in natural nutrient availability in the soil. It may also lead to nutrient loss. For instance, nitrogenous fertiliser often undergoes great losses of ammonia by volatilisation and of N 2 by nitrification (Granhall et al., 1987). Most importantly, fertiliser has contributed in adding foreign chemical species such as heavy metals into agricultural soils (Dissanayake and Chandrajith, 2009). Health risks are known to arise from the presence of impurities such as heavy metals in commercial fertiliser. Especially rock phosphates are a potential source for the transport of metals in the world (Kponblekou and Tabatabai, 1994). Phosphate fertiliser, according to their origin may contain various trace elements. Such elements, when applied to the soil, may persist in soils as for their long life times. In acidified soils, they may also be readily available for plants (Kponblekou and Tabatabai, 1994). This bioavailability may create a potential risk of accumulation in plants and consequently in human body through the food chain (McLaugli and Mineau, 1995). These heavy metals can be toxic to living systems when present in high concentrations (Kponblekou and Tabatabai, 1994). There is a wide variety of retail fertiliser applied in the Sri Lankan agricultural sector, but most of them come as a blend between a limited numbers of single-nutrient fertiliser types. The use and effect of each fertiliser type on the crop depends on the proportion of each raw material fertiliser, containing nutrients such as nitrogen phosphorous and potassium. There are more than seven types of raw material fertiliser used in the blending process. This study considered only seven types, which are the ones with the most use. Urea, Muriate of Potash (MOP), Ammonium Sulfate (SA), Eppawala Rock Phosphate (ERP), High Grade Rock Phosphate (HERP), Imported for Ca, Sr, Fe, Mn and Cu was µg/g. All XRF readings were converted to percentage values for the ease of interpretation. The Rock Phosphate (IRP) and Triple Super Phosphate (TSP) were analysed in the study. Some physical and chemical characteristics of these fertiliser types are given in Table 1. Most of the fertiliser used in Sri Lanka, apart from ERP and HERP, are imported due to the unavailability of natural deposits and lack of industrial technology for manufacturing. ERP and HERP are locally produced and urea, MOP, SA, IRP and TSP are imported from various continents of the world, by a number of companies operating in Sri Lanka. METHOD OF STUDY Three companies were selected for sampling, which had a cumulative share of over 70% of the total fertiliser imported to Sri Lanka. The sampling was done such that the overall fertiliser usage in Sri Lanka was represented to the highest extent as possible. The identities of the three companies will be kept undisclosed, hence they will be denoted as A, B and C. Samples of all seven fertiliser types were obtained within a time period of a month, employing the standard composite sampling method. Fifteen 50 g samples were obtained from each warehouse stack of each fertiliser type. All Fifteen samples were mixed together on a clean plastic sheet and well homogenised. The same sampling method was repeated to make up a total of 21 samples from each company (Table 2). An exception was made at Company-A by obtaining four samples of each fertiliser. Collected samples were sealed in airtight polythene wrapping, transported to the laboratory and stored in rigid airtight plastic bottles away from direct sunlight. The XRF instrument used, was a FISCHER XAN-FD, with a beam diameter of 1 mm and a X-ray energy of 8-10 kev (Helmutfischer Instruments, 2010). A calibration was done using a series of mixtures of AnalaRgrade calcium phosphate, manganese dioxide, cuprous potassium sulfate, ferric phosphate, strontium phosphate and silicone. The calibration standard was prepared to resemble the physical characteristics of the sampled fertiliser. Limits of detection (LOD) were estimated by measuring respective peak heights of spectrums. Approximate LOD AAS was a THERMO SCIENTIFIC SOLAAR-M series Spectrometer with a nitrous oxide/acetylene fuel gas system (Nitrous Ox-

