The Distribution of Zinc Fractions in Surface Samples of Selected Agricultural Soils of Zambia
Olusegun A. Yerokun
Soil parent material, pedo-chemical transformations and anthropogenic interventions play important roles in the distribution of zinc (Zn) into its various forms in the soil and ultimately, its bio-availability for plant uptake. Therefore knowledge of the soil Zn forms is useful for its management. This study defined five mechanistic Zn pools in 11 cultivated agricultural soils of Zambian and their uncultivated pairs. A batch extraction scheme was used to estimate exchangeable (Ex-Zn), carbonate (CO3-Zn), organic (Org-Zn), sesquioxide (Ox-Zn) and residual (Res-Zn) Zn pools in each soil. Total Zn was calculated as a sum of the pools and it ranged from 13.11 to 108.02 mg kg-1 with an average of 52.26 mg kg-1. The distribution of Zn in the soils on the basis of average concentrations was in the order 22.99 mg kg-1 Ox-Zn (44%)>14.97 mg kg-1 Res-Zn (29%)>7.51 mg kg-1 CO3-Zn (14%)>4.81 mg kg-1 Org-Zn (9%)>1.98 mg kg-1 Ex-Zn (4%). Cultivation depressed Tot-Zn and Ex-Zn concentrations in several of the soils. Correlation analysis (n = 44) showed that Tot-Zn increased along with soil clay content (r = 0.50, p = 0.03) as well as soil Cation Exchange Capacity (CEC) (r = 0.60; p = 0.007), CO3-Zn increased with CEC (r = 0.68, p = 0.001) and Res-Zn increased with soil clay content (r = 0.53; p = 0.02). The fact that the chemistry of Zn in this set of soils appears to be dominated by the more stable fractions offers an explanation for the common notion held that majority of Zambian soils have low soil Zn test levels which accounts for low Zn bio-availability.
Received: November 04, 2011;
Accepted: January 21, 2012;
Published: February 11, 2012
Zinc (Zn) is an important essential element which is required only in limited
amounts by plants, animals and humans for various physiological and reproductive
functions. It influences the quality and yield of crops (Madyiwa
et al., 2002; Alloway, 2003; Chidanandappa
et al., 2008) and in humans it is a necessary cofactor in more than
300 enzymes and numerous transcription factors (FAO/WHO/IAEA,
1996; Haug et al., 2010). The deficiency
of Zn in human diet has been recognized to commonly cause impaired growth (stunting)
in children and consequently poor human development (Hambridge
et al., 1986). This phenomenon is common in the southern Africa region
and requires urgent attention. Previous concern about Zn focused more on the
potential for heavy metal toxicities to humans from high levels of pollution
emitted by mining operations (Tembo et al., 2006)
than in its role in crop production. Clinical symptoms including headaches,
nausea, loss of appetite and diarrhea have been recorded from high amounts of
Zn intake by humans (Panel on Micronutrients, 2001).
In a recent review of soil analysis data from the University of Zambia Soil
Analysis Laboratory a large number of agricultural soils across the nation were
observed to have tested low in bio-available Zn. However, there have not been
consistent concurrent visual observations of widespread incidence of Zn deficiency
in crops grown on these soils. This is plausibly because of the higher likelihood
for deficiencies of nitrogen, phosphorus and potassium to appear on crop grown
in low fertility soil with moderate to advanced weathering, while Zn deficiency
will more likely manifest as hidden hunger and contribute to low crop productivity
(Mapiki and Phiri, 1995). On the other hand, aluminium
and manganese toxicities manifested in acid soils also act to negatively affect
crops before the influence of Zn is visually observed. This hidden form of Zn
deficiency has been reported to contribute as much as 40% reduction in crop
yields (Alloway, 2003).
The Zn requirement of crops is largely met from soluble portions released through
chemical transformations of native soil Zn (Shuman, 1991).
In addition Zn may be supplied to plants from soluble forms in synthetic or
organic sources, as well as anthropogenic atmospheric inputs (Iyengar
et al., 1981; Johnson and Petras, 1998).
