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Magnetic and spectral signatures of Cerrado soils in the state of Goiás, Brazil

Article (PDF Available) inPesquisa Agropecuária Brasileira 52(10):923-932 · October 2017with41 Reads
DOI: 10.1590/s0100-204x2017001000012
Kathleen Fernandes at São Paulo State University
  • 6.56
  • São Paulo State University
Adriana Aparecida Ribon at Universidade Estadual de Goiás, Câmpus Palmeiras de Goiás
  • 15.59
  • Universidade Estadual de Goiás, Câmpus Palmeiras de Goiás
Marques Jr José at São Paulo State University
  • 34.22
  • São Paulo State University
João Tavares Filho at Universidade Estadual de Londrina
  • 27.84
  • Universidade Estadual de Londrina
Abstract
The objective of this work was to estimate the iron oxide contents (hematite and goethite) and to characterize the color and the spectral and magnetic signatures of Cerrado soils in the state of Goiás, Brazil. Six Oxisols and one Inceptisol were studied. Spectral and magnetic signatures were determined by diffuse reflectance spectroscopy (DRS) and magnetic susceptibility, respectively. Then, the spectral curves and the second derivative calculations were used to determine hematite and goethite contents, as well as soil color after conversion into tristimulus values. Hematite and goethite contents were also obtained by x-ray diffractometry, and soil color was also defined in the field (Munsell color chart). The values for the isomorphic substitution of iron by aluminum and the degree of redness were also determined. DRS can be used to estimate hematite and goethite contents, as well as the color of Cerrado soils in the state of Goiás. The spectral signature can point out the main soil properties related to the contents of organic matter, iron oxides, kaolinite, and gibbsite. The magnetic signature, characteristic of soils rich in iron oxides (hematite and goethite), shows the predominance of pedogenic minerals.
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Assinaturas magnéticas e espectrais de solos do Cerrado de Goiás

Abstract
Resumo: O objetivo deste trabalho foi estimar os teores de óxidos de ferro (hematita e goethita) e caracterizar a cor e as assinaturas espectrais e magnéticas de solos do Cerrado de Goiás. Foram estudados seis Latossolos e um Cambissolo. As assinaturas espectrais e magnéticas foram determinadas por espectroscopia de reflectância difusa (ERD) e suscetibilidade magnética, respectivamente. Posteriormente, as curvas espectrais e os cálculos de derivativos secundários foram utilizados para determinar os conteúdos de hematita e goethita, além da cor dos solos após conversão em valores tristimulus. Os teores de hematita e goethita também foram obtidos por difratometria de raios-x, e a cor dos solos também foi avaliada em campo (carta de Munsell). Também foram determinados os valores de substituição isomórfica do ferro por alumínio e o índice de avermelhamento dos solos. A ERD pode ser utilizada para estimar os conteúdos de hematita e goethita, bem como a cor dos solos do Cerrado de Goiás. A assinatura espectral é capaz de apontar as principais características dos solos relacionadas aos conteúdos de matéria orgânica, óxidos de ferro, caulinita e gibbsita. A assinatura magnética, característica de solos ricos em óxidos de ferro (hematita e goethita), mostra o predomínio de minerais pedogenéticos.
Pesq. agropec. bras., Brasília, v.52, n.10, p.923-932, out. 2017
DOI: 10.1590/S0100-204X2017001000012
Magnetic and spectral signatures of Cerrado
soils in the state of Goiás, Brazil
Kathleen Lourenço Fernandes(1), Adriana Aparecida Ribon(2), José Marques Junior(1),
Angélica Santos Rabelo de Souza Bahia(1) and João Tavares Filho(3)
(1)Universidade Estadual Paulista Júlio de Mesquita Filho, Faculdade de Ciências Agrárias e Veterinárias, Campus de Jaboticabal,
Via de Acesso Prof. Paulo Donato Castellane, s/no, CEP 14884-9 00 Jaboticabal, SP, Brazil. E-mail: kathleen_agro@hotmail.com,
marques@fcav.unesp.br, angelicasantosrabelo@yahoo.com.br (2)Universidade Estadual de Goiás, Campus Palmeiras de Goiás, Rua S -7,
s/no, Setor Sul, CEP 76190-000 Palmeiras de Goiás, GO, Brazil. E-mail: adriana.ribon@ueg.br ( 3)Universidade Estadual de Londrina,
Rodovia Celso Garcia Cid, Pr 445, Km 380, Caixa Postal 10.011, CEP 86057-970 Londrina, PR, Brazil. E-mail: tavares@uel.br
Abstract – The objective of this work was to estimate the iron oxide contents (hematite and goethite) and to
characterize the color and the spectral and magnetic signatures of Cerrado soils in the state of Goiás, Brazil.
