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Communications in Soil Scienceand Plant AnalysisPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lcss20
Trace Elements Extractedby DTPA and Mehlich‐3 fromAgricultural Soils with andwithout Compost AdditionsEva Vidal‐Vázquez a , Raquel Caridad‐Cancela a , M.
Mercedes Taboada‐Castro a , Antonio Paz‐González a
& Cleide Aparecida de Abreu ba Facultad de Ciencias, Universidade da Coruña, ACoruña, Spainb Centro de Solos e Recursos Agroambientais,Instituto Agronómico, Campinas (SP), BrazilPublished online: 31 Oct 2011.
To cite this article: Eva Vidal‐Vázquez , Raquel Caridad‐Cancela , M. MercedesTaboada‐Castro , Antonio Paz‐González & Cleide Aparecida de Abreu (2005): TraceElements Extracted by DTPA and Mehlich‐3 from Agricultural Soils with and withoutCompost Additions, Communications in Soil Science and Plant Analysis, 36:4-6,717-727
To link to this article: http://dx.doi.org/10.1081/CSS-200043354
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Trace Elements Extracted by DTPA andMehlich-3 from Agricultural Soils with and
without Compost Additions
Eva Vidal-Vazquez, Raquel Caridad-Cancela,
M. Mercedes Taboada-Castro, and Antonio Paz-Gonzalez
Facultad de Ciencias, Universidade da Coruna, A Coruna, Spain
Cleide Aparecida de Abreu
Centro de Solos e Recursos Agroambientais, Instituto Agronomico,
Campinas (SP), Brazil
Abstract: Risks of soil contamination when waste materials are used as fertilizers have
been a matter of frequent concern. The effect of compost from municipal organic waste
on trace element status was examined in short-term field trials at neighboring areas of A
Coruna (northwest Spain). The study sites were characterized as medium textured soils,
with a range of pHs, organic matter content, and cation exchange capacity. The
objective of this work was to compare two extraction methods (DTPA and Mehlich-
3) to determine micronutrient contents in soils with and without compost additions.
DTPA and Mehlich-3 extractions were carried out, and then analyses for Fe, Mn,
Cu, Zn, Ni, Cr, Pb, and Cd contents were performed by ICP-AES. Overall, DTPA
was less efficient than Mehlich-3 for Fe and Mn extraction. Lead, Ni, and Cd were
extracted more effectively by DTPA than by Mehlich-3. In general, the efficiency of
the extractants was related to compost addition, a trend that was most apparent for
Zn and Cu. Levels of trace metals extracted by DTPA and Mehlich-3 from soils
without compost addition were already highly variable, because of traditional
farmyard manure and slurry fertilization. Diagnostic criteria for allowable loading
Address correspondence to Eva Vidal-Vazquez, Facultad de Ciencias, Universidade
da Coruna, Campus de a Zapateira s/n, 15071 A Coruna, Spain; E-mail: [email protected]
Communications in Soil Science and Plant Analysis, 36: 717–727, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0010-3624 print/1532-2416 online
DOI: 10.1081/CSS-200043354
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limits of heavy metals during compost application should take into account direct
measurements of background levels and relevant soil properties such as soil acidity.
INTRODUCTION
The use of waste materials as fertilizers is a matter of frequent concern, given
the risks of soil contamination. In particular, trace element and/or micro-
nutrient accumulation in agricultural soils from point and diffuse sources of
different origins has been widely studied (1, 2).
Frequently, heavy metal status and metal hazard levels have been
assessed by chemical analysis, mainly based on total soil element concen-
trations (3, 4). On the other hand, soil-testing laboratories are commonly
using extractants to evaluate the “available” nutrient content for fertilizer
recommendations. To avoid having to use several different extractants,
multielement extractants have been widely tested (5–7) and are now
preferred. Soil testing for both assessment of an “available” fraction and
for metal contamination is a continually evolving process. There is also
recognition that appropriate extractants depend on specific properties of
particular soils and that local knowledge needs to be applied to any general
guideline for management of micronutrient and/or heavy metal status of the
soil (8).
Disposal of municipal sewage sludge and waste of urban origin on
agricultural land near urban areas (e.g., home gardens) is carried out in
many places as a way of recycling city refuses, which may contain
variable, and even sometimes unacceptable, loads of heavy metals with
longtime residence in soils. Composting organic materials of municipal
origin is considered a safer practice; however, the accumulation problem
still remains.
