Sanjib Kumar Behera A,C, Dhyan Singh B, B. S. Dwivedi B, Sarjeet Singh B, K. Kumar B, and D. S. Rana B

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1 CSIRO PUBLISHING Australian Journal of Soil Research, 2008, 46, Distribution of fractions of zinc and their contribution towards availability and plant uptake of zinc under long-term maize (Zea mays L.) wheat (Triticum aestivum L.) cropping on an Inceptisol Sanjib Kumar Behera A,C, Dhyan Singh B, B. S. Dwivedi B, Sarjeet Singh B, K. Kumar B, and D. S. Rana B A Indian Institute of Soil Science, Nabibagh, Berasia Road, Bhopal , Madhya Pradesh, India. B Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi , India. C Corresponding author. sanjib bls@rediffmail.com Abstract. Intensive farming with high yielding cultivars, application of high analysis NPK fertilisers, and reduced use of organic manures caused a decrease in the availability of zinc (Zn) in Indian soils. We collected soil and plant samples from an ongoing long-term experiment at Indian Agricultural Research Institute, New Delhi, to study the distribution of different fractions of Zn in an Inceptisol and their contribution towards the Zn availability in soil and Zn uptake in maize wheat crop rotation. The treatments used for the study were NPK, NPK + FYM, NPK + Zn, and control (no fertiliser or manure). The DTPA-Zn concentration in soil was higher where Zn had been applied and declined with an increase in soil depth. The distribution of different fractions of Zn under various treatments and depths was inconsistent, and varied in a cropping year. The average concentration of total Zn (mg/kg) was 183, 183, 171, and 211 in , , , and m depth, respectively. Residual Zn was the dominant portion of total Zn at all soil depths. Grain and stover yield of maize ranged from 1.10 to 2.43 t/ha and 1.22 to 2.46 t/ha, respectively, under different treatments, whereas, the yield of wheat grain varied from 2.25 to 4.69 t/ha and that of wheat straw from 2.56 to 5.20 t/ha. Highest uptake of Zn by both the crops occurred in Zn-treated plots. Zinc associated with easily reducible manganese, carbonate and iron and aluminum oxides contributed directly towards DTPA-extractable Zn. Sorbed Zn (SORB-Zn) and Zn associated with organic matter (OM-Zn) contributed significantly towards Zn uptake by the 2 crops. Additional keywords: zinc fractions, DTPA-extractable Zn, long-term experiment, maize wheat sequence, Zn availability, Zn uptake. Introduction Micronutrient deficiencies, particularly zinc (Zn) deficiency, in field crops are widespread all over the world because of increased Zn demands of intensive cropping systems and adoption of high-yielding cultivars with relatively greater Zn demand. Other reasons for the increase in Zn-deficient areas are enhanced production of crops on soils that contain low levels of Zn, increased use of high analysis fertilisers containing low amounts of Zn, decreased use of animal manures, composts, and crop residues, and involvement of natural and anthropogenic factors that limit adequate plant nutrient availability and create nutrient imbalances (Fageria et al. 2002). Although the amount of zinc needed for crop growth is far less than that of macronutrients, Zn deficiency in soil has been reported widely from different parts of the world (Thorne 1957; Katyal and Vlek 1985; Welch et al. 1991). The problem is more acute in India, where enhancing productivity through intensive cropping has occurred in past 4 decades. One study has found Zn deficiency in nearly 47% soil samples collected from agricultural crop fields (Singh 1998). In soils, Zn exists in 5 discrete pools: (1) water soluble, (2) easily exchangeable, (3) adsorbed, chelated, or complexed, (4) associated with secondary minerals, and (5) held in primary minerals (Viets 1962). The contribution of these pools towards Zn availability to the plants depends on the dynamic equilibrium among different fractions. The amount and rate of transformation of these pools of zinc determine the size of labile Zn pool. Previous studies attempted to evaluate the availability of Zn pools between the size of the fractions and