Six Oxisols and one Inceptisol were studied. Spectral and magnetic signatures were determined by diffuse
reectance spectroscopy (DRS) and magnetic susceptibility, respectively. Then, the spectral curves and the
second derivative calculations were used to determine hematite and goethite contents, as well as soil color after
conversion into tristimulus values. Hematite and goethite contents were also obtained by x-ray diffractometry,
and soil color was also dened in the eld (Munsell color chart). The values for the isomorphic substitution of
iron by aluminum and the degree of redness were also determined. DRS can be used to estimate hematite and
goethite contents, as well as the color of Cerrado soils in the state of Goiás. The spectral signature can point
out the main soil properties related to the contents of organic matter, iron oxides, kaolinite, and gibbsite. The
magnetic signature, characteristic of soils rich in iron oxides (hematite and goethite), shows the predominance
of pedogenic minerals.
Index terms: diffuse reectance spectroscopy, goethite, hematite, magnetic susceptibility, Oxisols.
Assinaturas magnéticas e espectrais de solos do Cerrado de Goiás
Resumo – O objetivo deste trabalho foi estimar os teores de óxidos de ferro (hematita e goethita) e caracterizar
a cor e as assinaturas espectrais e magnéticas de solos do Cerrado de Goiás. Foram estudados seis Latossolos e
um Cambissolo. As assinaturas espectrais e magnéticas foram determinadas por espectroscopia de reectância
difusa (ERD) e suscetibilidade magnética, respectivamente. Posteriormente, as curvas espectrais e os cálculos
de derivativos secundários foram utilizados para determinar os conteúdos de hematita e goethita, além da cor
dos solos após conversão em valores tristimulus. Os teores de hematita e goethita também foram obtidos por
difratometria de raios-x, e a cor dos solos também foi avaliada em campo (carta de Munsell). Também foram
determinados os valores de substituição isomórca do ferro por alumínio e o índice de avermelhamento
dos solos. A ERD pode ser utilizada para estimar os conteúdos de hematita e goethita, bem como a cor
dos solos do Cerrado de Goiás. A assinatura espectral é capaz de apontar as principais características dos
solos relacionadas aos conteúdos de matéria orgânica, óxidos de ferro, caulinita e gibbsita. A assinatura
magnética, característica de solos ricos em óxidos de ferro (hematita e goethita), mostra o predomínio de
minerais pedogenéticos.
Termos para indexação: espectroscopia de reectância difusa, goethita, hematita, suscetibilidade magnética,
Latossolos.
Introduction
The soil is a complex system that requires proper
management to avoid possible risks of degradation,
ensuring food security and sustainable development.
Understanding soils, considering all their attributes
and processes of formation, is essential to monitor them
(Viscarra Rossel et al., 2016). However, laboratory
analyses are generally costly and time-consuming,
restricting the number of samples representative of
a region (Viscarra Rossel, 2011; Bahia et al., 2015;
Demattê et al., 2016). In this context, the use of spectral
signature emerges as an alternative methodology for
this task.
924 K.L. Fernandes et al.
Pesq. agropec. bras., Brasília, v.52, n.10, p.923-932, out. 2017
DOI: 10.1590/S0100-204X2017001000012
Diffuse reectance spectroscopy (DRS) and
magnetic susceptibility (MS) are alternative techniques
that can be used to estimate soil properties such as:
iron oxides (Viscarra Rossel et al., 2010; Bahia et al.,
2015); silicate minerals, including kaolinite, illite, and
smectite (Viscarra Rossel, 2011; Mulder et al., 2013);
chemical attributes and grain size (Marques Jr. et
al., 2014; Dematet al., 2016; Viscarra Rossel et al.,
2016); phosphorus adsorption (Camargo et al., 2015;
Peluco et al., 2015); and soil color (Aquino et al., 2016;
Carmo et al., 2016).