The purpose of this article is not to discuss the above issues in detail but
rather to determine which decisions about land use have to consider nutrient
constraints, either deficiency or toxicity, for different soil types because the
final outcome of soil testing is the interpretation for farmer advice under a
given natural environment.
Traditionally, the agricultural system of Galicia (northwest Spain) has
included the application of large quantities of organic manure to agricultural
fields. Cattle, pig, and poultry manure, either as solids or slurries, has
commonly been used to improve the nutrient status of crops and pasture
soils. Natural soils of this region are acid and rich in organic matter. In agri-
cultural soils, organic fertilizers have been shown to be suitable both as
nutrient sources and to maintain high levels of soil organic matter. The
ability of the soil to retain heavy metals has been recognized to depend on
properties such as the cation exchange capacity (CEC), so that the higher
the CEC, the better the soil serves as an environmental buffer (9). In the
E. Vidal-Vazquez et al.718
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prevailing types of soils of the study area, organic matter has been found to
improve soil condition by increasing the CEC and, hence, the buffering
function of the soil (10). Under conventional tillage, these soils are also
degraded structurally by loss of soil organic matter (11). Notwithstanding,
these long-term additions of farmyard organic manure may have produced
an increase in heavy metals (12, 13).
Recently, in the area being studied, the transformation of organic wastes
of urban origin into compost is now being carried out. The compost from
municipal refuse will be used mainly as fertilizer. As a consequence, field
trials are being conducted to examine the effect of compost on soil trace
element status in neighboring areas of A Coruna (northwest Spain).
Environmentally sound studies on metal accumulations in soils require
long-term trials, as frequently quoted in the literature (14, 15). However,
initial soil survey may help to guide research. Comparison of analytical results
from soils with and without compost should provide insights into the hazards
of heavy metal loading in the study region. Moreover, large amounts of
organic matter added to agricultural soils caused a shift between the different
pools of nutrients in the soil. Using compost as an additional external input in
long-term cultivated soils, where management was not modified over
prolonged periods, may have consequences for the available micronutrient pool.
The main objective of this study was to compare the micronutrient levels
of fields with long-term traditional fertilization, including manure but not
compost, with those of fields having a traditional fertilization history where
compost was also applied in short-term trials. Furthermore, the efficiency of
two extractants widely used for characterizing trace element availability
(DTPA and Mehlich 3) was tested.
MATERIALS AND METHODS
The study fields were located within a 30-km radius of A Coruna (NW Spain).
Twenty-four soil samples were collected from traditional agricultural fields
where long-term manure additions had been applied. In addition, 17 soil
samples were taken from agricultural fields with similar characteristics, but
that were also fertilized with compost for a short-term period (1–2 yr). The
sampling depth was 0–20 cm.
Samples were air-dried at room temperature and ground to pass through a
2-mm sieve. Soil physicochemical properties were characterized by routine
methods (16, 17). The pH of the soil was measured in water (1 : 2.5 v/v).
Particle size distribution was determined by the pipette method. Organic
matter content was measured by a modified Walkley-Black procedure. CEC
was determined by extraction with NH4OAc solution at pH 7.
Macro- and microelements were extracted from duplicate samples with
acid solution of Mehlich-3 (M3 : 0.2 N CH3COOH, 0.25 N NH4NO3, 0.015
Trace Elements Extracted by DTPA and Mehlich-3 719
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N NH4F, 0.013 N HNO3, and 0.001 N EDTA adjusted to pH 2.5) using a 1 : 10
(v : v) volume ratio (18). In addition, microelements were extracted by DTPA
(DTPA: 0.005 M DTPA, 0.1 M triethanolamine (TEA) and 0.01 M CaCl2 at
pH 7.3.) with a 1 : 2 (v : v) volume ratio (19). Available forms of Fe, Mn,
Cu, Zn, Ni, Pb, and Cd were analyzed by inductively coupled plasma
atomic emission spectroscopy (ICP-AES).
Differences between experiments were analyzed with routine statistical
procedures, including linear regression and histogram characterization, and
significant differences between soil groups with and without compost were
determined by using the computer software SPSS. For assessing the signi-
ficance of differences between soil groups, a nonparametric Mann-Whitney
U test was carried out.
