Evaluation of Five Extraction Methods for Predicting Available Forms of Iron in Soils of Southeastern, Nigeria

IOSR Journal of Agriculture and Veterinary Science (IOSR-JAVS)
e-ISSN: 2319-2380, p-ISSN: 2319-2372. Volume 8, Issue 1 Ver. II (Jan. 2015), PP 01-08
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DOI: 10.9790/2380-08120108 www.iosrjournals.org 1 | Page
Evaluation of Five Extraction Methods for Predicting Available
Forms of Iron in Soils of Southeastern, Nigeria
Ernest U. Eteng1
and Damian O. Asawalam2
Department of Soil Science and Meteorology, Michael Okpara University of Agriculture, Umudike, Nigeria
Abstract: Available forms of iron in soils can be assessed through the use of chemical extraction methods. Soil
test provide an indication of nutrient level in the soil and together with plant analysis are important agronomic
tools for determining crop nutrient needs, predicting the nutrient-deficient areas and preventing the deficiency.
This study was therefore conducted in laboratory and greenhouse environment to evaluate suitable extraction
method for predicting available forms of iron in soils of southeastern Nigeria, using five extraction procedures
(Coca-cola, 0.05M EDTA, 1N NH4OAC +0.05M EDTA, 1N NH4OAC and O.1N HCl). Iron uptake by maize (Zea
mays L.) plants was analyzed together with the Fe content in the soils in which maize plants were grown under
greenhouse conditions. Iron content in maize plants were determined by AAS. The results obtained in this study
shows that Coca-cola method extracted the highest amount of the Fe (12.77 mgkg-1
) while, 1N NH4OAC +0.05M
EDTA extracted the least amount of Fe (4.48 mgkg-1
) with an average of 7.33 mgkg-1
. Coca-cola-extractable Fe
correlated positively and significantly with Fe concentration (r = 0.650**) and Fe uptake (r = 0.712***) while,
O.1N HCl- and 0.05M EDTA-extractable Fe had significant positive correlations with dry matter yield and Fe
uptake. Their r values being, (0.630** and 0.676**) and (0.601** and 0.588*), respectively. Moreover, the
result obtained from the regression equations corresponded with the correlations values determined.
Accordingly, the study indicates that, the order of significance for the extracting solutions is as follows: Coca-
cola > O.1N HCl > 0.05M EDTA > 1N NH4OAC > 0.05M EDTA+1N NH4OAc. Thus, the high correlations (r)
values of extractable Fe with soil properties and yield parameters suggests the superiority of Coca-cola, O.1N
HCl and 0.05M EDTA extractants over others extractants but, the Coca-cola method was determined to be the
most suitable extractant for predicting available Fe content for soils of southeastern Nigeria. However, in the
absent of Coca-cola extractant, HCl and EDTA methods can equally be used in the determination of available
Fe in these soils.
Keywords: Iron, extractants, maize, acid, salt, chelate, Fe uptake, acid sands.
I. Introduction
Iron is an essential plant nutrient that is required in small quantity for growth and development of plant.
It is harmful when present in large quantity in soils. Iron is taken up by plants at a much higher rate than is the
case with other trace elements (Sahrawat, 2003). One of the reasons Fe is rarely tested is that, the Fe availability
is generally presumed to be sufficiency in soils. Ponnamperuma et al. (1967) noted that the level of available Fe
in the soils is extremely pH dependent and that, at very low pH levels, Fe is reduced from its oxidized Fe3+
form
– which it is generally found in dry conditions – to its highly soluble and readily available Fe2+
form.
Conversely, Nayak (2008) reported that, where a soil is well aerated and well drained, with good structure and
porosity, Fe toxicity is unlikely to be a problem. According to Norvell and Lindsay (1982), Fe chelates aid in the
movement of Fe to plant roots, but they are neither absorbed to any great extent nor do they raise the activity of
Fe3+
or Fe2+
in the bulk soil solution. To be effective, Fe chelates must be stable in soil environments.
Knowledge of available forms of Fe in soil is fundamental for suitable fertilization recommendations,
in other to avoid deficiency or toxicity problems. However, the deficiencies of Fe have constituted a major soil
fertility problem in many parts of Nigeria (kang et al., 1976) particularly in acid sandy soils of Southeastern
Nigeria where, Fe deficiency has been widely reported (Enwezor et al., 1990). Fe deficiency partly caused by
continuous and intensive farming, leading to nutrient mining (Aduayi et al., 2002 and FAO, 2010) has however
led to continuous declining yields of major food crops such as maize crop (Chude et al., 2004), especially in
soils where Fe deficiency occurs (Sillanpaa, 1990 and FAO, 2010).