plant uptake of Zn under field (Soon and Bates 1982) or greenhouse (Murthy 1982; Mandal and Mandal 1986) conditions by using correlation analysis. The conclusions drawn on the basis of correlation studies in different soils led to several ambiguities. For example, Mandal and Mandal (1986) reported that Zn bound to hydrous oxides contributed as much to plant uptake as exchangeable Zn, whereas Murthy (1982) attributed a smaller role to hydrous oxide-zn. However, Iyengar et al. (1981) reported negative partial correlations between Zn associated with oxides and Zn uptake. Information on the distribution of different fractions of Zn in a soil under intensive cropping systems, and their CSIRO /SR /08/010083

2 84 Australian Journal of Soil Research S. K. Behera et al. contribution towards Zn availability for uptake by plants, is lacking. We undertook the present investigation to study the effect of continuous cropping and fertiliser applications on different fractions of Zn in soil and their availability for uptake of Zn by crops. Materials and methods Experimental site An ongoing long-term experiment (LTE) commenced in 1971 on a Typic Haplustept soil at the Indian Agricultural Research Institute Farm, New Delhi (28 N, 77 E, 250 m above mean sea level), was used for this study. The climate of New Delhi is semi-arid, subtropical with dry hot summers and cold winters. The average annual rainfall is 650 mm, of which nearly 80% is received through the south-west monsoon during July September. The average monthly minimum and maximum temperatures are 18.3 and 30.4 C, respectively, with May the hottest month and January the coolest. The soil of the experimental field is sandy clay loam (29% clay, 54% sand, 17% silt) of alluvial origin, non-calcareous, very deep (>2 m), well-drained, flat (about 1% slope), and belongs to hyperthermic family. The original soil ( m depth) was mildly alkaline (1 : 2.5 water ph 8.3) and non-saline (EC 0.45 ds/m) and contained 0.44% organic carbon (Walkley and Black 1934), 16 kg/ha Olsen-P (Olsen et al. 1954), 155 kg/ha NH 4 OAc-K (Hanway and Heidel 1952), and 1.11 mg/kg DTPA-extractable Zn (Lindsay and Norvell 1978). Treatment details The LTE consists of 10 treatments, comprising of different nutrient management options in a randomised block design with 4 replications. Crop rotation consists of irrigated maize (Zea mays L.) wheat (Triticum aestivum L.) cropping system since 2002 (earlier pearl millet-wheat-forage cowpea cropping system was followed until 1982, and maize-wheat-forage cowpea until 2002)..However, 4 treatments were chosen for this study: C, unfertilised, unmanured control; NPK, recommended N, P, and K fertiliser to both the crops; NPK + FYM, recommended N, P, and K fertiliser to both the crops, and 15 t FYM/ha.year to maize only; NPK + Zn, recommended N, P, and K fertiliser to both the crops, and 10 kg zinc sulphate/ha to wheat only. The recommended fertiliser for maize and wheat was 120 kg N, 26 kg P, and 33 kg K/ha since Fertiliser N, P, K, and Zn were applied through urea (46% N), diammonium phosphate (DAP, 18% N, 20% P), muriate of potash (KCl, 50% K), and zinc sulfate (22% Zn), respectively. The entire amount of P, K, and Zn and half of the recommended N were applied as basal dressing at sowing. The remaining N was topdressed at tillering in wheat and at knee-high stage in maize. Maize cv. Ganga Safed 2 was sown in rows 0.75 m apart at 20 kg/ha during the last week of June, and subsequent wheat cv. HD 2329 at 100 kg/ha in rows 0.20 m apart during second week of November. The crops were harvested manually at the maturity, and the aboveground biomass was removed from the field. Soil and plant analysis Soil samples were collected with a screw type auger from , , , and m depths before sowing of the maize crop (2003) and after harvest of wheat ( ). The subsamples collected from 4 randomly chosen positions in each plot were mixed thoroughly to obtain a representative soil sample. The samples were air-dried, ground in wooden pestlemortar, and passed through a 2-mm sieve. The soil samples were analysed for different fractions of Zn following the fractionation scheme of Ma and Uren (1995): Form/association Step Operational definition Zn in soil solution 1 Distilled water, 1 : 5, shaking 2 h (WS) Sorbed (SORB) Zn 2 1% NaCaHEDTA in 1 M NH 4 OAc, ph 8.3, 1 : 10, shaking 2 h Zn associated with easily reducible Mn 3 0.2% quinol in 1 M NH 4 OAc, ph 7. 