Although these techniques have been adopted in
several studies, there are few local researches on soils
in the Cerrado in the state of Goiás, Brazil. Demattê
et al. (2016) assessed models for the prediction of soil
attributes that were generated using a spectral database
from multiple locations. According to these authors,
the more complete and broad a spectral library of
soils is in a given region, the better will be the quality
of the evaluation of the attributes, processes, and
management of such soils.
The objective of this work was to estimate the
iron oxide contents (hematite and goethite) and to
characterize color and the spectral and magnetic
signatures of soils in the Cerrado in the state of Goiás,
Brazil.
Materials and Methods
The study was conducted in the Cerrado biome, in
the municipality of Campestre de Goiás, in the south
of the state of Goiás, Brazil (146'39"S, 49°44'38"W).
The climate type of the region, according to Köppen’s
classication, is tropical wet (Aw), with dry winters and
rainy summers (Ribeiro & Walter, 2008). The source
material from the site is composed of metamorphic
metagranites of the Jurubatuba formation from the
Proterozoic age, as characterized by Fernandes et al.
(2016).
For the experiment, seven soil proles were selected,
classied according to Santos et al. (2013): S1, Latossolo
Amarelo distrocoeso típico (Xanthic Hapludox)
(LAdx-1); S2, Cambissolo Háplico Tb eutróco
latossólico (Eutrochrept) (CXbe); S3, Latossolo
Amarelo eutróco típico (Xanthic Eutrudox) (LAe-
1); S4, Latossolo Vermelho eutróco chernossólico
(Rhodic Eutrudox) (LVe); S5, Latossolo Amarelo
distrocoeso típico (Xanthic Hapludox) (LAdx-2); S6,
Latossolo Vermelho-Amarelo eutróco típico (Typic
Eutrudox) (LVAe-1); S7, Latossolo Amarelo eutróco
típico (Xanthic Eutrudox) (LAe-2). Table 1 shows the
grain size and chemical characteristics of the evaluated
soils.
Total iron oxide content – dithionite (Fed) – was
determined by extraction by sodium dithionite-citrate-
bicarbonate, following the methodology of Mehra &
Jackson (1960), and the dosage of iron was measured
by atomic absorption spectrophotometry. The content
of iron oxide of low crystallinity (Feo) was obtained
by extraction by ammonium oxalate, according to
Camargo et al. (1986).
For the mineralogical analysis, the clay fraction was
separated only in the diagnostic horizons. The samples
were subjected to mechanical agitation with NaOH
0.5 mol L-1 for 10 min for particle dispersion. After
this treatment, the sand fraction was removed with
a 0.05-mm sieve. The silt fraction was separated by
centrifugation at 1,600 rpm, and the running time for
the procedure varied according to the temperature of
the samples at the time of analysis. The clay suspension
was occulated with concentrated HCl and centrifuged
at 2,000 rpm for 2 min.
For the characterization of the minerals hematite
(Hm) and goethite (Gt), the concentration of oxides was
determined with NaOH 5 mol L-1 (1 g clay per 100 mL
solution), according to the method of Norrish & Taylor
(1961) modied by Kämpf & Schwertmann (1983). To
maintain the minimum concentration of silicic acid
in the solution of NaOH 5 mol L-1, 10% by weight
of ground silica gel was added, avoiding changes in
aluminum replacement and goethite crystallinity
(Kämpf & Schwertmann, 1983). To prevent sodalite
from hampering the reading of the diffractograms, the
samples were washed with a solution of HCl 0.5 mol L-1
(1 g clay per 100 mL solution) and stirred for 4 hours.
Deviations in the positions (d) of the reections being
studied were corrected by adding to the samples 10%
by weight of sodium chloride, which was ground and
sieved in a 0.10-mm mesh before diffraction.
Subsequently, the minerals were characterized by
X-ray diffraction (XRD) in a MiniFlex II unit (Rigaku
Americas Corporation, Woodlands, TX, USA), using
a copper cathode, with a nickel lter and Kα radiation
(20 mA, 30 Kv), and the dust method. Scanning speed
was 1º2θ per minute with an amplitude of 23 to 49°2θ
for the characterization of Hm and Gt.
Magnetic and spectral signat ures of soils in the Cerrado 925
Pesq. agropec. bras., Brasília, v.52, n.10, p.923-932, out. 2017
DOI: 10.1590/S0100-204X2017001000012
To calculate t he isomorphous replacement (mol mol-1)
of iron by aluminum in Gt, the procedures suggested
by Schulze & Schwertmann (1984) were followed.