3 ide/acetylene L/min, flame temperature ~ 2600 C) and an absorbance range of A (Thermo Scientific Instruments, 2010). The wavelength range was nm and the deuterium lamp background correction was applied. Background signals were corrected with < 2 % error. LOD was 0.2 ppm for Cd and 1 ppm for Cr. All fertiliser samples were well ground in to a fine powder for the XRF analysis. After the XRF analysis, samples were subjected to an acid digestion for absorbance measurements by AAS. Nitric acid (molecular weight g/mol) was AnalaR-grade, produced by Surechem products LTD, UK. Manufacturer specification indicated following impurities: Cl - < 0.005%, SO 4 2- < 0.01%, As %, Fe < 0.005%, Pb < 0.01% and trace non-volatile residue. Cd and Cr impurities were absent according to manufacturer specification. 15 g samples were treated with concentrated nitric acid and kept in heat for 30 minutes. The resultant solution was filtered using a sintered glass crucible of porosity 4.0, centrifuged for 10 minutes and kept standing for 6 hours. 2.5 ml of each solution was extracted from the top and subjected to a four-fold dilution before being aspirated to the AAS for analysis. One, five, eight and ten ppm calibration standards were prepared from Cd and Cr stock solutions. Flame atomisation method was used and measured absorbance values were subjected to a blank correction. RESULTS AND DISCUSSION X-Ray fluorescence analysis The XRF analysis was conducted using solid samples in fine powder form. In samples of urea and SA analysed, presence of metals was not indicated. However, there is a high possibility for concentrations to be below the LOD of the instrument. Graphical plots of mean values were drawn for only metals showing significant levels. Sample numbers 1-4, 5-7 and 8-10 belonged to companies A, B and C respectively. The XRF analysis revealed a group of less toxic metals in potassium and phosphate fertiliser (Table 3). No metals were detected in urea and SA by the XRF analysis. MOP is a fertiliser generally used to supply Potassium (K) to crops. Most MOP fertiliser are derived from potassium minerals such as Sylvite (KCl) and other potassium silicates (Manning, 2010). The K level in MOP ranged between % and usual K percentages range between % in potash fertiliser (Dept. of Agriculture, Sri Lanka, 2000). The tested samples did not follow a consistent pattern (Figure 1). Cu, which was the only impurity metal detected by the XRF, was present in substantially low levels, ranging between %. Due to the diverse nature of potash ores around the world, a standard level for impurities has not been established. Availability of impurities is highly source dependent for mining products such as MOP. The major element in ERP was Ca which was present in a range: % (Figure 2). ERP which is mined from the Eppawala rock phosphate deposit, contained apatite (Ca 5(PO 4) 3.OH,F,Cl) as the main constituent mineral (Dissanayake and Chandrajith., 2009). Every fertiliser company in Sri Lanka utilises Eppawala rock phosphates for their long term plantation crop fertiliser blends. ERP is considered to be useful only on long term crops owing to low solubility of phosphate (Dept of Agriculture, Sri Lanka, 2000). Commercial ERP consists of both apatite and the matrix and is considered to contain % Ca (Hewawasam and Dahanayake, 1999). P percentages were calculated assuming that Ca is present only in the form of Ca 3(PO 4) 2 (Figure 2). Percentages of Ca and P obtained were below the levels reported by Dahanayake and Hewawasam who used X- ray diffractometry as the analytical method. (Hewawasam and Dahanayake, 1999). This could be due to lower sensitivity of the XRF used in this study. Fe, Cu and Sr were present in percentages: %, % and % respectively. Cu and Sr are naturally available in earth and can be assumed to be present in the apatite matrix. Early studies also indicate a presence of % Fe, in secondary apatite and the matrix, in the form of hematite (Fe 2O 3) (Dahanayake et al., 1995). The HERP contains finely ground high grade apatite crystals. Therefore, HERP naturally contains more Ca and fewer impurities. Ca and P were indicated to be % and % respectively (Figure 3a).