RESULTS AND DISCUSSION
Soil Properties and Compost Elemental Composition
The main physicochemical soil properties of the soils in this study are listed in
Table 1. The 24 fields without any compost addition were characterized as
medium textured soils (i.e., silty-loam, loam, sandy-loam, and clay-loam)
with a range in pHH20 (4.2–7.87%), organic matter content (1.48–9.96%)
and CEC (12.5–24.70 cmolþ.kg21). The texture of the 17 fields where
compost was used in place of manure was similar to that of the first group
and also had broad ranges of pHH2O (4.9–7.7), organic matter content
(2.18–11.20%) and CEC (9.51–21.70 cmolþ.kg21).
Table 1. Selected properties of the soil used in this study
Soil property Range Mean Median
Without compost (n ¼ 24)
pH (H2O) 4.2–7.8 5.9 6.0
OM (%) 1.48–9.96 4.81 5.24
CEC (cmolþkg21) 12.50–24.70 16.42 16.48
Sand (%) 26.24–61.61 40.9 38.7
Silt (%) 22.07–55.02 38.5 37.1
Clay (%) 13.85–30.57 20.6 19.8
With compost (n ¼ 17)
pH (H2O) 4.9–7.7 5.8 5.7
OM (%) 2.18–11.20 5.37 4.73
CEC (cmolþkg21) 9.51–21.70 14.7 14.4
Sand (%) 33.26–65.86 49.2 49.6
Silt (%) 16.18–45.00 29.1 27.4
Clay (%) 10.77–28.75 21.7 21.8
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To analyze whether differences between both groups were statistically
significant, a non-parametric Mann-Whitney U test was carried out. For this
type of sampling, this test is better suited than a parametric Student’s t-test
or a nonparametric Wilcoxon W test for indicating significant differences.
The Mann-Whitney U test was used with 24 cases in Group 0 (without
compost) and 17 cases in Group 1 (with compost addition). This analysis
led to the conclusion that there were significant differences between the two
groups for organic matter content and CEC. On average, soils without
compost additions showed lower organic matter contents. However, the two
groups of fields were not statistically different with respect to their pH,
sand, silt, and clay content. Thus, the soil in fields with and without
compost addition may be considered homogeneous with regard to pH and
texture.
Average composition of compost is shown in Table 2. Although the major
nutrient that confers value to a compost is N (4, 9), the high Ca content of the
applied compost is noteworthy, considering the acid nature of the soils in the
region. An increase in soil pH due to compost addition is expected to reduce
risks of metal hazards. Note also the high P contents because accumulation of
P could pose a potential hazard to surface waters by overland flow (20). Heavy
metal concentrations extracted by M3 were not elevated, with the exception of
Zn (61.2 mg kg21) and Pb (18.6 mg kg21), whose content was highly variable.
At the present time, there are no limitations on compost use. Assuming a
maximum application rate of 100 tm ha21, the heavy metal loading over 20
yr as assessed by Mehlich-3-extractable contents, would be (kg ha21) as
follows: Zn: 122; Pb: 37; Cu: 6; Cr: 1; Ni: 1; and Cd: 0.48. Total metal
content loading would at least double these amounts.
Micronutrients Extracted by DTPA and Mehlich-3
Table 3 summarizes DTPA- and M3-extractable contents of the four micro-
nutrients: Fe, Mn, Cu, and Zn. For the set of samples without any compost,
Mehlich-3-extractable concentrations of Fe, Zn, and Cu were lower than the
extractable concentrations measured by DTPA. However, DTPA extracted
Table 2. Mean Concentrations for macro- and micronutrients extracted by Mehlich-3
from the compost
Element
Macronutrients (g kg21) Micronutrients (mg kg21)
Ca Mg P Zn Pb Cu Cr Ni Cd
11.12 1.06 0.89 61.2 18.6 3.0 0.52 0.61 0.24
Trace Elements Extracted by DTPA and Mehlich-3 721
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lower concentrations of Fe, Mn, Cu, and Zn than did Mehlich-3 for the group
of soils where compost was added.
In general, mean micronutrient concentrations extracted by both solutions
were of the same order of magnitude as values previously quoted (5, 21, 22).
As expected, extractable values for Fe, Mn, Zn, and Cu of the soils in this
study were higher than background or reference values measured in soils of
the region (23).