Generally, soil extractants used for predicting available forms of micronutrients in soils included the
weak replacement ion salts (CaCl2, NH4OAc, etc.) (Whitney, 1988; Kabata-Pendias, 2001); weak acids (acetic
acid and hydrochloric acid) (Shittu et al., 2010) and weak chelating agents; EDTA (Lindsay and Norvel, 1978;
Agarwal and Sastry, 1995; Gronflaten and Steinnes, 2005) and DTPA (Lindsay, 1995; Aydemir, 1981 and
Katyal and Shuman, 1991). Accordingly, Gronflaten and Steinnes (2005), reported that, neutral salt solutions are
capable of extracting the easily soluble and exchangeable i.e. potentially accessible forms of trace elements in
soils. When diluted solutions of the concentrated acids-HCl and HNO3, was used for determining the mobile and
accessible forms of trace elements in acid soils. However, due to the high active concentration of hydrogen ions

Evaluation of Five Extraction Methods for Predicting Available Forms of Iron in Soils of …
in these acids, they extracted not only the mobile, but also the steadily bound forms of the elements in the soils
(Kabata-Pendias, 2001; Dahnke and Olson, 1990). Besides, chelating agents such as EDTA and DTPA have also
been used to estimate plant available Zn, Cu, Fe and Mn in soils (Haddad and Evans, 1993; Agarwal and Sastry,
1995; Dolui and Chattopadhyay, 1997). Comparatively, Knezek and Ellis (1980) and Norvel (1984) reported
that, of the commonly used chelated agents, only EDTA can be used successfully as extractant for estimating
mobile forms of Fe in soils of different types whereas, DTPA was observed to be unsuitable for use as extractant
in acid soils.
As a proper choice of an extraction method for soil test analysis, a positive correlation between the
nutrient concentration determined by the method and the nutrient quantity taken up by plants is fundamental
(Adiloglu, 2003; Menzies et al., 2007). The use of acid extractants is based on lowering the pH and the
consequent solubilization of some compounds containing these elements (Grava, 1980). On the other hand,
chelating extractants have the capacity of reducing the activity of dissolved metals, resulting in release of more
soluble compounds in buffered pH (Adiloglu, 2003).
Though in Nigeria, only a few numbers of chemical extractants are being used to determine plant-
available Fe in soils, the most commonly used extraction methods are the dilute acid, HCl (Kparmwang et al.,
1995), and the chelating agent, DTPA (Mustapha and Loks, 2005; Tening and Omueti, 2011; Ibrahim et al.,
2011; Oluwadare et al., 2013), no satisfactory extractants has so far been reported or documented for evaluating
available Fe in soils of southeastern Nigeria. Moreover, a reliable soil test is essentially to determine the Fe
needs of the various crops grown in this region, under various ecological conditions.
Keeping in view the important role of maize in the economy of this country and the effect of Fe on the
performance and quality of maize product as well as limited information about this nutrient element condition in
the soil, there is an urgent need for an accurate estimation of Fe in soils of southeastern Nigeria where, the
content of Fe had been considered poorly related to Fe supply. The objective of this research was to evaluate the
suitability of 5 extractants for predicting available forms of Fe in soils of southeastern Nigeria.
II. Materials And Methods
2.1. Location of study area
The sampling sites selected covered a wide range of soil types from 20 locations representing soils
across Southeastern Nigeria (Table 1) which is located at Latitude 4o
20’ and 7o
25 North of the Equator and
Longitude 5o
25’ and 8o
51’ East of the Greenwich meridian (Ojanuga, 2006). The mean minimum and
maximum temperatures ranged from 21-30o
c in the Coast and from 29-33o
c in the interior. The rainfall pattern is
bimodal and decreases from over 3000 mm in the south to 1,700 mm in the north, this has given rise to double
cropping (early and late) seasons. The vegetation stretches from mangrove swamp in the coastal region through
rainforest to derived savanna in the interior.
Table 1: Location of sampling site showing soil types
Sample
No.
