Received 7 October 2015; accepted 4 December 2015; published 7 December 2015
1. Introduction
The micronutrients i.e. Fe, Cu, Zn, Mn, Co, Ni, Mo, and S in soil play a very important role in plant growth, productivity, soil fertility and animal nutrition [1] . The main functions of the micronutrients in living organism are structural components of cell constituents and its metabolically active compounds, in the maintenance of cellular organization, in energy transformation, in enzyme action, etc. [2] . The increment in nutrient supply beyond a certain limit resulting in the decreased yield of plants is often be associated with the production of specific toxic effects [3] . The incidence of micronutrient deficiencies in soil and plants is increasing due to high and multiple plant yields. The quantification of both total and available (active form) of nutrients in soil is important [4] . The main sources of micronutrients in soils are rock weathering and atmospheric deposition in form of dust, precipitates, volatile compounds, etc. The micronutrients in soil occur in different chemical forms i.e. water soluble, exchangeable, specifically adsorbed, chelated or complexed, secondary clay minerals or oxide, primary minerals, etc. [5] .
Their available contents were leached out from soil with various extractants i.e. diethylenetriaminepenta acetic acid (DTPA), ammonium bicarbonate-DTPA (AB-DTPA), triethanolamine-DTPA (TEA-DTPA), Mehlich-1 (0.05 N HCl + 0.025 N H2SO4), Mehlich-3 (0.2 N CH3COOH + 0.25 N NH4NO3 + O.O15 N NH4F + 0.013 N HNO3 + 0.001 M EDTA), acid ammonium acetate-EDTA (AAA-EDTA), MgCl2, HCl (0.05 N), HNO3 (0.31 N), ammonium acetate, water, etc. [6] . However, most of these extractants were suffered from some shortcoming i.e. unable to extract several trace elements present in soil, not always efficient for all nutrients, etc. Thus now a days, multi-nutrient extractants i.e. Mehlich-3, AB-DTPA, TEA-DTPA, acid ammonium acetate-EDTA etc. were widely used for the extraction of micronutrients and trace elements from the soils [6] . Among them, AB-DTPA and TEA-DTPA were claimed better extractant than Melich-3 for Zn and Cu, but they extracted only Zn, Cu, Mn, and Fe. However, the AAA-EDTA leached out several nutrients i.e. Mn, Fe, Co, Ni, Cu, Zn, Al, Cd, Mo, Cr, Pb, Sr, P, etc. Ammonium oxalate, ammonium acetate, hot water, etc. were reported for the leaching of the available Mo from the soil. Of these oxalate is widely used for the extraction of available Mo from soil but it required prolonged extraction period (≈ 24 hr). The hot water extraction was recommended for leaching of the available Mo from the soil. Calcium chloride, Bray-1 (0.03N NH4F + 0.025 N HCl), Morgan’s reagent (sodium acetate-acetic acid, pH 4.8), deionized water, etc. were reported for the extraction of S from soil. The total content of micronutrients in soil was leached out with acids i.e. aqua-regia, HClO4, HF, HClO4 + HNO3, H2SO4 + HCl + HNO3, etc. [6] .
The micronutrient status in surface soils of some parts of India was reported [7] - [23] . However, the information on the levels of micronutrient i.e. Co, Ni and Mo in the soil is lacking. In this work, the status of eight micronutrients i.e. Fe, Cu, Zn, Mn, Co, Ni, Mo, and S in surface soils of 100 villages of Raipur district is described. The concentration variations, deficiencies and toxicities of the micronutrients in the soil are discussed.
2. Materials and Methods
2.1. Study Area
The most of urbanization and industrialization in central India has been marked nearby capital city, Raipur, Chhattisgarh state, India. Raipur area includes Raipur district (22˚33'N - 21˚14'N and 82˚6' - 81˚38'E) and surrounding districts i.e. Balodabazar and Gariabandh. They are situated in the fertile plains of Chhattisgarh region of the country. Hundred city, town and villages of Raipur area (»2.0 × 104 km2) were selected for determining the micronutrient status of the soil.
2.2. Sample Collection
Generally, three different types of soil i.e. red laterite, gray, yellow soils occurred in this region. Three different types of soil from 100 villages of Raipur block were collected, Figure 1. Soils were taken from horizon of 0 - 15 cm depth. A total 300 soil samples were collected in February 2013 as described in the literature [24] .