To estimate Gt content, crystalline iron content was
multiplied by the Gt/(Gt+Hm) ratio and by 1.59. For
Hm, crystalline iron content was multiplied by 1.43,
after subtracting from this value the amount of iron
corresponding to Gt (Dick, 1986).
To obtain the DRS spectra, approximately 1 g air-
dry ne earth (ADFE) – from the diagnostic horizons
was ground (Barron & Torrent, 1986; Torrent
& Barrón, 1993) in agate mortar until obtaining a
constant color, and the content was placed in a sample
holder with an inner cylinder (16 mm in diameter). The
reectance values were measured in a Lambda 950
UV/Vis/NIR spectrophotometer (Perkin Elemer do
Brasil, São Paulo, SP, Brazil) coupled with a 150-mm
integrating sphere. The spectra were recorded at every
0.5 nm, with integration time of 2.43 nm s-1 over the
range of 380 to 780 nm (visible region) and the NIR.
The contents of Hm and Gt were estimated by
applying the second derivative of the Kubelka-Munk
function (Kubelka & Munk, 1931). The absorption
bands typical of iron oxides were identied in the
curves of the second derivative (Kosmas et al., 1984;
Scheinost et al., 1998). For the identication of Gt and
Hm, the minimum ranges adopted were 415–425 and
530–545 nm, respectively, and the maximum ranges
were 440–450 and 575–590 nm (Bahia et al., 2014).
The value of the amplitudes of the absorption bands of
Gt and Hm were used to calculate the correlation with
the estimated quantity of these minerals, according
Tab le 1. Grain size, chemical properties, and sulfuric acid content of the studied soils(1).
Soil Depth Horizon pH
H2O
Base
saturation
SOM Grain size SiO2Al2O3Fe2O3Ki
Sand Silt Clay
(m) (%) ----------------------------------(g kg-1)----------------------------------
S1 ( L Ad x-1)
0.00–0.28 A5.30 2 7.0 0 2.40 186 127 687 ----
-0.63 A/B 5.0 0 26.00 1.50 166 107 727 ----
-1.21 Bw15.50 15.00 6.00 146 167 687 100.00 135.00 70.25 1.30
-1.60 + Bw26.20 63.00 7.00 186 247 567 ----
S2 (C X be)
0.00–0.25 A5.67 66.27 33.41 39 0 240 370 ----
-0.65 A/B 5.78 65.53 35.90 147 300 553 ---
-1.5 Bi16.41 55.7 9 12.28 327 280 393 116.0 0 90.00 59. 05 2.20
-2.00+ Bi26.98 58.31 6.15 260 220 520 ----
S3 (LAe-1)
0.00–0.29 A5.67 52.73 16.9 400 80 520 ----
-0.37 A/B 5.26 50.66 16.12 300 80 620 ----
-0.66 Bw14.93 60.53 12.07 260 120 620 101.50 85.00 40.50 2.0 0
-1.10+ Bw25.15 61.71 8.95 300 80 620 ----
S4 ( LVe)
0.00–0.36 A6 .41 78.43 38 .71 520 120 360 ----
-0.58 A/B 5.60 65.09 11.2 9 360 120 ----
-1.30 + Bw15.69 63.25 10. 51 400 50 550 2 5.50 170.00 44.07 0.30
S5 ( LAdx-2)
0.00–0.13 A5.64 4 0.19 19.70 700 60 240 ----
-0.24 A/B 4.23 52.5 4 19.08 560 120 320 ----
-0.38 B /A 5.45 45.61 14.56 460 100 440 ----
-1.50 + Bw 5.67 50.33 8.93 484 11 2 404 42.0 0 90.00 62.31 0.80
S6 (LVAe-1)
0.00–0.23 A6.31 55.95 30.92 460 100 440 ----
-0.42 A/B 6.25 72.43 19. 55 300 80 620 ----
-0.92 Bw15.55 74.54 13.63 340 180 480 25.50 150.00 57. 25 0.30
-1.56 + Bw24.95 54.8 8.95 260 120 620 ----
S7 ( LAe-2)
0.00–0.19 A6.19 72.30 19.55 600 60 340 ----
-0.59 Bw15.60 52.38 10.51 460 80 460 33.00 70.00 56.89 0.80
-1.08 Bw25. 37 67.09 8.95 480 100 420 ----
-1.50 + Bw34.85 37.16 8.18 380 160 460 ----
(1)S1 (LAdx-1), Latossolo Amarelo distrocoeso típico (Xanthic Hapludox); S2 (CXbe); Cambissolo Háplico Tb eutróco latossólico (Eutrochrept); S3
(LAe-1), Latossolo Amarelo eutróco típico (Xanthic Eutrudox); S4 (LVe), Latossolo Vermelho eutróco chernossólico (Rhodic Eutrudox); S5 (LAdx-2),
Latossolo Amarelo distrocoeso típico (Xanthic Hapludox); S6 (LVAe-1), Latossolo Vermelho-Amarelo eutróco típico (Typic Eutrudox); S7 (LAe-2),
Latossolo Amarelo eutróco típico (Xanthic Eutrudox). SOM, soil organic matter.