Sample location Soil types Classification*
1 Bende Shale Aquic Hapludults
2 Okigwe Shale Aquic Ustiflurents
3 Afikpo Shale Typic Ustifluents
4 Odukpani Shale Aquic Haplustalfs
5 Igbariam Shale Udic Haplustalfs
6 Abiriba Sandstone Psamment Paleudults
7 Etiti Sandstone Udipsamments
8 Uturu Sandstone Psammentic Paleudults
9 Nsukka Sandstone Typic Psammaquent
10 Ofodua Sandstone Typic Ustpsamments
11 Owerri Coastal plain sand Udic Haplustalfs
12 Umudike Coastal plain sand Typic Paleudults
13 Uyo Coastal plain sand Typic Paleudults
14 Ikot Ekpene Coastal plain sand Psammentic Paleudults
15 Akpabuyo Coastal plain sand Psammentic Paleudults
16 Oban Basement complex Typic Hapludults
17 Awi Basement complex Typic Paleudults
18 Uyanga Basement complex Typic Paleudults
19 Betem Basement complex Typic Paleudults
20 Akpet Basement complex Typic Hapludults
* FDALR (1995).
Source: Eteng et. al. (2014)

2.2. Sample collection and laboratory study
A laboratory study was conducted on these surface (0-20) soils to determine some physical and chemical
properties (Table 3). The soil samples were air-dried and screened through a 2 mm sieve. The 2 mm sieved soils
were analysed for particle size analysis by the hydrometer method (Gee and Bauder, 1986); Soil pH (soil: water
ratio of 1: 2.5) was determined with a combined electrode pH meter (Thomas, 1996); Soil organic carbon was
determined by the potassium dichromate wet oxidation method (Nelson and Sommer, 1996) and the content of
the organic carbon was converted to organic matter (OC x 1.724) while, the effective cations exchange capacity
(ECEC) was determined by the method described by Sumner and Miller (1996). For the estimation of available
Fe contents in soils, the following chemical properties of the extractants as shown in Table 2 were used.
Table 2: Chemical properties of the extractants used
S/N
o
Extractant Groups pH
Soil-
solution
ratio
Shaking
time
Reference
1 Coca-cola Acid 2.7 1:10 10 mins. Schnug et al. (2001)
2 0.01N HCl Acid 4.8 1:10 1 hour Lindsay (1995)
3 0.05M EDTA Chelate 7.0 1:2 30 mins. Gronflaten and Steinnes, 2005
4 1N NH4OAc Salt 7.0 1:10 1 hour Olsen, 1992
5 0.05M EDTA+1N NH4OAc Chelate+salt 4.8 1:10 1 hour Adiloglu, 2003
Soil samples were shaken with respective extractants of various properties (Table 2). After shaking, the
soil-solution was centrifuged and filtered through Whatman filter paper No. 42. The quantity of Fe content in
the soil sample was performed with an atomic absorption spectrophotometer (AAS) (UNICAM model SOLAAR
32: Fe ASTM D1068).
2.3. Greenhouse study
The greenhouse study was conducted on the same soils to assess the performance of maize parameters.
One kilogramme of the air-dried soils was weighed into plastic containers of 2 liter capacity, placed on flat
plastic receiver. A total number of 400 plastic containers (20 soil samples x 5 levels of Fe x 4 replications) were
arranged in a complete randomized design (CRD). The plastic containers were randomly distributed on the
bench in the greenhouse growth chamber and supplied with distilled-deionized water as required during the
growth period. Before planting, the soils were watered to about field moisture capacity (adjusted to 70 %) with
distilled water and allowed to stand for about 48 hours in the greenhouse. Maize (Zea mays L.) variety; Oba
Supper II (yellow), was used as a test plant and sown at the rate of six seeds per plastic container and later
thinned to five seedlings a week after germination.
Five levels of Fe (0, 4, 8, 12 and 16 kg ha-1
) converted to mg kg-1
was applied as FeSO4.5H2O (33% Fe)
in solution and recommended basal dosage of 120 kg N ha-1
(mg N kg-1
soil) as Urea, 60 kg P2O5 ha-1
(mg P kg-1
soil) as single super phosphate (SSP) and 60 kg K2O ha-1
(mg K kg-1
) as muriate of potash (MOP) respectively,
as a basal N P and K fertilizer, were respectively applied uniformly to all the containers in solution form, at two
weeks after planting (WAP) as recommended for this region (Enwezor et al., 1990; Aduayi et al., 2002).