2.3. Analysis of pH and Extraction
The soils were dried, ground and sieved through a 2-mm sieve. All samples were stored in a 500-mL wide mouth polythene bottles for the analysis. A 10.0 g weighed amount of soil was taken in a 100-mL polythene conical flask by mixing with 20 mL deionized water. The mixture was shaken for 6 hrs, and their pH and electrical conductivity (EC) values were measured with the Hanna sensor-HI 991300N.
A mixed solution of reagents (E. Merck) i.e. AAAA-EDTA for the extraction of nutrient i.e. Mn, Fe, Co, Ni, Cu, Zn and PO43−was used by dissolving 38.5 g ammonium acetate, 9.5 g Na2EDTA and 29 mL acetic acid (17 M) into 1 L deionized water [25] . A 10 g dried and ground soil sample was taken into a 250-mL polyethylene flask with subsequent addition of 100 mL AAAA-EDTA solution. The mixture was equilibrated for 1 hr with a shaker, and solution was filtered through a 0.45 µm glass fiber filter in a 100-mLpolyethylene volumetric flask.
For Mo, a 10 g soil sample was taken into 250-mL conical polyethylene flask by mixing with 100 mL deionized water [26] . It was heated at boiling temperature for 10 min by subsequent filtering the cold solution as above. The activated charcoal and hot water were employed for the extraction of the available content of sulfate. A 10 g soil sample was mixed with 1 g activated charcoal and 100 mL deionized water into a 250-mL conical
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Figure 1. Representation of sampling locations in Chhattisgarh state of the country.
polyethylene flask. The mixture was shaken for a duration of 1 hr by a shaker and solution was filtered through a 0.45 µm glass fiber as above.
The mixed acid (H2SO4 + HCl + HNO3) was used for extraction of the total content of micronutrients i.e. Mn, Fe, Co, Ni, Cu and Zn. A 1.0 g dried and powdered soil sample was taken in a 100-mL Teflon beaker. Into it, 30.0 ml of mixed acid solution (H2SO4 + HCl + HNO3) was added. The mixture was heated until white fumes were no longer emitted. The residue was washed with hot dilute hydrochloric acid (0.01 N) and the hot water (50˚C). The mixture was filtered through a 0.45 µm glass fiber as above.
For leaching of total content of sulfate, a 1 g soil sample was mixed with 50 mL solution of acids: (HNO3 + HClO4) into a 100-mL Teflon beaker by keeping overnight (12 hr). The solution was concentrated to 20 mL by gentle heating, and the cold solution was filtered through a glass fiber as above. The filtrate was evaporated to the dryness by subsequent dissolving with deionized water in a 50-mL polyethylene flask.
2.4. Analysis of Micronutrients
The Flame GBC 932AAwas used for analysis of metals i.e. Mn, Fe, Co, Ni, Cu, Zn and Mo in the soil. The Dionex ion chromatography-1100―was employed for analysis of 
and
.
3. Results and Discussion
3.1. Soil Characteristics
The agricultural land includes » 50% area of the total land of the studied area. Three types of soils i.e. gray, yellow and red are available in the studied area. The red and yellow soil was originated by weathering of various rocks i.e. quartz, feldspars, mica and iron coated quartz formed over different geological periods. The yellow color was ascribed to the higher degree of hydration of the ferric oxide in these soils. The color shaded was also varied from reddish yellow to yellowish brown with often fine textured. The pH and EC of soil was ranged from 4.7 -7.7 and 100 - 900 µS/cm.
3.2. Iron
The levels of available micronutrients in the soils of Raipur area is presented in Table 1. Iron comprises about 5% of the earth’s crust and is the fourth most abundant element in the lithosphere [27] . The most of the soil iron was found in primary mineral, clays, oxides and hydroxides. The available and total content of Fe in soils of the studied area were varied widely and ranged from 30 - 8253 and 11676 - 40,928 mg/kg with mean value of 642 ± 186 and 19930 ± 5979 mg/kg, respectively. Considering 6 mg/kg as the critical value of Fe, the soils of studied area was found to be contaminated with a very high level of Fe.