0, 1 : 10, shaking 2 h (ERMn) Zn associated with carbonate (CA) M Na/H acetate, ph 4.74, 1 : 10, soaking 15 h/shaking 3 h Zn associated with organic matter (OM) Zn associated with Fe and Al oxides (FeOx) 5 5 ml 30% H 2 O 2, ph 4.74, digested twice at 85 C, and extracted as for carbonate fraction M (NH 4 ) 2 C 2 O M H 2 C 2 O 4, ph 3.25, 1 : 10, soaking 15 h/shaking 2h Residue (RES) 7 Total minus sum of the extractable amounts The extractions were conducted in 50-mL polypropylene centrifuge tubes. Between each successive extraction, the supernatant was obtained by centrifuging at 3000 r.p.m. for 15 min and filtering. The total concentration of Zn was determined after digestion of the soil with H 2 SO 4 HClO 4 HNO 3 HF mixture. The concentration of Zn in the supernatant was measured using GBC 904 AA atomic absorption spectrophotometer (AAS). DTPA-extractable Zn was also determined (Lindsay and Norvell 1978). For Zn uptake studies, grain and straw/stover samples were collected from each plot at the time of harvest of maize and wheat. The thoroughly washed plant samples were dried in oven at 70 C for 48 h, ground in a stainless steel Wiley mill, and digested in a di-acid mixture of HNO 3 and HClO 4 (Jackson 1973). Zn concentration Table 1. Influence of fertiliser applied on the DTPA soil extractable Zn (mg/kg) in pre-maize and post-wheat soil after 32nd cropping cycle of maize wheat rotation Treatment Soil depth m m m m Pre-maize sampling Control NPK NPK + FYM NPK + Zn CD A (P = 0.05) Post-wheat sampling Control NPK NPK + FYM NPK + Zn CD A (P = 0.05) A Critical difference.

3 Zinc fractions and plant uptake of zinc Australian Journal of Soil Research 85 was then determined in aqueous extracts of the digested plant material by AAS. Total Zn uptake was calculated as dry weight of stover/straw multiplied by the Zn concentration and added to the grain yield multiplied by Zn concentration in grain. Statistical analysis Data on soil Zn concentration and Zn uptake by the crops were subjected to F-test using the procedure for randomised block design (Gomez and Gomez 1984). Path coefficient and correlation studies were carried out using SPAR 1 and SPSS softwares, respectively. Results and discussion DTPA soil extractable zinc The DTPA-extractable Zn concentration of NPK + Zn was significantly higher in surface ( m) and subsurface ( m) soil for both the pre-maize and post-wheat sampling times than for other treatments (Table 1). It indicates that continuous application of Zn results in accumulation in the surface and migration of Zn to the subsurface layer. Annual application of organic manure FYM resulted in higher DTPA soil extractable Zn, which may be due to mineralisation of organically bound Zn in FYM and formation of organic chelates of higher Table 2. Effect of nutrient management on depth-wise distribution of different forms of Zn in soil under a long-term experiment at New Delhi PM, Pre-maize; PW, post-wheat; ND, not detected; CD, critical difference; n.s., not significant Treatment m m m m Overall PM PW Mean PM PW Mean PM PW Mean PM PW Mean mean Sorbed zinc (SORB-Zn) (mg/kg) C NPK NPK + FYM NPK + Zn Mean CD (0.05) Sampling stage (SS) : 0.14 Fertiliser treatment (FT) : 0.25 Sampling depth (SD) : 0.20 FT SD : 0.50 SS FT SD n.s. Zinc associated with easily reducible manganese (ERMn-Zn) (mg/kg) C ND ND 0.55 NPK ND ND 0.68 NPK + FYM ND ND 0.82 NPK + Zn ND ND 0.83 Mean CD (0.05) Zinc associated with carbonate (CA-Zn) (mg/kg) C ND ND 1.18 NPK ND ND 1.53 NPK + FYM ND ND 1.30 NPK + Zn ND ND 1.77 Mean CD (0.05) n.s. Zinc associated with organic matter (OM-Zn) (mg/kg) C ND ND 0.94 NPK ND ND 1.24 NPK + FYM ND ND 1.19 NPK + Zn ND ND 1.45 Mean CD (0.05) Zinc associated with Fe and Al oxides (FeOx-Zn) (mg/kg) C NPK NPK + FYM NPK + Zn Mean CD (0.05) Residue zinc (RES-Zn) (mg/kg) C NPK NPK + FYM NPK + Zn Mean CD (0.05) n.s.