The plants (shoots and roots) were harvested 42 DAS, rinsed in distilled water, pre-dried under shade to
remove excess water and later packed in large brown envelopes and oven-dried at 70o
C for 72 hours. The oven-
dried plants and ground materials were digested using sulphuric acid, nitric acid and perchloric acid
(H2SO4:HNO3:HClO4) method (Shuman, 1985). The Fe contents in maize plants were determined by Atomic
Absorption Spectrophotometer (AAS: Pre UNICAM model 939). Dry matter yield, Fe concentration and Fe
uptake values were used as yield parameters of maize plants. Iron uptake was calculated as the product of the
concentration of Fe in the plant tissue and the dry matter yield.
2.4. Statistical Analysis
The data obtained were subjected to simple correlation and regression analyses at different probability
levels using general linear model of Genstat (2013) and PASW Statistics 18 for Window 7.0, to show the
relationships between the different extractants and the plant Fe uptake. The extractant which displayed the
highest correlation coefficient (r) with the maize uptake was recommended for the determination of available Fe
content of the soils of southeastern Nigeria.
III. Results And Discussion
3.1. Soil properties of the study area
Some physical and chemical properties of the soil samples determined are indicated in Table 3. In the
table, pH values of the soil samples ranged from 4.01 to 5.89 with an average of 4.62, suggesting that the soils
are very strongly acidic; the percent organic matter content were between 0.71 and 2.47% with an average of
1.59%. Higher percent of the soils were low in organic matter content (Tandon, 1995). The ECEC values varied

widely which ranged between 0.78 and 14.31 cmol kg-1
with an average of 7.15 cmol kg-1
. However, ECEC of
most of the soils tested were below 12 cmol kg-1
and are rated low (Melsted et al., 1969), and this is the range of
critical value for soils that are dominated by oxide and hydroxide clays (Omueti and Lavkulich, 1988; Bimie
and Paterson, 1991). This suggests that most of the soils may have few exchange sites. For instance, the high
values of ECEC obtained in some of the soil samples could be obvious as these samples were collected from
shale derived soils (a hydromorphic soil) as shown in Table 1, with high clay content which ranged from 7 and
34 %, respectively. Most of the soils samples determined were coarse texture and the textural class was in the
average of sandy loam (SL).
Fig 3: Some physical and chemical properties of the soil samples (N=20).
Soil Samples
pH
(1:2.5)
Org.
matter
%
ECEC
Cmolkg-1
Particle size distribution
Textural class
Sand % Silt % Clay %
Min. 4.01 0.71 0.78 39 6 7
Max. 5.89 2.47 14.31 85 30 34
Means 4.62 1.59 7.15 62 18 20 Sandy loam
3.2. Iron contents of soils according to different extraction methods.
Five extraction methods were used to extract available Fe content of the soil samples (Table 4). There
were substantial variations in the amount of extractable Fe in the soils. Available Fe varied widely depending on
the extraction method used, reasons for which could be due to type, concentration, pH, shaking time and
solution ratio of the extraction solution (Sorensen et al., 1971; Whitney, 1988) and variability observed in the
physical and chemical properties of the soils used (Loeppert and Iskeep, 1996 and Elinc, 1997).
Coca-cola-extractable Fe ranged from 3.57 to 21.31 mg kg-1
with a mean of 12.77 mg kg-1
. 0.05M EDTA-
extractable Fe varied widely between 1.99 and 16.28 mgkg-1
with a mean of 8.81mg kg-1
. 0.01N HCl-extractable Fe ranged
from 3.29 to 18.63 mg kg-1
.While the 0.05M EDTA+1N NH4OAc and 1N NH4OAc extraction
methods have significantly (P<0.05) lower extractable Fe relative to Coca-cola, 0.01N HCl and 0.05M EDTA methods.
Thus, the quantity of available Fe by 0.05M EDTA+1N NH4OAc method was three time lower than the amount extracted by
Coca-cola method and twice lower than the amount extracted by 0.05M EDTA and HCl methods, respectively. This trend
was the same in the case of 1N NH4OAc-extractable Fe. However, the values of 0.05M EDTA+1N NH4OAc and 1N
NH4OAc-extractable Fe have values which varied widely from 1.27-8.63 mg kg-1
and 2.02-9.86 mg kg-1
, respectively.