4 Table 1: Selected physico-chemical characteristics of fertiliser (Ceylon Fertilizer Corporation, 2012; Dahanayake and Subasinghe, 1991) Type Recommended composition Main chemical components Bulk density (kg/m 3 ) Water solubility (at 20 C) Urea 46% nitrogen NH 2CONH ~ 100 % MOP 60% K 2O KCl % SA 20.6% nitrogen (NH 4) 2SO ~ 75 % ERP 28% phosphorous P 2O % HERP 40% phosphorous P 2O % IRP 28% phosphorous P 2O % TSP 46% phosphorous P 2O > 90% Table 2: Sampling scheme used for fertiliser Fertiliser Type and No of samples Company UREA MOP SA ERP HERP IRP TSP A B C Table 3: Content of some metals in fertiliser samples determined by XRF (BD Blow Detection limit) Type % K % Cu % Ca % Sr % Mn % Fe MOP BD BD BD BD ERP BD HERP BD BD BD IRP BD BD TSP BD BD Urea BD BD BD BD BD BD SA BD BD BD BD BD BD The anticipated behavior for levels of Ca was observed in the HERP sample series. Cu was not detected and Sr was detected in very low levels ( %). Fe was present in low levels with a range of %. Levels 0.5 % were reported by Dahanayake and Subasinghe (1991). Sr ranged between % resembling the levels observed in ERP (Figure 3b). The IRP is imported by commercial fertiliser blenders in Sri Lanka from a wide selection of Cu were not detected in HERP and Fe levels were lower than that of ERP. Therefore, the higher purity of HERP was confirmed. It was in agreement with previously published reports (Hewawasam and Dahanayake, 1999). Trace levels of Fe could be attributed to contamination by the matrix. Both rock phosphates of Sri Lankan origins exhibited decent purity in terms of nutrient-proportion and burden of impurity metals. Also no hazardous heavy metals were detected in Sri Lankan rock phosphates. The IRP is imported to Sri Lanka from countries such as Brazil, China, Morocco and Russia. The composition of each IRP type depends on the mineralogical make-up of the source ore. Figure 3c illustrates the variation of Ca and P percentages in IRP samples. First four samples exhibited significantly low Ca and P levels. In addition, they were high in impurities such as Fe, Cu and Mn (Figure 3d). The reset of the sample series showed a consistent pattern with adequate Ca and P levels and lower Fe, Cu and Mn levels. The first group of samples (1 4: Company - A) was indicated to be highly substandard in this context, with higher Fe percentages than Ca. Imported rock phosphates were found to

5 Ratnayake and Navaratne/Ceylon Journal of Science- Physical Sciences 18 (2014) 31-xx Figure 1: Cu and K in Muriate of Potash Figure 2: Ca, P, Fe, Cu and Sr in ERP Table 4: Elemental composition based on AAS analysis (as ppm )(BLD-below detection limit) Fertiliser Company/ Mean SD 10-2 Mean SD 10-3 Sample Cr (ppm) Cd (ppm) MOP A (1-4) BLD - B (3-7) BLD - C (8-10) BLD - ERP A (1-4) BLD - B (3-7) BLD - C (8-10) BLD - HERP A (1-4) BLD - B (3-7) BLD - C (8-10) BLD - IRP A (1-4) B (3-7) BLD - C (8-10) TSP A (1-4) contain varying and higher levels of trace metals than rock phosphates of Sri Lankan origin. TSP is yet another popular fertiliser type currently being imported to Sri Lanka. It maintains a high demand in the fertiliser market owing to it s higher solubility. Commercial makes consist % of soluble P (Mullins and Sikora, 1994). TSP is produced by subjecting rock phosphate to a chemical reaction with phosphoric acid (New Zealand Institute of Chemistry, 2008). TSP can be chemically denoted as Ca(H 2PO 4) 2.XH2O (International Plant nutrient Institute, 2013). Any impurities present in the original rock phosphate ore could remain in TSP after the acidulation process as well. Ca and P percentages in TSP were % and % respectively. Fe, Cu and Mn were found to be present in low levels with Fe: %, Cu: % and Mn: %. Cu and Mn levels were consistent throughout the series while Fe levels showed a considerable between sample variability (Figure 4).