Typically soil total Zn ranges between 10-300 mg kg-1 with a mean
value of 55 mg kg-1 (Kiekens, 1995), however
knowledge of total Zn provides only limited information about its transformations
and bio-availability. For a better understanding, total soil Zn can be broadly
described in five mechanistic fractions which can be quantified using sequential
or batch fractionation schemes (Zerbe et al., 1999;
Hseu, 2006; Fedotov and Spivakov,
2008; Saffari et al., 2009). Generally these
are: (1) water soluble Zn in the soil solution, (2) easily exchangeable Zn in
electrostatic reaction with soil particles, (3) organic Zn complexed, chelated
or adsorbed to organic ligands, (4) inorganic Zn associated with secondary minerals
such as carbonates or insoluble metal oxides and (5) residual Zn held in primary
minerals (Sposito et al, 1982; Alloway,
2003; Saffari et al., 2009). These fractions
provide broad information on the biological, geological and chemical processes
which have occurred in a soil and are useful for predicting the availability
of Zn for plant uptake. The extent to which each fraction is present and the
transformations in equilibrium between fractions is influenced by soil properties
such as pH, cation exchange capacity, presence of metal oxides and soil organic
matter. It has been widely reported that the residual Zn and oxide bound Zn
are the more stable fractions while the exchangeable Zn and water soluble Zn
fractions are rather more soluble (Saffari et al.,
Low levels of bio-available Zn found in soils has been attributed to one or
a combination of low native Zn, very slow solubilization of Zn from soil minerals,
strong adsorption of Zn on soil surfaces, or co-leaching of Zn with dissolved
organic matter (Zimdahl and Skogerboe, 1977; Rieuwerts
et al., 2006). These mechanisms appear to be active in many soils
such that incidences of widespread soil Zn deficiency have been reported in
several regions of the world including Australia (McDonald
et al., 2001), Spain (Obrador et al.,
2007), India (Karak et al., 2006), the Savannas
(Agbenin, 2003), Brazil (Furlani
et al., 2005), Turkey (Cakmak et al.,
1999) and Iran (Maftoun and Karimian, 1989). Soil
Zn has not been well studied in Zambia and there is insufficient information
that could be used in predicting Zn bio-availability. Therefore to the extent
that our communities have low meat and milk protein intake and Zn requirements
are largely met from food crop supply, it is important to have a better understanding
of the transformations of soil Zn fractions and their potential to supply plant
requirement. The objective of this study was to characterize the distribution
of Zn in its various fractions in selected Zambian soils and to determine if
cropping affects their distribution.
MATERIALS AND METHODS
Soils: The soils used for this study were collected from eleven locations
across Zambia where samples were obtained from both a cultivated and an adjacent
uncultivated field (Table 1). The lengths and intensities
of cultivation and management of crops at the various locations were not uniform
but they were in excess of five years with fertilizer application. Maize production
was common to the farms.
Laboratory analyses: Soil samples were collected from the 0-20 cm depth
at ten random spots per field and mixed together to obtain one composite sample.
All samples were air dried and crushed to pass through a 2 mm sieve. Soil analysis
was done using standard procedures (Van-Ranst et al.,
1999). Particle size distribution was determined by the hydrometer method.
The pH was measured in a 1:2.5 (w/v) ratio of soil to 0.01 M CaCl2
solution. Soil organic matter was determined using the Walkley-Black chromate
reduction method. Cation Exchange Capacity (CEC) was determined in neutral 1
N NH4OAc with steam distillation.
Zinc fractionation: A modified version of the batch or single extraction
scheme (Johnson and Petras, 1998) was used to define
the various Zn fractions in the soils. Rather than using the same soil residue
in the next extraction step, fresh sample was weighed into the next reagent,
||Extractable Zn (Ex-Zn): Representing the water soluble
and exchangeable fraction: Twenty grams of soil was extracted in 40 mL 0.005
M DTPA for 2 h
||Carbonate bound Zn (CO3-Zn): Representing the inorganically
bound fraction: One gram soil was extracted in 20 mL 1 M CH3COONH4/CH3COOH
mixture at pH 5 for 5 h
||Organic bound Zn (Org-Zn): Representing the fraction complexed,
chelated or adsorbed to
||Organic ligands: One gram soil extracted in 40 mL 0.1 M K2P2O7
for 17 h
||Sesquioxide Zn (Ox-Zn): Representing the amorphous bound fraction:
one gram soil was extracted in 50 mL acid Oxalate at pH 3 (four parts 0.2
M ammonium oxalate and three parts 0.23 M oxalic acid) for 17 h
||Residual Zn (Res-Zn): One gram soil sample was digested in 25 mL
aqua regia (one part HNO3 to three parts HCl) for twenty minutes
on a hot plate and then allowed to cool
||Total Zn (Tot-Zn): Total Zn was calculated as a sum of all the
Each soil suspension was filtered after shaking or digestion. The concentration of Zn in the extracts was determined using the atomic absorption spectrophotometer (Analyst 400 Perkin Elmer). All the soils were analyzed in triplicates.