Similarly, the result presented in Table 4 indicates that Fe extracted by Coca-cola, 0.01N HCl and 0.05M EDTA methods
had values contents which are relatively above the mean value. On the other hand, Fe extracted in most soils by 0.05M
EDTA+1N NH4OAc and 1N NH4OAc had values that are less than the overall mean.
Extractable Fe of the soils varied widely from 5.52 to 12.77 mg kg-1
(Table 4). Coca-cola-extractable Fe was significantly higher followed by 0.01N HCl and 0.05M EDTA-
extractable Fe. On the other hand, the lowest available Fe content of soil sample was determined with 1N
NH4OAc and 0.05M EDTA+1N NH4OAc methods, respectively. This result also show that higher available Fe
was determined using acid (Coca-Cola and 0.01N HCl methods) and chelate (0.05M EDTA method) in
comparison to the methods using neutral salt (1N NH4OAc method) and chelate-salt (0.05M EDTA + 1N
NH4OAc method). Meanwhile, the mean available Fe content of the soils were determined to be 12.77; 10.17;
8.81; 5.52 and 4.48 mgkg-1
, using the extractants; Coca-cola, 0.01N HCl, E0.05M DTA, and 1N NH4OAc,
0.05M EDTA+1N NH4OAc methods, respectively. In line with this study, Coca-Cola has been reported to be an
excellent extractant for Fe, for many soils (Schnug et al., 1996; 1998).
Table 4: Available forms of Fe content in soils obtained by 5 extractants
Soil
Sample
No.
Extractable iron (Fe)
Coca-Cola EDTA HCl EDTA+ NH4OAc NH4OAc
mgkg-1
1 14.46 14.05 13.90 6.77 8.78
2 16.27 6.28 14.55 3.87 5.33
3 19.93 13.59 9.24 4.07 4.39
4 17.54 15.89 17.19 3.02 3.01
5 9.41 6.37 8.33 4.54 9.86
6 4.94 4.17 4.24 3.69 4.83
7 15.7 2.08 12.29 1.99 2.15
8 10.41 1.99 9.69 2.09 2.02
9 8.70 6.20 5.49 2.03 5.16
10 14.97 13.35 3.29 7.48 8.06
11 9.87 3.57 4.73 2.83 3.06
12 3.57 6.98 3.20 6.78 3.81
13 7.32 7.59 4.33 2.76 2.88
14 8.14 3.57 9.30 2.32 2.35
15 7.44 5.71 4.36 1.27 3.59
16 15.28 9.90 18.03 6.74 7.15
17 14.24 11.38 12.05 5.86 8.98

18 18.23 11.67 13.62 8.63 7.23
19 17.6 15.42 18.63 5.23 6.38
20 21.31 16.28 16.68 7.60 9.44
Min. 3.57 1.99 3.29 1.27 2.02
Max. 21.31 16.28 18.63 8.63 9.86
Mean 12.77 8.81 10.16 4.48 5.52
3.3. Dry matter, Fe concentration and Fe uptake by maize (Zea mays L.) grown on soils of southeastern
Nigeria:
Maize (Zea mays L.) yield parameters obtained from the soils of southeastern Nigeria are presented in
Table 5. In the table, dry matter yield varied widely from 5.45 to 17.82 g/plant with a mean of 12.79 g/plant
(Table 5). Higher dry matter were produced from soils 2, 9, 15 and 17 while lower dry matter were obtained
from soils 6, 11, 13, 19 and 20, respectively. However, dry matter yield of the maize plants were found to be
significantly (P<0.05) higher at 12 kg Fe ha-1
levels of application, relative to other rates of application.
Generally, dry matter yield increased from control to 12 kg Fe ha-1
levels of application and thereafter declined
in yield to 16 kg Fe ha-1
levels, respectively in all the soil samples. This result suggests that the 12 kg Fe ha-1
levels of application could be the optimum level of Fe in these soils. Similar results were reports by Adiloglu
(2003); Lisuma et al. (2006) and Tening and Omuiti (2011).