926 K.L. Fernandes et al.
Pesq. agropec. bras., Brasília, v.52, n.10, p.923-932, out. 2017
DOI: 10.1590/S0100-204X2017001000012
to Scheinost et al. (1998) and Bahia et al. (2014). The
spectral curves for the diagnostic B horizons of each
prole are shown in Figure 1.
After obtaining the spectra, the XYZ tristimulus
values were determined, as dened by Commission
International de L’Eclairage (Wyszecki & Stiles,
1982). The Munsell values for hue, value, and chroma
were calculated based on the XYZ coordinates
using the software Munsell Conversion, version 6.4,
according to Barrón et al. (2000) and Viscarra Rossel
et al. (2010). Subsequently, the degree of redness
(DOR) was calculated (Barrón et al., 2000), as:
DOR = [(10 - Hue)xChroma]/Value.
Soil color in the eld was determined by visual
observation, at the time of the morphological
description of the prole, with the aid of the Munsell
color system (Munsell, 1994). MS was determined in
the ADFE, in a Bartington MS2 system, coupled to a
MS2B dual-frequency sensor (Dearing, 1999); the MS
values were at a low frequency of 0.47 kHz (Dearing,
1994; Costa et al., 1999).
Results and Discussion
Spectral signature is an intrinsic property of each
soil, as shown by the curves of the diagnostic B
horizons (Figure 1). The S1, S4, and S6 soils presented
the highest reectance values in the range of the
visible region (250–780 nm) and at the beginning of
the NIR (780–2,500 nm), besides marked concavities
(Figure 1). According to Melo Filho et al. (2004), the
greater the organic matter (OM) contents in the soil,
the higher the concavity values of the spectral curve.
However, this was not the case in the present study
(Figure 1 and Table 1), nor in that of Demattê et al.
(2003). These authors evaluated the effect of OM on
spectral curves in soil samples in the state of São
Paulo and found that the samples without OM showed
higher reectance values along the spectral curve.
This opposite behavior was also observed for the S2,
S3, and S5 soils, which showed higher OM contents
(Table 1) and lower reectance along the spectral
curve (Figure 1).
OM affects various physical and chemical processes
in soils, as well as the expression of their color,
evidencing its close relation to their spectral signature
(Dalmolin et al., 2005). For the studied soils, the
greater proximity between the spectral curves, at the
end of the NIR image (Figure 1), can be justied by the
association of OM with minerals from the soil, which
can also affect reectance in certain bands. Clearly,
in the band at 1,400 nm, there are OH groups that are
typical of the illite, smectite, and kaolinite minerals
(Goetz et al., 2001). In the bands at 1,900 nm (Hunt &
Salisbury, 1970) and 1,920 nm (Bishop et al., 2008),
there are water molecules linked to minerals and
impurities; therefore, soil reectance is characterized
by various interferences and variations.