6 As emphasised before, types of impurities vary according to the source. For instance, Mullins and Evans reported that Al, Fe and Mg were significantly present in TSP of Moroccan origin (Mullins, Evans, 1990). Fe was detected in all samples except urea and SA, which were of synthetic origin. Presence of Fe could be due to the unrefined nature of phosphate fertiliser. Rock phosphates are raw fertiliser, from either open pit or underground mines. Contamination by natural Fe containing minerals such as hematite (Fe 2O 3) and magnetite (Fe 3O 4) or top soil may have enhanced the Fe content. Mn was only present in foreign rock phosphates. ERP and HERP which were from Sri Lankan phosphate deposits were indicated to be far superior in quality compared to IRP. TSP which is a chemical derivative of foreign rock phosphate was indicated to be lower in Ca and P compared to untreated rock phosphates. However it is assumed to be compensated by its high solubility and the faster release of P in a more bioavailable form. Among fertiliser tested, urea, MOP, SA and TSP produced nutrients in readily available forms to the plants. Little research has been done on the combined effect of fertiliser, soil minerals and microorganisms on crops. Flame atomic absorption spectrometric (AAS) analysis As revealed by the AAS analysis, Cr was detected in five types of fertiliser except in urea and SA. Cd was only detected in IRP and TSP (Table 4). The levels could have been below the LOD of the instrument. Cr in MOP ranged from 1 14 ppm throughout the sample series (Figure 5a). First four samples (1-4) exhibited higher levels, mostly above 10 ppm. The next two sets (5 7 & 8-10) showed lower levels ( ppm). Literature reports that Cr levels in fertiliser, obtained from the retail market, to be in the same order of magnitude (Chandrajith et al., 2012). An irregular pattern was observed throughout the series implying the chemically diverse nature of ore based fertiliser such as MOP (Figure 5a). Cr levels in the overall the ERP series exhibited a rage of ppm. The pattern was rather coarse yet was within the range reported in literature. In HERP, detected Cr levels were at the boundary of the LOD ( ppm). The higher purity of HERP was yet again confirmed. However Cr presence in HERP could have been due to the contamination by the matrix. There is no evidence to support the presence of Cr in pure apatite crystals. In samples of IRP and TSP, levels of Cr were in the range of ppm and ppm respectively (Figure 5b). Detected Cd levels are illustrated in figure 5C and 5d. The highest Cd content was 1.7 ppm and was observed in samples 8 10 (Company C). Sample group - 2 did not indicate a presence of Cd and the group 1 indicated a level around 1 ppm. Cd levels in the overall TSP series were ranging between ppm. Levels of same order of magnitude have been reported (Chandrajith et al., 2012), (Atafar et al., 2010). Among fertiliser types analysed using the AAS, Cd was below the LOD in urea, MOP, SA, ERP and HERP. Highest levels of Cd were observed in foreign rock phosphates and, as suggested by earlier reports, rock phosphates of Sri Lankan origin did not seem to harbor Cd impurities (Illeperuma, 2000). Cd levels in rock phosphate ores around the world are known to vary depending on the mineralogy (Anon., 1989). Cadmium naturally occurs in soils at levels up to about 20 ppm (Wixson, 1977). A survey conducted of 35 sedimentary phosphate rock deposits worldwide indicated that 43 % had concentrations <10 ppm, 25 % had concentrations ppm and 11 % had ppm of Cd (Kauwenbergh, 2002). The average cadmium content in European fertiliser is 138 mg/kg of P. Phosphate ores in Morocco, which is one of the fertiliser suppliers to Sri Lanka, are reported to have had Cd over 20 ppm(finnish Environment Institute., 2000). In Chile, which again is an occasional supplier to Sri Lanka, Cd in rock phosphate is at ppm (Carnelo, 1997). The maximum tolerance level for Cd in fertiliser in Australia is 450 mg/kg and the minimum is 35 mg/kg in Netherlands (Mortvedt, 1996). However a global recommended level has not been set at the moment. In the field, fertiliser is applied synchronised with the crop seasons. It is usually applied in bulk quantities such as kilos. Therefore, excessive application is usually common. According to observed levels of Cd in IRP and TSP fertiliser, the calculated gross mass of Cadmium in 1 kg of IRP is about 1.7