4 86 Australian Journal of Soil Research S. K. Behera et al. stability (Kher 1993; Verma and Subehia 2005). The DTPA-Zn concentration of the subsurface soil was lower than that of the surface soil in all treatments, which is in agreement with findings of Kher (1993) for a long-term maize wheat system. Depth-wise distribution of various fractions of zinc The fractions of Zn in soil at 4 depths during the 32nd cycle of cropping were measured by sequential fractionation analyses (Table 2). The water-soluble Zn (WS-Zn) concentration was <0.01 mg/kg in all 4 depths of the soil. A range of soils generally have negligible or very low levels of Zn in the water-soluble fraction (Iyengar and Deb 1977; Edward Raja and Iyengar 1986; Dhane and Shukla 1995). An average of 3.9 mg/kg of SORB-Zn was recorded in m soil depth across the treatments and the sampling stages, and it was higher than other soil depths. In the pre-maize and post-wheat samples, SORB-Zn varied from 2.6 to 3.1 mg/kg for different fertiliser treatments. ERMn-Zn and CA-Zn were too low to be detected in m depth. The average value of CA-Zn over the soil depths was in the range mg/kg under various treatments, and declined with increasing soil depth. The OM-Zn was in the range of 2.1 (control) to 3.1 mg/kg for NPK + Zn in surface soil. The OM-Zn values declined with increasing soil depth, and were undetectable at m depth as organic matter content of lower layers of the soil declined. Similarly, Iyengar and Deb (1977) and Singhal and Rattan (1995) reported lower values of OM-Zn in soils with low organic matter content. Averaging treatments and sampling stages, the FeOx-Zn was 6.6, 7.0, 3.2, and 2.8 mg/kg at , , , and m soil depth, respectively. The highest FeOx-Zn was measured in the NPK + Zn treatment (5.6 mg/kg) (Table 2). The sampling stage and treatment effects on total Zn were not significant, and its average values were 183, 183, 171, and 211 mg/kg at , , , and m depth, respectively (data not given). Averaged across the soil depths, residual Zn ranged between 173 and 183 mg Zn/kg for different treatments. Residual Zn associated with the minerals fraction constituted the major amount of native soil Zn (Mandal and Mandal 1986; Chandrashekhar and Kedlaya 1988; Liang et al. 1990; Suresh Kumar et al. 2004; Verma and Subehia 2005). Yield and zinc uptake by component crops The crop yields and Zn uptake during the 32nd year are given in Table 3. Grain and stover yields of maize ranged from 1.10 to Table 3. Influence of continuous manuring and fertiliser use on yields and zinc uptake by component crops in 32nd cycle of maize wheat cropping rotation Treatment Maize yield Wheat yield Total Zn uptake in grain plus (t/ha) (t/ha) stover/straw at maturity (g/ha) Grain Stover Grain Straw Maize Wheat Control NPK NPK + FYM NPK + Zn CD A (P = 0.05) A Critical difference. Table 4. Intercorrelations among zinc fractions (WS-Zn was not detected) in soil ( m) collected at pre-maize and post-wheat stages WS-Zn, Zn in soil solution; SORB-Zn, sorbed Zn; ERMn-Zn, Zn associated with easily reducible Mn; CA-Zn, Zn associated with carbonate; OM-Zn, Zn associated with organic matter; FeOx-Zn, Zn associated with Fe and Al oxides, RES-Zn, residue Zn. *P < 0.05 SORB-Zn ERMn-Zn CA-Zn OM-Zn FeOx-Zn RES-Zn Pre-maize sampling SORB-Zn ERMn-Zn CA-Zn * OM-Zn * 0.28 FeOx-Zn RES-Zn 1.00 Post-wheat sampling SORB-Zn * 0.84* ERMn-Zn CA-Zn * OM-Zn FeOx-Zn RES-Zn 1.00

5 Zinc fractions and plant uptake of zinc Australian Journal of Soil Research t/ha and 1.22 to 2.48 t/ha, respectively (Table 3). The highest maize yield was obtained under NPK + FYM. The grain yield of wheat ranged from 2.25 to 4.69 t/ha and yield of wheat straw from 2.56 to 5.20 t/ha for different treatments. In the control, where neither fertilisers nor manures were applied, the grain yield was the lowest (2.25 t/ha). The uptake of Zn by maize ranged from 120 g/ha for the control to 269 g/ha for NPK + Zn. The Zn uptake by maize for NPK alone or with FYM was in the range g Zn/ha, which was significantly less than that under NPK + Zn. Compared with control, the uptake of Zn by wheat