When all samples were included in the data set, mean values extracted by
DTPA were as follows: Fe: 99.54 mg kg21; Mn: 36.82 mg kg21; Zn: 6.75 mg
kg21; and Cu: 3.47 mg kg21. The corresponding concentrations extracted by
Mehlich-3 were as follows: Fe: 133.95 mg kg21; Mn: 49.90 mg kg21; Zn:
5.16 mg kg21; and Cu: 2.22 mg kg21. Overall, M-3 extracted more Fe and
Mn and less Zn and Cu than DTPA.
Results for Mn and Fe extracted by DTPA and Mehlich-3 are in agreement
with findings in previous works (5, 8, 21, 22). This was not the case for Zn and
Cu. In general, Mehlich-3, due to the presence of acid reagents and chelates
such as EDTA, is expected to extract higher amounts of macro- and micro-
nutrients than DTPA. However, there is also evidence that DTPA has the
highest efficiency in extracting these elements from near neutral soils (24),
whereas Mehlich-3 is thought to be more suitable for acid conditions (8, 22).
The ranges of the four micronutrients extracted by DTPA and Mehlich-3
are also differently ranked in the two groups of soils, indicating good corre-
spondence with the above results for mean values. As shown in Table 3,
these ranges were higher with DTPA for Fe, Zn, and Cu, but not for Mn in
Table 3. Micronutrients (Fe, Mn, Zn, and Cu) extracted by DTPA and Mehlich 3
from soils with and without compost addition
Element
extractant
Without compost (mg/kg) After compost (mg/kg)
Range Mean Median Range Mean Median
Fe
DTPA 31.5–207.8 96.0 92.1 40.0–274.8 104.5 88.1
Mehlich-3 18.0–95.0 58.6 61.0 116.2–360.2 240.4 251.0
Mn
DTPA 5.0–83.3 33.0 24.6 9.0–112.8 42.2 30.9
Mehlich-3 2.7–105.0 42.1 45.5 25.5–141.9 60.9 49.1
Zn
DTPA 0.7–49.8 8.6 4.3 0.8–12.6 4.1 4.0
Mehlich-3 0–24.7 3.9 2.1 1.1–24.6 6.9 5.1
Cu
DTPA 0.1–15.2 4.1 3.1 0.9–6.1 2.6 2.5
Mehlich-3 0–4.6 1.4 0.9 1.3–10.2 3.4 2.9
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the group of soils without any compost. In soils with a history of compost
addition, Mehlich-3 gave the highest ranges for all four elements.
In general, the range of micronutrient contents was of the same order of
magnitude for the soils without and with additional compost application.
Moreover, the frequency distribution of extractable contents exhibited a
similar trend for the studied elements in the two data sets.
Micronutrient contents were further analyzed by regression analysis,
comparing values extracted by DTPA with those by Mehlich-3. A high corre-
lation would indicate that both methods probably are extracting from the same
pool in the soil. When all samples were included in the data set, the highest
determination coefficients were for Mn (r2 ¼ 0.67) and Zn (r2 ¼ 0.41),
which were significant at the p , 0.01 level. Iron and Cu extracted by
DTPA and Mehlich-3 were not significantly correlated for the whole set of
samples. However, when considering sample sets with and without
compost, correlation between micronutrient contents extracted by DTPA
and Mehlich-3 were higher. For the group of soils without compost, the coeffi-
cients of correlation varied as follows: Fe: 0.14; Mn: 0.44; Zn: 0.82; and Cu:
0.89. After compost addition, they were as follows: Fe: 0.45; Mn: 0.90; Zn:
0.77; and Cu: 0.85. Thus, when both groups of samples were considered as
a unique set, correlations between micronutrient contents extracted by both
solutions were much weaker or not significant.
The results illustrate difficulties in selecting a single multielement extrac-
tant for soil samples with different physicochemical properties. As quoted in
previous works (5, 8, 23, 24), DTPA and Mehlich-3 cannot be recommended
as general test procedures for extraction of the micronutrients studied from
soil samples having various chemical and physical properties. These two
groups of study soils, with and without compost, even if they are developed
over relatively similar parent material, present a range of pHs, organic
matter content, and CEC. Thus, the efficiency of DTPA and Mehlich-3
solutions in extracting microelements showed an important variability (was
expected to and did within and between each of the soil groups studied),
which was an expected result.
In general, the efficiency of the extractant was strongly impacted by the
addition of compost and was most apparent for Zn and Cu (Fig. 1). Zinc
and Cu were extracted more easily by DTPA than by Mehlich-3 in the
group of soils where compost was not applied, whereas the opposite was
true after compost addition.