7 Ratnayake and Navaratne/Ceylon Journal of Science- Physical Sciences 18 (2014) 31-xx Figure 3: (a) Percentages of Ca and P in HERP; (b) Percentages of Fe and Sr in HERP; (c) Percentage of Ca and P in IRP; (d) Percentages of Fe, Cu and Mn in IRP. mg and for TSP it is about mg/kg. The fertiliser use for paddy/rice cultivation alone was 0.35 million tons in 2002 (Ekanayake, 2006). General paddy fertiliser, which is a blend of urea/sa, MOP and TSP, consists of phosphorous as the most abundant nutrient (Wijewardena, 2006). The usual paddy fertiliser blend has the ratio 10: 27.4: 23 of N, P and K respectively (Dissanayake and Wijayatilleka, 2000). Almost half the weight of a unit of paddy fertiliser is comprised of TSP and therefore; the quantity of TSP applied in rice fields is remarkably high. These trace heavy element such as Cd and Cr available in fertiliser, are much more available to biota than those amounts bound to soil particles (Sager, 1997).Consequently, there seem to be a high risk of soil being irreversibly contaminated by impurity metals such as Cd present in fertiliser. The current increasing trend of fertiliser application can be expected to continue. As a result of this, local agricultural fields might get destabilised of their natural chemical composition and nutrient availability. Rare-earth metals such as Mn, Sr, Ti, and Cu exert a weaker impact, compared to toxic metals such as Cd and Cr, which may give rise to long lasting health issues. The World Health Organization (WHO) has implicated Cd as a major contributor for CKDu, prevalent in the north central region of Sri Lanka. WHO tolerance limits for Cd and Cr in drinking water, are respectively 3 and 50 ppb (World Health Organization, 2011). The average total Cd content of cultivated soils in Europe is 0.5 mg/kg (Finnish Environment Institute, 2000). No such general values for heavy metals have been established for Sri Lankan soils. Currently in most rural agricultural areas, the use of highly soluble TSP for paddy cultivation is common. Water purification is only available in a handful of areas and it too does not guarantee removal of heavy metals. Therefore, in these agricultural areas, Cd is likely to be ingested above tolerance limits. Addition of heavy metals such as Cd to the soil may cause emergence of diseases within agricultural communities of Sri Lanka. Furthermore crop products contaminated with heavy metals may endanger the total population. If the current trend continues without intervention, incidence of epidemics such as CKDu may increase within the country and additional complex health issues may be led to rise.

8 Figure 4: Content of Ca, P, Fe, Cu and Mn in TSP Fig 5. (a) Concentrations of Cr in MOP and ERP; (b) Concentrations of Cr in IRP and TSP; (c) Concentrations of Cd in TSP; (d) Concentrations of Cd in IRP CONCLUSIONS Very few studies have sampled raw material fertiliser, directly from manufacturer warehouses, to investigate on their composition. During this study, metals such as Ca, K, Sr, Cu, Fe and Mn were quantitatively determined by XRF analysis with decent precision. Ultra trace level presence of metals such as Ni and W were indicated but were not quantifiable. In urea and SA no metals were identified. In MOP, ERP, HERP, IRP and TSP, less hazardous impurities such as Fe, Mn, Cr and Cu were identified. In IRP and TSP, Cd was detected in significant quantities. It can be concluded that the nutrient quality and level of impurities in most fertiliser used in Sri Lanka are highly source dependent. Recent studies have produced evidence supporting Cd as a contributory nephrotoxic agent for the mysterious endemic renal disease of Sri Lanka. Since Cd has been indicated to be a constituent of imported phosphates, in this research and other peer studies, the need for a better fertiliser standardisation protocol, especially for imported phosphate fertiliser, is highly recommended. ACKNOWLEDGEMENTS The authors wish to thank Professor Namal- Priyantha Mr. Anushka Bandaranayake, Mr. Aravinda Niriella, Mr. Mahinda Paranawitharana and the analytical chemistry research group of the Department of Chemis-