Statistical analysis: The data obtained was analyzed using the SAS Statistical Program (SAS, 6.12) to obtain the means and standard deviations of Zn concentration in the different pools. In addition, correlation analysis was done to obtain the relationship between Zn concentrations in the various pools and soil properties.
RESULTS AND DISCUSSION
Soil properties: The soil samples used in this study varied in classification
(Table 1) and their pH (CaCl2) values ranged from
4.1 to 7.5 (Table 2). Half of them were acidic while the other
half were alkaline.
|| Description of soils used in the study
|| Some chemical and physical characteristics of soils used
in the study
|l: Loam, ls: Loamy sand, sl: Sandy loam
Although there was no significant difference (t-test, p = 0.05) in mean soil
reaction between cultivated and uncultivated soil samples, cultivation generally
had the tendency to reduce soil pH (Table 2). It is known
that the nature of soil reaction is influenced by parent material, extent of
weathering, erosion and leaching and soil management. In this case, the strongly
acidic soils, comprising Mpongwe, Misamfu red, Mufulira, Nakambala and Liempe
are alfisols or ultisols belonging to the oxic sub-group (Table
1) and located in moderate to high rainfall agro-ecological regions. The
UNZA, York and Kashima soils are formed on relatively less weathered parent
material and have high pH values probably owing to continuous irrigation with
alkaline water from aquifer sitting in limestone bedrock.
|| Soil Zn fractions of the 11 Zambian soils collected from
|T: Trace value, Nd: Not determine, Ex: Extractable, Carbo:
Carbonate, Org: Organic, Sesq: Sesquioxide, Resi: Residual
The soil samples were dominated by coarse textured soils ranging between loamy
sand to loam (Table 2). Soil organic matter was highly variable,
being very low or very high (<2.5%>) and uncultivated fields were more
likely to have higher values than their cultivated analogs (Table
2). The soil cation exchange capacities were between 4.9 and 44.8 cmol kg-1
(Table 2) with most observed to be low (<15 cmol kg-1),
probably due to relatively high sand and low organic matter contents of many
of these soils.
Zinc distribution in soils: Total Zn in the soils ranged between 13.11-108.02
mg kg-1 (Table 3) and many of the soils can be
characterized as having low Tot-Zn, evidenced by concentrations below the group
average of 52.26 mg kg-1. These values generally reflect the influence
of soil parent material, age, weathering and texture wherein the more weathered
and coarser soils have lower Tot-Zn concentrations. Nevertheless the average
concentration compares well to several reported in literature such as 34 mg
kg-1 in Australia 56.5 mg kg-1 in the USA (Holmgren
et al., 1993), 68 mg kg-1 in Europe (Angelone
and Bini, 1992) and 45-59 mg kg-1 in Asia (Katyal
and Vlek, 1985). The Australian soils are older and more weathered hence
have lower average Tot-Zn concentrations. Tagwira et
al. (1993) in Zimbabwe and Chahal et al.
(2005) in India demonstrated that finer textured soils contain higher concentrations
of Zn in all the fractions when compared to coarser textured soils. The lower
concentrations can be explained by the presence of fewer exchange sites for
Zn adsorption or its loss from mineralization in the older and coarser soils.
In the current study, evaluation of the fractional distribution of soil Zn
(Table 3) showed that Ox-Zn (44%)>Res-Zn (29%)>CO3-Zn
(14%)>Org-Zn (9%)>Ex-Zn (4%). The more stable Ox-Zn and Res-Zn fractions
accounted for more than 70% of soil Zn. This is similar to observations reported
by Tagwira et al. (1993), Dvorak
et al. (2003), Tehrani (2005), Milivojevic
et al. (2005), Behera et al. (2008),
Chidanandappa et al. (2008) and Saffari
et al. (2009), who found residual Zn to be predominant in soils.