The Fe concentration in maize shoots increased with increasing Fe application (Table 5). Accordingly,
Fe concentrations varied widely between 31.90 and 118.48 mg/kg with a mean of 102 mg/kg, and these were
sufficient (Melsted et al., 1969; Tandon, 1995; Menzies et al., 2007). Higher Fe concentration were obtained in
soils 6, 11, 14, 16, and 17 while lower Fe concentration were produced in soils 1, 2, 3, 5, 8, 18 and 20. The wide
variability in observed in this study may be as a result of higher available Fe content in some of the soils. The
effect of Fe application on the yield parameter of maize plants was determined to be significant at 5% level and
the results obtained in this study are in agreement with those reported by Adiloglu (2003); Lisuma et al. (2006);
Tening and Omueti (2011).Accordingly, the Fe uptake of the maize shoots at determined at 6 WAP increased
with increasing Fe application (Table 5) and varied widely from 0.23 to 2.39 mg/plant with an average of 1.38
mg/plant. However, on the average, soils 9, 14, 15 and 17 have significant (P<0.05) higher Fe uptake (1.538,
1.582, 1.658 and 1.752 mg plant-1
) relative to other soils while, soils 1, 5, 19 and 20 yielded the lowest Fe
uptake (0.98, 1.298, 1.266 and 1.148 mg plant-1
) of maize plant shoots, respectively. Similar results were
obtained by Tening and Omuiti (2011) in soils of humid zone of southeastern Nigeria and Adiloglu (2003) in
soils of Edirne Province, Turkey for barley plants.Generally, the wide variability observed in maize yield
parameters may be due to higher or lower available Fe content in some locations of the soils of southeastern
Nigeria. However, Ponnamperuma et al (1967) reported that higher available Fe in soil is pH dependent, while
Sahrawat (2008) and Nayak (2008) noted that availability of Fe content in soil depends on drainage nature of the
soils. At low pH levels, Fe is reduced from its oxidized Fe3+
form which is generally found in dry conditions to
its highly soluble and readily available Fe2+
form. This is confirmed by Nayak (2008) who reported that, where a
soil is well aerated and well drained, with a good structure and porosity, Fe toxicity is unlikely to be a problem.
Norvell and Lindsay (1982) had earlier noted that Fe chelates may or may not raise the activity of Fe3+
or Fe2+
in
the bulk soil solution. Whichever is the case may have resulted to the wide variability in Fe uptake of maize
plants grown in different soils of southeastern Nigeria. Similar results were obtained by Tening and Omuiti
(2011) in soils of humid zone of southeastern Nigeria; Adiloglu (2003) in soils of Edirne Province, Turkey for
barley plants; Lisuma et al (2006) in volcanic soil of Mpangala, Tanzania for maize and also McBridge et al.
(2004).
Table 5: The effect of Fe application on dry matter yield, Fe concentration and Fe uptake by maize
grown on soils of southeastern Nigeria

3.4. Prediction of Iron availability in the soils
The usefulness of any soil extractant to predict the availability of micronutrient element is dependent
on the ability to predict (from the extractant) the extent to which plants will accumulate a given nutrient
element. Although, comparisons between various treatments can be problematic due to variations in soil types,
properties and treatment duration (Menzies et al., 2007), an effective extractant will be able to assess the
availability of these trace elements under a variety of soil conditions (McBridge et al., 2004 and Menzies et al.,
2007).Significant correlation coefficients (r) were observed between all the extractants and Fe uptake at 5 %
level (Table 6). The highest correlation coefficients (r) were determined between Coca-Cola and HCl methods
and Fe uptake of maize. However, Coca-Cola-extractable Fe had significant positive correlations with Dry
matter yield, concentration and uptake of maize. Their r values are 0.528*, 0.650** and 0.712***, respectively.
HCl-, EDTA-, and NH4OAc-extractable Fe had significant positive correlations with Dry matter yield and
uptake of maize plants. Their r values are (0.630** and 0.676**), (0.601** and 0.588*), (0.508* and 0.475*),
respectively. The EDTA+NH4OAc-extractable Fe had significant positive correlations with uptake (0.463*).
The significant positive correlation with plant parameters suggests that the amount of Fe extracted by these
extractants has strong association with plant uptake (Lindsay and Schwab, 1982). This may be due to Fe
transformation and availability in soils which depends on various forms of this nutrient element with which Fe
have significant and positive correlation (Dahnke and Olson, 1990). The results obtained from EDTA method
followed the above methods regarding the correlation coefficients (r). Accordingly this study indicates that, the
order of significance for the extracting solutions is as follows: Coca-Cola > HCl > EDTA > NH4OAc >
EDTA+NH4OAc.