3.3. Copper
The concentration of Cu in the earth’s crust is averaged 28 mg/kg [27] . The available and total status of Cu in soils of the studied area was ranged from 2.0 - 8.0 and 45 - 69 mg/kg with mean value of 4.3 ± 0.3 and 53 ± 16 mg/kg, respectively. A 0.2 and 50 mg/kg Cu were reported as critical and threshold value for Cu-deficiency and Cu-toxicity to plant growth. Almost all soil of this region was found to be contaminated with sufficient amount of Cu for the healthy growth of plants.
3.4. Zinc
The Zinc content of the lithosphere is 67 mg/kg [27] . Zinc has a strong tendency to combine with sulfide ores, and it occurs most frequently in the lithosphere as sphalerite. The available and total status of Zn in soils of this region was ranged from 0.7 - 5.0 and 27 - 56 mg/kg with mean value of 2.3 ± 0.2 and 38 ± 14 mg/kg, respectively. Critical limit for Zn-deficiency in different type of soils for different crops were ranged from 0.4 to 0.8 mg/kg. A few soils of studied area was found to be deficient in available Zn for the plant growth if the value 0.80 mg/kg was considered as a critical limit. A value of 50 mg/kg Zn was reported as threshold value for the plant toxicity. None of soil of the studied is contaminated Zn at the toxic level.
3.5. Manganese
Manganese concentration in the earth’s crust is 1000 mg/kg [27] . The available and total level of Mn in soils of this region lie in the range of 205 - 2800 and 2737 -10,122 mg/kg with mean value 1178 ± 119 and 6889 ± 2274 mg/kg, respectively. A 5.7 and 55 mg/kg were reported as the critical limit for Mn-deficiency and threshold value of Mn-toxicity for plant growth, respectively.
3.6. Cobalt
The average total cobalt concentration in the earth’s crust is 40 mg/kg [27] . The available and total concentration of cobalt in soils of this region were varied from 2.2 - 31.2 and 64 - 139 mg/kg with mean value 12.8 ± 1.3 and 119 ± 30 mg/kg, respectively. Considering the 2.5 mg/kg as the critical limit for Co deficiency in soil, almost all soils of this region may be rated as contaminated with sufficient level of Co for plant growth.
3.7. Nickel
The natural abundance of nickel in the earth’ crust is 47 mg/kg [27] . The available and total level of Ni in soils of this region were varied from 0.1 - 13.4 and 15 - 70 mg/kg with mean value of 3.9 ± 0.6 and 35 ± 11 mg/kg, respectively. A 0.1 and 50 mg/kg Ni in soil were considered as the critical limit, and threshold value of toxicity for plant growth, respectively. All type of soils were found to be contaminated with a sufficient level of Ni for plant growth.
3.8. Molybdenum
Molybdenum occurs in the soils in extremely small quantities, is usually found in concentrations of less than 1 mg/kg [27] . The available, and total Mo content in soils of the studied area varied from 0.1 - 8.9 and 3.4 - 9.2 mg/kg with mean 1.5 ± 0.3 and 4.6 ± 2.6 mg/kg, respectively. A 0.1 mg/kg was considered as the critical limit for Mo deficiency in soil. A 20.0 mg/kg Mo in soil was considered as threshold value for toxicity. None of the soil in this region was found to contain Mo at the toxic level.
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Table 1. Mean (n = 3) micronutrient status of soil, mg/kg.
3.9. Sulphur
The earth’s crust contain about 0.06% Sulphur [27] . It is mostly present as sulfides, sulfates and organic combinations with C and N. The available and total 
in this region was lie in the range of 41 - 747 and 294 - 1782 mg/kg with mean value of 281 ± 25 and 631 ± 180 mg/kg, respectively. The critical limit of 
in the soil was reported to be 10 mg/kg. No soils of the studied area were found to be deficient in sulfur level.
3.10. Concentration Variations and Statistics
The range, mean, median, kurtosis and skewness values of micronutrient concentration in soils of 100 villages of the studied area are presented in Table 2. The highest value of Fe or
, Cu, Zn, Mn, Co, Ni and Mo was observed in site i.e. Khamtarai, Arang, Murabangoli, Mandir Hasaud, Tilda, Abanpur and Dharsiva, respectively, Figure 2. The content of six micronutrients i.e.