However in their studies they reported higher residual Zn than oxide Zn fraction
due to more alkaline soil pH and low sesquioxide concentrations. In other studies,
Zerbe et al. (1999), Ramos
et al. (1999), Hseu (2006) and Margui
et al. (2007) reported that the sesquioxide Zn fraction was dominant
in the soils studied. According to Chileshe and Wen (1985)
and Magai (1985) soil formation in Zambia has been characterized
by a mixing of old and new deposits whereby Precambrian Basement Complex has
been mixed in with or covered by the more recent deposits of the Kalahari sands
and Karoo deposits of various sedimentary rocks including sandstones and calcareous
mudstones. This has caused the soils to have variable amounts of sesquioxide
and weatherable minerals that can retain significant amounts of Zn. Therefore,
whereas the fractional average Ox-Zn in the soils used in this study was higher
than Res-Zn, eight of the samples contained more Res-Zn than Ox-Zn. Higher Zn
concentrations of these stable fractions denotes their importance as the storage
fractions for soil Zn, although their solubilities will determine how available
they are for plant uptake. There was no consistent effect of cultivation on
Tot-Zn or its fractional distribution in the soils.
The concentrations of the CO3-Zn and Org-Zn fractions were intermediate
(Table 3). This is probably indicative of their role as transitional
fractions in the bioavailability of soil Zn. Behera et
al. (2008) showed that these fractions declined with years of cropping.
Hence, associated soil parameters of such fractions, in this case carbonates
and organic matter are important in their soil buffering capacity for Zn (Dvorak
et al., 2003). The influence of organic matter can be attributed
to formation of organo-Zn complexes producing buffer zones for Zn (Udom
et al., 2004). In soils of Iran with high carbonates content, Saffari
et al. (2009) reported CO3-Zn values similar to these
The Ex-Zn fraction in the Zambian soils studied always contained the lowest
concentrations of Zn (Table 3). As it has been proposed that
plant Zn uptake is largely from this fraction (Shuman, 1991)
it will therefore be expected that it relies on the other fractions, especially
those with intermediate concentrations (CO3-Zn and Org-Zn) to replenish
it. However there was no correlation between and among any of the fractions
obtained. Elsokkary (1979), Dvorak
et al. (2003), Milivojevic et al. (2005)
and Margui et al. (2007) also made similar observations
that the exchangeable pool had the lowest concentration of the fractions that
they defined. Going by the critical index of 0.8 mg kg-1 used by
the soil analysis laboratories in Zambia at least 60% of the samples test low
Correlation between soil Zn and soil characteristics: The Tot-Zn concentration
of the soils increased with soil pH, organic matter content, soil clay content
and CEC (Table 4). It would appear that even with the dominance
of coarse texture in this set of soils with wide ranging pH, the clay type and
organic matter and increasing amounts of them provided sufficient charge points
for incremental Zn retention. Similarly, Elsokkary (1979)
in Egypt showed that Zn adsorption by soil was highly associated with CEC, Fe2O3
and clay. Mclaren et al. (1997) also concluded
that CEC and organic matter influenced the adsorption and desorption of Zn.
|| Correlation coefficients between soil zinc fractions and
the properties of selected Zambian soils
|*Significant at 0.05
The Ox-Zn concentration decreased as soil pH increased. This can be explained
by the natural reduction in oxide solubility and concentrations as pH increases.
Shiowatana et al. (2005) made similar observation
when they analyzed soils in Thailand. They concluded that soil pH significantly
determined the adsorption and sorption of metals and that its reduction increased
the concentration of metal ions in the soil solution. Correlation analysis showed
that CO3-Zn increased with soil pH and CEC (Table 4)
Calcium carbonate tends to adsorb Zn or form complexes such as CaCO3:
ZnCO3, a double salt, under favorably high pH values (Ramos
et al., 1999). The correlation coefficient between pH and the exchangeable
soil Zn was 0.42 (p = 0.12), suggesting that Zn bioavailability is probably
more from those soils with higher pH values among the set studied.
The fractional distribution of Zn in Zambian soils was in the order Ox-Zn>Res-Zn>CO3-Zn>Org-Zn>Ex-Zn and they can be clustered into stable (Ox-Zn and Res-Zn), intermediate (CO3-Zn and Org-Zn) and available (Ex-Zn) forms. Extractable Zn concentrations of these soils are generally low because the stable fraction is the dominant form in soils. Given low levels of bio-available Zn in many Zambian soils, the implication is that in the transformation of soil Zn, the stable form is only very slowly released into the available fraction and the intermediate forms offer a strong buffering to replenishing the soluble form albeit at a slow equilibrium. Soil Zn concentrations were influenced by the colloidal properties of the soils such that soils with high pH and those with finer texture were more likely to have higher Zn concentration in the various fractions. Land cultivation will probably result into lower soil Zn concentration compared to fallow land.
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