The low values obtained by NH4OAc and EDTA+NH4OAc may be attributed to the soil factors due to
redox potential conditions of the soils which determined the final behavior of Fe in the soils and its availability
to maize plants. Moreover, chemical properties of the soils showed that they were strongly acidic, low in
organic matter and coarse in texture (Eteng et al., 2014). Thus, the use of salt and chelate mix (NH4OAc+
EDTA) extraction method was not adequate in the determination of available Fe, but the used of acid method
(Coca-Cola) was shown to be more suitable in the prediction of available Fe content in the soils. The result
obtained in this study with Coca-Cola method is similar to the result reported by Schnug et al. (2001) in soils on
the Island of Ruegen. This is supported by the result with higher correlation coefficients (r) which were obtained
from acid methods (Table 6) as a result, when considered the physical and chemical properties of the soils
studied; the acid method can be used with satisfaction in the determination of available Fe contents in soils of
southeastern Nigeria. Besides the better results obtained with Coca-Cola method, in extracting available Fe
fractions from these soils, the correlations with plant uptake was superior to the other extracting solutions.
Further advantages of Coca-Cola as an extractant are its ubiquitous availability and readiness for us but also its
easy and safe handling and the fact that the procedure has no harmful impacts as compared to the other
extracting solutions (Schnug et al., 1996; 1998).
Table 6: Correlation of extractable Fe with yield parameter of maize (N=20)
S/No.
Extraction methods
Yield parameters of maize plants
(non-application of Fe in soils)
Dry matter
yield of plant
Fe concentration
of plant
Uptake of Fe from
soil
1. Coca-cola 0.578* 0.650** 0.712**
2. 0.01N HCl 0.630** 0.281 0.676**
3. 0.05M EDTA 0.601** 0.131 0.588*
4. 1N NH4OAc 0.508* 0.029 0.475*
5. 0.05M EDTA+ 1N NH4OAc 0.313 0.274 0.462*
Linear regression analysis conducted on Fe uptake revealed that all the equations were statistically
significant with exception of EDTA+NH4OAc (Table 7). In these evaluations, uptake results are very similar to
those for dry matter yield of maize plants. The equation that is not significant indicates that the material did not
significantly increase Fe uptake. Though, EDTA and HCl methods performed equally well but, the Coca-cola
method performed better. The order of importance for uptake are Coca-cola> HCl> EDTA> NH4OAc. However,
regression analysis carried out in this study suggests that plant available Fe extracted by Coca-cola method is
highly recommended.
Table 7: Regression of extractable Fe with Fe uptake (N=20).
Regression equation R2
Fe uptake = 174.27 + 36.0 Coca-Cola-Fe 0.675***
Fe uptake = 209.51+47.34 HCl-Fe 0.602**
Fe uptake = 95.33 + 21.44 EDTA-Fe 0.574**
Fe uptake = 110.89 + 25.52 NH4OAc-Fe 0.406*
Fe uptake = 185.89 + 16.45 NH4OAc+EDTA-Fe 0.313ns

Ns = not significant at P < 0.05. * = significant at P < 0.05. ** = significant at P < 0.01
*** = significant at P < 0.001
IV. Conclusion And Recommendation
4.1. Conclusion
Soil test provide an indication of nutrient level in the soil and together with plant analysis are important
agronomic tools for determining crop nutrient needs, predicting the nutrient-deficient areas and preventing the
deficiency. In this study, substantial variations in the amount of available Fe in soils of southeastern Nigeria
were determined. Coca-Cola method was determined to be the most suitable extractant among others in the
determination of available Fe in the soils. However, in the absent of Coca-cola extractant, HCl and EDTA
methods can equally be used in the determination of available Fe in the soils. This is supported by the higher
correlations coefficients (r) exhibited by the nature of these three extractants with maize yield parameters (Table
6) and the regression equations for Fe uptake (Table 7). As a result, when considering the chemical properties of
the soils studied, the used of acids and chelate methods were found to be satisfactory for the prediction of
available Fe contents in southeastern Nigeria soils. Nevertheless, these methods are suitable to certain physical
and chemical properties of the soils in this region.
V. Recommendation
Further studies are expected to be carried out in field experiments for at least two years to determine
critical limit and optimum levels of iron for fertilizer recommendation, in soils of southeastern Nigeria.
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