The less marked concavities observed in the band
at 800–900 nm indicate the highest iron content in the
samples (Dalmolin et al., 2005). The S3 soil showed
the lowest total iron content of 40.50 g kg-1 and the
highest reectance, which decreased in the following
soil order: S1, with 70.25 g kg-1 iron content; S6, with
57.25 g kg-1; S7, with 56.89 g kg-1; S5, with 62.31 g kg-1;
Figure 1. Spectral curves obtained by diffuse reectance
spectroscopy (DRS) for the following diagnostic B horizons
of the evaluated soils: Bw1 of S1 (LAdx-1), Lat ossolo Amarelo
distrocoeso típico (Xanthic Hapludox) ; Bi1 of S2 (CXbe),
Cambissolo Háplico Tb eutróco latossólico (Eutrochrept)
Bw1 of S3 (LAe-1), Latossolo Amarelo eutróco típico
(Xanthic Eutrudox) ; Bw1 of S4 (LVe), Latossolo Vermelho
eutróco chernossólico (Rhodic Eutrudox); Bw of S5
(LAdx-2), Latossolo Amarelo distrocoeso típico (Xanthic
Hapludox) ; Bw1 of S6 (LVAe-1), Latossolo Vermelho-
Amarelo eutróco típico (Typic Eutrudox) ; and Bw1 of
S7 (LAe-2), Latossolo Amarelo eutróco típico (Xanthic
Eutrudox).
500 1000 1500 2000 2500
0
10
20
30
40
50
60
70
Concavity of iron
oxide contents
Lowest organic matter content is indicated
by concavity because of iron oxides Edges influenced
by water
Gibbsite
Kaolinite
%R
nm
S1 (LAdx-1) S2 (CXbe) S3 (LAe-1) S4 (LVe )
S5 (LAdx-2)
S6 (LVAe-1)
S7 (LAe-2)
Magnetic and spectral signat ures of soils in the Cerrado 927
Pesq. agropec. bras., Brasília, v.52, n.10, p.923-932, out. 2017
DOI: 10.1590/S0100-204X2017001000012
S2 with 59.05 g kg-1, and S4, with total iron content of
44.07 g kg-1 and the lowest reectance (Table 1). The
differences among reectance values and iron contents,
greater between the S4 and S1 soils, can be explained
by the effect of other attributes on the spectral curve.
Demattê et al. (2003) reported that the removal of
OM allows the characteristic concavity of iron oxides
to become more evident at 850 nm, for soils with low
iron content. In the present study, the lower OM content
favored the higher reectance of the S1 soil with higher
iron content, whereas the higher OM content favored
the lower reectance of S4 with lower iron content,
which should have encouraged greater reectance.
These results indicate that the spectral signature must
be analyzed in association with other soil attributes
in order to avoid erroneous interpretations. The
crystallinity of iron oxides can also affect the spectral
signature of each soil. When studying the effect of
synthetic iron oxides on soil spectral signature, Cezar
et al. (2013) observed that the greater specic surface
area of more crystalline oxides causes a greater spread
of photons by changing the nal reectance of the
samples.
In the bands from 2,100 to 2,200 nm (Viscarra
Rossel, 2011; Demattê et al., 2016), valleys that are
characteristic of kaolinite (Kt) were observed in the
spectral curves, and, in the band at 2,265 nm (Demat
et al., 2000; Pizarro et al., 2001), valleys that are
typical of gibbsite (Gb). All the studied soils showed
valleys in the spectral curves that are characteristic of
Kt (Figure 1), but only some had valleys characteristic
of Gb. In the S2 and S3 soils, the more prominent
valley was that of Kt, and in S4, S5 and S7, that of Gb
(Fi g ure 1).
The occurrence of Gb in the above mentioned soils
is linked to their weathering rate (Pavelhão et al.,
2016). Gb is easily formed in soils with good drainage
conditions, rst by rapid weathering and alteration
of aluminosilicates, then by longer weathering that
involves the complete desilicatization of Kt (Ker,
1994). Therefore, the structural grids of Kt and Gb
have similar elements, such as Al, H, and O (with
the exception of Si for Kt), and may be associated in
the spectral curves. Oliveira et al. (2013) studied the
characterization of the spectral band in which Kt and
Gb occur, and found great proximity between the
valleys of the spectral curve, which makes it difcult
to interpret one without the effect of the other.
Table 2 shows data from Hm and Gt obtained by the
XRD and DRS techniques, as well as the correlation
coefcients. High levels of correlation (close to 1) were
obtained and can be used to estimate Hm and Gt for
all soils, except Hm for S1, which showed a coefcient
of 0.79.
The results obtained in the present study indicate
that DRS is an efcient technique for the indirect
quantication of oxides, allowing a large number of
samples to be evaluated. When studying Oxisols in
the state of São Paulo, Brazil, Bahia et al. (2015) used
DRS to accurately estimate the levels of Hm and Gt,
also observing that the technique allows working with
a higher sample density. Viscarra Rossel et al. (2010)
validated DRS to estimate Hm and Gt for mapping
soils in Australia. It should be noted that one of the
major obstacles to the advancement in soil science in
Brazil is detailed soil mapping, which is still scarce
(Marques Jr. et al., 2014).