9 try, Faculty of Science, University of Peradeniya, Sri Lanka. REFERENCES Anon. (1989) Cadmium in phosphates: One part of a wider environmental problem. Phosphorus and Potassium 162 (4); Atafar, Z., Mesdaghinia, A., Nouri, J., Homaee, M., Yunesian, M., Ahmadimoghaddam, M. and Hossein Mahvi, A. (2010) Effect of fertilizer application on soil heavy metal concentration. Environmental Monitoring and Assessment 160 (1-4); Carnelo, L.G.L., de Miguez, S.R. and Marbán, L. (1997) Heavy metals input with phosphate fertilizers used in Argentina. Science of The Total Environment 204 (3); Ceylon Fertiliser Corporation. (2012) Accessed on: Assessed &Itemid=50&lang=en Chandrajith, R., Seneviratna, S., Wickramaarachchi, K., Attanayake, T., Aturaliya, T.N.C. and Dissanayake, C.B. (2010) Natural radionuclides and trace elements in rice field soils in relation to fertilizer application: study of a chronic kidney disease of Sri Lanka. Environmental Earth Sciences 60(1); Dahanayake, K., Ratnayake, M.P.K. and Sunil, P.A. (1995) Potential of Eppawala apatite as a directly applied low-cost fertiliser for rice production in Sri Lanka. Fertilizer Research 41; Dahanayake, K. and Subasinghe, S.M.N.D. (1991) Mineralogical, chemical and solubility variations in the Eppawala phosphate deposit of Sri Lanka - a case for selective mining for fertilizers. Fertilizer Research 28; Department of Agriculture of Sri Lanka. (2000) Integrated plant nutrient systems- Training manual. Dissanayake, C.B. and Chandrajith, R. (2009) Phosphate mineral fertilizers, trace metals and human health. Journal of the National Science Foundation of Sri Lanka 37 (3); Dissanayake, S.T. and Wijayatilleka, D.R. (2000) Incentives for increasing rice production; an outlook, Rice congress 2000.Abeysiriwardena, D.S., Dissanayaka, D.M.N., Nugaliyadde, L. (eds.) Department of Agriculture, Sri Lanka. Ekanayake, H.K.J. (2006) The Impact of Fertilizer Subsidy on Paddy Cultivation in Sri Lanka: Staff Studies-Central Bank of Sri Lanka 36: Finnish Environment Institute. (2000) Cadmium in fertilizers risks to human health and the environment: Study report for the Finnish Ministry of Agriculture and Forestry. Granhall,U., Kulasooriya, S.A., Hirimburegama, W.K., Silva, R.S.Y and Lindberg, T. (1987) Nitrogen fixation in some rice soils in Sri Lanka. MIRCEN Journal 3; Helmut-fischer Instruments (2010) Accessed on: fischer.com/globalfiles/broc_ AN_ _0606.pdf Hewawasam, T. and Dahanayake, K. (1999) A note on chemistry and mineralogy of apatite of Eppawala and Ridigama phosphate deposits. Sabaragamua University Journal 2; Illeperuma, O.A. (2000) Environmental pollution in Sri Lanka: a review. Journal of the National Science Foundation of Sri Lanka 28(4); International Plant nutrient Institute (2013) Accessed on: a4b8be72a35cd d9001a18da/ 467db10b83fe960d b005779d4! OpenDocument Kauwenbergh, S.J.V. (2002) Cadmium content of phosphate rocks and fertilizers. IFDC, USA. Kponblekou, A. andtabatabai, M. (1994) Metal contents of phosphate rocks.communications in Soil Science and Plant Analysis 26; Manning, A.C. (2010) Mineral sources of potassium for plant nutrition.a review.agronomy for Sustainable Development 30; McLauglin, A. and Mineau, P. (1995) The impact of agricultural practices on biodiversity. Agriculture, Ecosystems & Environment 55; Mortvedt, J. J. (1996) Heavy metal contaminants in inorganic and organic fertilizers. Fertilizer Research 43;

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