was significantly higher in all other treatments. The Zn uptake under NPK was significantly less than that under NPK + Zn (325 g/ha) and NPK + FYM (291.7 g/ha). The lowest Zn uptake by wheat was obtained in control plots followed by that for NPK. It reveals that continuous cropping with or without NPK fertilisers and in the absence of Zn led to lower removal of Zn compared with treatments having Zn. Increased uptake of Zn for wheat with addition of Zn has also been reported by Bajwa and Paul (1978) and Singh and Nand Ram (2005). Relationships among different fractions of zinc in soil and their contributions towards availability in soil and uptake by crops A significant and positive correlation between CA-Zn and OM- Zn (r = 0.83*) and between OM-Zn and FeOx-Zn (r = 0.86*) was observed in pre-maize soil (Table 4), whereas, significant and positive correlations existed between SORB-Zn and CA- Zn (r = 0.84*), between SORB-Zn and OM-Zn (r = 0.84*), and between CA-Zn and RES-Zn (r = 0.89*) in post-wheat soil. These results suggest the existence of dynamic equilibrium among different soil Zn fractions (Sarkar and Deb 1982; Singh and Abrol 1986; Ghanem and Mikkelsen 1987; Singhal and Rattan 1995). The depletion of Zn from fractions that are readily available to plants is replenished from the other fractions of soil Zn. DTPA soil extractable Zn in soil for the pre-maize soil samples was not correlated with either fractions of zinc, whereas in post-wheat soil it was correlated with SORB- Zn (r = 0.86*) and CA-Zn (r = 0.96**) (Table 5). Similarly, Singhal (2003) reported better correlation of DTPA-extractable Zn with sesquioxides occluded and organically bound Zn. Path coefficient analyses revealed that in pre-maize soil, ERMn- Zn, CA-Zn, FeOx-Zn, and RES-Zn contributed directly and positively towards the soil DTPA Zn concentration. However, the effect of sorbed and organic matter bound Zn fractions was negative and direct (Table 5). For post-wheat soil, the effect of all the fractions except RES-Zn on DTPA-extractable Zn was direct and positive. The reason for variation in contribution of different fractions of Zn towards the soil DTPA-Zn at different stages of sampling is unknown. However, easily reducible manganese, carbonate, and sesquioxide Zn contributed directly to the DTPA- Zn in both the pre-maize and post-wheat soil. The results suggest that the fractions of soil Zn are in equilibrium as indicated earlier by Viets (1962). This agrees with Edward Raja and Iyengar (1986), Liang et al. (1990), and Verma and Subehia (2005) that soil Zn fractions contribute to DTPA soil extractable Zn. Different fractions of Zn in soil were not correlated with Zn uptake by maize (Table 6). However, SORB-Zn (P = 0.26) and OM-Zn (P = 2.23) contributed directly and positively towards the uptake of Zn by maize, and ERMn-Zn (P = 0.68), CA- Zn (P = 1.11), FeOx-Zn (P = 1.93), and RES-Zn (P = 0.62) influenced via OM-Zn. Overall correlations between different fractions of Zn and the uptake of Zn by wheat were not significant (Table 6). However, path coefficient analysis revealed that SORB-Zn (P = 0.36) and OM-Zn (P = 1.60) contributed directly towards the uptake of Zn by wheat, whereas it was indirectly influenced by SORB-Zn (P = 0.49) via ERMn-Zn, Table 5. Path coefficients indicating direct and indirect effects of Zn fractions (WS-Zn was not detected) on DTPA-extractable Zn in surface soil ( m) Direct effects (bold values) are on main diagonal, values on the horizontal rows are indirect effects, and r values on the last column are simple correlation coefficients of factors listed horizontally with dependent variables. WS-Zn, Zn in soil solution; SORB-Zn, sorbed Zn; ERMn-Zn, Zn associated with easily reducible Mn; CA-Zn, Zn associated with carbonate; OM-Zn, Zn associated with organic matter; FeOx-Zn, Zn associated with Fe and Al oxides, RES-Zn, residue Zn. *P < 0.05; **P < 0.01 SORB-Zn ERMn-Zn CA-Zn OM-Zn FeOx-Zn RES-Zn r Pre-maize SORB-Zn ERMn-Zn CA-Zn OM-Zn FeOx-Zn RES-Zn Post-wheat SORB-Zn * ERMn-Zn CA-Zn ** OM-Zn FeOx-Zn RES-Zn