, Cu, Zn, Co, Ni and Mo are symmetrically distributed in all locations, Figure 3. However, large variations in the content of two micronutrients i.e. Fe and Mn was seen, may be due to input by the industrial effluents, Figure 3. The data for seven micronutrients i.e. Cu, Zn, Mn, Co, Ni, Mo and 
were found to be distributed normally and symmetrically with comparable median and mean values. A large variation in the case of Fe was noticed, may be due to asymmetric distribution of iron minerals in the soil. The available content of the micronutrients in soils of the studied area was found in the following decreasing order: Mn > Fe > SO42−> Co > Cu > Ni > Zn > Mo, Figure 4. Among them, manganese was found to be at the highest level followed by Fe. However in the case of total levels, a different trend (Fe > Mn > SO42−> Co > Cu > Zn > Ni > Mo) was observed, Figure 4. A large fraction of S was found to be available for the plant growth, Figure 5. However, low to moderate fractions of other micronutrients were available, Figure 5. All of them showed a positive correlation between the available and total metal content with the highest and lowest values for Ni and Cu, respectively, Figure 6.
The available content of all micronutrients except Zn and 
was found to decrease as the soil profile was increased from 0 to 120 cm, may be due to strong adsorption of the cations by the geomedia, Figure 7. The total content of all micronutrients except 
was found to increase as the soil depth profile was increased from 0 to 120 cm due to their poor adsorption by the geo-media, Figure 8.
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Figure 2. Representation of locations showing maximum micronutrient concentration in studied area.
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Table 2. Statistics for distribution of available micronutrients in the soil, (n = 100).
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Figure 3. Representation of distribution pattern of micronutrients in surface soil of Raipur area.
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Figure 4. Mean value of total (T) and available (Av) content of micronutrient in the surface soil of Raipur area.
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Figure 5. Fraction of available content of micronutrient in surface soil of Raipur area.
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Figure 6. Correlation coefficient of total and available content of micronutrient in surface of Raipur area.
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Figure 7. Depth profile studies of available content of micronutrient in soil of Raipur area.
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Figure 8. Depth profile studies of total content of micronutrient in soil of Raipur area.
3.11. Micronutrient Deficiency and Toxicity
The micronutrients i.e. Fe, Mn, Co, Ni, Cu, Zn, Mo and S are used in very small amounts. The presence of micronutrients below critical limit often causes adverse effects in plant growth and in their yields. For each micronutrient, the critical levels, limit of deficiency, toxicity and optimal growth vary with the genotype and are profoundly affected by plant metabolism and by edaphic and environmental factor that affect the absorption of nutrients. In the studied area, five micronutrients i.e. Co, Ni, Cu, Mo and S are found in soils at the sufficient levels. Two micronutrients i.e. Fe and Mn are present at toxic levels in all types of soils of this region. In some area, the Zn deficiency in the soils is observed. The iron chlorosis was commonly seen in the plants of this region which may be due to very high amount of Fe content in soil. The brown or purplish spots on leaves, on lower part of the stem and leaf margins are commonly marked in this region that may be due to manganese toxicity.The most visible zinc deficiency symptoms i.e. short internodes and a decrease in leaf size were observed in the plants of this region.
3.12. Comparison of Micronutrient Status
The concentration of available maximum amount of Fe, Mn, Zn, ![]()
and Cu reported was 12234, 269, 121, 70, and 30 mg/kg, respectively [8] [11] [21] . The levels of micronutrients i.e. Fe, Mn, and S were found to be at the highest levels in this region. The content of Cu was present at moderated levels similar to other part of country [7] . The Zn content was present low levels as occurred in other parts of the Country [7] . The level of Mo was present at moderate levels but high content of Fe, Mn, and S may cause Mo deficiency.
4. Conclusion
All types of soils in Raipur area are found to be associated with high levels of Fe, Mn, S; moderate levels of Cu and low level of Mo and Zn. The relative abundance of free form of micronutrients (available content/total content) in soils of this region is found in following decreasing order: S >> Mn > Mo > Cu >> Zn ≈ Fe. The adverse effects i.e. chlorosis of young leaves, premature fall of fruits, narcotics, stunned growth of plants/crops in plants of this region are frequently seen may be due to either Fe and Mn toxicities or Zn deficiency or their combination. The Zn deficiency could be corrected by application of Zn compounds e.g. Zinc sulfate, Zinc oxide, Zinc phosphate, etc. in the soil.
Acknowledgements
We are thankful to the Madhya Pradesh Council of Science & Technology, Raipur for granting scholarship to one of the author: SC.
NOTES
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*Corresponding author.