Levels of extractable micronutrients in agricultural soils without compost
addition were already highly variable due to long-term farmyard manure
and, in some cases, slurry additions. In both groups of soils, the highest vari-
ability was observed with extractable Zn and Cu. DTPA-extractable Zn was
.10 mg kg21 in 5 out of the 41 samples studied and DTPA-extractable Cu
was .6 mg kg21. These results are attributed to long-term traditional agricul-
ture and not to short-term compost addition.
Trace Elements Extracted by DTPA and Mehlich-3 723
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Other Trace Elements Extracted by DTPA and Mehlich-3
The amount of Cr extracted by DTPA and Mehlich-3 was below the detection
limit. A statistical summary of amounts of Ni, Pb, and Cd extracted by DTPA
and M-3 appears in Table 4. Mean Ni-, Pb-, and Cd- extractable values were
always low with both extractants. The presence of high levels of Pb or Ni in
both soil groups of this study limited rather to some of the individual
samples.
Figure 1. Relationship between micronutrients extracted by DTPA and Mehlich 3.
E. Vidal-Vazquez et al.724
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For the two data sets of this study, mean values of Pb, Ni, or Cd extracted
by DTPA were much higher than those obtained by the Mehlich-3 solution.
Note that in most of the soil samples where compost was not added, Pb, Ni,
and Cd contents extracted by Mehlich-3 were below the detection limit.
This was also true for Ni and Cd after compost addition. Thus, DTPA is
more efficient than Mehlich-3 in extracting Pb, Ni, and Cd and seems to be
the most adequate extractant for analyzing the availability of these heavy
metals in agriculture soils.
Again, trace metal contents (Pb, Ni, and Cd) other than micronutrients
extracted by DTPA and Mehlich-3 resulted in relatively low correlation co-
efficients when samples from fields with and without compost were
included in the data set. This is an indication that both extractants are not
acting over the same metal species.
The low levels of heavy metals measured above show that the impact
caused by heavy metals in agricultural soils of the study area is still small.
This is in accordance with previous findings where DTPA was used as diag-
nostic criteria for assessing heavy metal contamination (25). Moreover,
there seems to be no risk of soil contamination by toxic trace elements
when using composts as fertilizers if recommended statutory thresholds for
heavy metal contents are respected.
Diagnostic criteria for allowable loading rates of heavy metals during
compost application should take into account background soil contents, due
to the differences in total levels and extractability between individual fields.
Lime addition to prevent the build up of acidity for the soils of this region
should be one of the aspects of compost management because of the
well-known relationship between acidity increase and heavy metal availability
(2, 4).
Table 4. Pb, Ni, and Cd extracted by DTPA and Mehlich 3 from soils without and
with compost
Element
extractant
Without compost (mg/kg) After compost (mg/kg)
Range Mean Median Range Mean Median
Pb
DTPA 0.19–4.37 1.93 1.47 1.16–4.22 2.40 2.30
Mehlich-3 0.0–4.50 0.56 0.00 0.0–2.59 0.47 0.18
Ni
DTPA 0.0–4.62 0.56 0.36 0.26–5.10 1.01 0.44
Mehlich-3 0.0–0.10 — — 0.0–3.3 0.40 0.00
Cd
DTPA 0.0–0.16 0.06 0.01 0.01–0.16 0.07 0.06
Mehlich-3 0.0–0.10 — — 0.0–0.24 0.06 0.00
Trace Elements Extracted by DTPA and Mehlich-3 725
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CONCLUSIONS
Levels of extractable micronutrients in the agricultural soils studied, close to
an urban area, were highly variable. Traditional agricultural practices have
resulted in high contents of extractable Zn and Cu in some fields. The
changes in micronutrients (Fe, Mn, Cu, and Zn) and other trace element
(Pb, Cr, Ni, and Cd) contents by recent compost addition (i.e., 1–2 yr) have
been minimal.
Micronutrient concentrations extracted by DTPA and Mehlich-3 resulted
in relatively low correlation coefficients when samples from fields with
and without compost were included in the data set. Correlation analysis
suggests that both solutions probably are not extracting from the same pool
in the soil.
ACKNOWLEDGMENTS
The authors thank Caixa Galicia for funding University of Corunna for
this work.
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Trace Elements Extracted by DTPA and Mehlich-3 727
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19:
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