6 88 Australian Journal of Soil Research S. K. Behera et al. Table 6. Path coefficients indicating direct and indirect effects of Zn fractions (WS-Zn was not detected) in pre-maize soil on Zn uptake by maize and wheat Direct effects (bold values) are on main diagonal, values on the horizontal rows are indirect effects, and r values on the last column are simple correlation coefficients of factors listed horizontally with dependent variables. WS-Zn, Zn in soil solution; SORB-Zn, sorbed Zn; ERMn-Zn, Zn associated with easily reducible Mn; CA-Zn, Zn associated with carbonate; OM-Zn, Zn associated with organic matter; FeOx-Zn, Zn associated with Fe and Al oxides, RES-Zn, residue Zn SORB-Zn ERMn-Zn CA-Zn OM-Zn FeOx-Zn RES-Zn r Zn uptake by maize SORB-Zn ERMn-Zn CA-Zn OM-Zn FeOx-Zn RES-Zn Zn uptake by wheat SORB-Zn ERMn-Zn CA-Zn OM-Zn FeOx-Zn RES-Zn by ERMn-Zn (P = 0.48) via OM-Zn, by CA-Zn (P = 0.80) via OM-Zn, by FeOx-Zn (P = 1.39) via OM-Zn, and by RES-Zn (P = 0.45) via OM-Zn. Sorbed and organic matter bound Zn contributed positively and directly to Zn uptake in both crops. Therefore, if the proportion of both fractions (SORB-Zn and OM-Zn) in soil is higher, the Zn uptake by the crops would be higher. Hence, zinc fractions contributed directly and/or indirectly to Zn uptake by crops (LeClaire et al. 1984; Sarkar and Deb 1985), indicating occurrence of reversible equilibrium among the fractions of Zn in soil (Sarkar and Deb 1982; Singh and Abrol 1986; Murthy and Schoen 1987; Chitdeshwari and Krishnasamy 2005). Conclusions The results of this study reveal that DTPA-extractable Zn in surface and immediate sub-surface soils of Zn treated plots was significantly higher than that under other treatments. It declined with increasing soil depth. Distribution of different fractions of Zn in soil profile was highly inconsistent. Residual Zn was the dominant portion of total Zn. DTPA soil extractable Zn, both in pre-maize and post-wheat soil, was prominently influenced by easily reducible manganese, carbonate, and sesquioxide bound fractions. Sorbed and organic matter bound zinc contributed directly and positively for the Zn uptake by both the crops. Acknowledgments This manuscript is a part of the first author s doctoral dissertation. The first author thanks the Indian Agricultural Research Institute and Council of Scientific and Industrial Research, New Delhi, for facilities and financial assistance. The authors appreciate the support provided by staff associated with the long-term experiment. We acknowledge the constructive suggestions provided by the reviewers in improving quality of the manuscript. References Bajwa MS, Paul J (1978) Effect of continuous application of N, P, K and Zn on yield and nutrient uptake by irrigated wheat and maize on available nutrients in a tropical acid brown soil. Journal of Indian Society of Soil Science 26, Chandrashekhar P, Kedlaya N (1988) Soil zinc fractions and their availability in an Oxisol. Journal of Indian Society of Soil Science 36, Chitdeshwari T, Krishnasamy R (2005) Path analysis of soil zinc fractions and rice yield as influenced by zinc enriched organic manures. Advances in Plant Sciences 18, Dhane SS, Shukla LM (1995) Distribution of different fractions of zinc in benchmark and other established soil series of Maharashtra. Journal of Indian Society of Soil Science 43, Edward Raja M, Iyengar BRV (1986) Chemical pools of zinc in some soils as influenced by sources of applied zinc. Journal of Indian Society of Soil Science 34, Fageria NK, Baligar VC, Clark RB (2002) Micronutrients in crop production. Advances in Agronomy 77, Ghanem SA, Mikkelsen DS (1987) Effect of organic matter on changes in soil zinc fractions found in wetland soils. Communications in Soil Science and Plant Analysis 18, Gomez KA, Gomez AA (1984) Statistical procedure for agricultural research. In International Rice Research Institute book. (John Wiley & Sons Inc.: Singapore) Hanway JJ, Heidel H (1952) Soil analyses methods as used in Iowa state college soil testing laboratory. Iowa Agric 57, Iyengar BRV, Deb DL (1977) Contribution of soil zinc fractions to uptake and fate of zinc applied to the soil. Journal of Indian Society of Soil Science 25, Iyengar SS, Martens DC, Miller WP (1981) Distribution and plant availability of soil zinc fractions. Soil Science Society of America Journal 45, Jackson ML (1973) Soil chemical analysis. (Prentice Hall of India Pvt. Ltd: New Delhi) Katyal JC, Vlek PLG (1985) Micronutrient problems in tropical Asia. Fertilizer Research 7, doi: /BF

7 Zinc fractions and plant uptake of zinc Australian Journal of Soil Research 89 Kher D (1993) Effect of continuous liming and cropping on DTPA extractable micronutrients in an Alfisol. Journal of Indian Society of Soil Science 41, LeClaire JP, Chang AC, Levesque CS, Sposito G (1984) Trace metal chemistry in arid-zone field soils amended with sewage sludge. IV. Correlations between zinc uptake and extracted soil zinc fractions. Soil Science Society of America Journal 48, Liang J, Stewart WB, Karamanos RE (1990) Distribution of Zn fractions in Prairie soil. Canadian Journal of Soil Science 70, Lindsay WL, Norvell WA (1978) Development of a DTPA soil test for zinc, iron, manganese and copper. Soil Science Society of America Journal 42, Ma YB, Uren NC (1995) Application of new fractionation scheme for heavy metals in soils. Communications in Soil Science and Plant Analysis 26, Mandal LN, Mandal B (1986) Zinc fractions in soils in relation to zinc nutrition of low land rice. Soil Science 142, doi: / Murthy ASP (1982) Zinc fractions in wetland rice soils and their availability to rice. Soil Science 133, doi: / Murthy ASP, Schoen HGA (1987) A comparative study of the soil zinc fractions determined by chemical methods and electro-ultrafiltration (EU) and their relations to zinc nutrition in rice. Plant and Soil 102, doi: /BF Olsen SR, Cole CV, Watanable FS, Dean LA (1954) Estimation of available phosphorus in soils by extracting with sodium bicarbonate. U.S. Department of Agriculture Circular No Sarkar AK, Deb DL (1982) Zinc fractions in rice soils and their contribution to plant uptake. Journal of Indian Society of Soil Science 30, Sarkar AK, Deb DL (1985) Fate of fertiliser zinc in a black soil (Vertisol). Journal of Agricultural Science, UK 104, Singh MV (1998) Micronutrient management. In 50 years of natural resource management research. (Eds GB Singh, BR Sharma) pp (Indian Council of Agricultural Research: New Delhi) Singh MV, Abrol IP (1986) Transformation and movement of zinc in an alkali soil and their influence on the yield and uptake of Zn by rice and wheat crop. Plant and Soil 94, doi: /BF Singh V, Nand Ram (2005) Effect of 25 years of continuous fertiliser use on response to applied nutrients and uptake of micronutrients by rice wheat cowpea system. Cereal Research Communications 33, doi: /CRC Singhal SK (2003) Available zinc and its relationship with soil zinc fractions in some alluvial soils. Annals of Agricultural Research 24, Singhal SK, Rattan RK (1995) Soil zinc fractions and their availability in some Inceptisols and Entisols. Journal of Indian Society of Soil Science 43, Soon YK, Bates TE (1982) Chemicals pools of cadmium, nickel and zinc in polluted soils and some preliminary indications of their availability to plants. Journal of Soil Science 33, doi: /j tb01782.x Suresh Kumar P, Rattan R, Singh AK (2004) Chemical fractions of zinc in soils and their contribution to available pool. Journal of Indian Society of Soil Science 52, Thorne DW (1957) Zinc deficiency and its control. Advances in Agronomy 9, Verma S, Subehia SK (2005) Zinc availability in an acid Alfisol as influenced by long term cropping in a wet temperate zone of western Himalayas. Agropedology 15, Viets FG Jr (1962) Chemistry and availability of micronutrients in soils. Journal of Agricultural and Food Chemistry 10, doi: / jf60121a004 Walkley AJ, Black IA (1934) An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Science 37, doi: / Welch RM, Allaway WH, House WA, Kubota J (1991) Geographic distribution of trace element problems, In Micronutrients in agriculture. 2nd edn (Eds JJ Mortvedt, LM Shuman, RM Welch) pp (Soil Science Society of America: Madison, WI) Manuscript received 1 June 2007, accepted 19 December