The MS values ranged from 0.01 to 0.46×10-3 m3 kg-1
(Table 2). The S4 soil showed the highest MS values
(0.28 to 0.46×10-3 m3 kg-1) along the prole, and S2
showed MS of 0.15×10-3 m3 kg-1 for the Bi2 horizon.
The other soils showed low MS, ranging between 0.01
and 0.08×10 -3 m3 kg-1. These ranges are in agreement
with those reported in the literature for soils rich in
iron oxides, Hm, and Gt. Dearing (1994) highlighted
that MS in both of these oxides is at detectable levels
in the order of 0.3 to 1.7 and 0.3 to 1.2×10-6 m3 kg-1,
respectively.
Vasconcelos et al. (2013), when studying Oxisols
and Inceptisols in the Cerrado of the state of Minas
Gerais, Brazil, found higher MS values for Inceptisols
and lower ones for Oxisols. According to these authors,
MS has a negative correlation with Gt contents, which
is attributed to the decrease of ferromagnetic minerals
as this mineral increased, given the advancement in
the weathering processes. This can explain the results
found for most of the studied soils.
A lower MS is indicative of the presence of pedogenic
magnetic minerals (Bartington Instruments, 2013),
i.e., formed in the soil, indicating a greater degree of
weathering. Silva et al. (2010), when studying samples
of the B horizon of soils in the state of Paraná, Brazil,
reported that higher MS values are related to soils
derived from igneous rocks, while lower values are
linked to metamorphic and sedimentary soils. In the
present study, this can explain the lower MS values
928 K.L. Fernandes et al.
Pesq. agropec. bras., Brasília, v.52, n.10, p.923-932, out. 2017
DOI: 10.1590/S0100-204X2017001000012
found in the evaluated soils, which are metamorphic,
even though they have high quantities of iron oxides.
As for soil color, the hues observed in the eld
ranged from 2.5 to 10 YR (Table 3). More grayish
colors were observed in S2 and more reddish ones in
S4. The other soils presented colors with yellowish to
brownish shades. The more grayish colors in S2 are
due to the lower iron oxide content in this soil, which
also showed Ki of 2.20 and higher contents of Feo, a
less crystalline type of iron; therefore, this is the least
weathered among the studied soils.
Most of the soil proles in the present study showed
variations in color and the presence of ferruginous
concretions, which makes it more difcult to observe
the color in the eld and also generates different DRS
results, since, for analyses in the device, the sample
is macerated until the complete homogenization of
the color (S2, S3, S5, S6, and S7). With the DRS
Tab le 2. Mineralogical characterization of the studied soils( 1).
Depth (m) Horizon GtXRD GtDRS r HmXRD HmDRS r FedFeoMS
--------(g kg-1)-------- ---------(g kg-1)------ --------(g kg-1)-------- (10-3 m3 kg-1)
S1 (LAdx-1)
0.00–0.28 A44.85 37. 67 0.95 30.55 37.01 0.79 52.9 2 3.35 0.08
-0.63 A/B 59.96 47. 42 2 2.7 0 33.98 58.15 4.57 0.06
-1.21 Bw157. 23 41.38 26.91 41.16 55.62 0.82 0.04
-1.60+ Bw227.81 30.94 46.47 43.65 56.35 6.36 0.05
S2 (CXbe)
0.00–0.25 A 1.25 1 .31 0.95 2 .12 2.07 0.91 19.32 17.05 0.02
-0.65 A/B 5.65 4.34 6.73 7.91 29.80 21.54 0.03
-1.50 Bi110.90 16.74 14 .30 9.0 5 24.20 7.34 0.01
-2.00+ Bi218.88 20.67 16.47 14.86 42.08 18.68 0.15
S3 (LAe-1)
0.00–0.29 A24.30 16.92 0.87 14 .20 20.87 0.91 27.27 2.04 0.03
-0.37 A/B 13.50 15.94 22.03 22.00 26.01 2.12 0.03
-0.66 Bw111.31 16 .25 2 4.77 20.33 33.41 7.02 0.04
-1.10+ Bw23 4.70 29.38 42 .11 28.80 43.70 1.71 0.01
S4 (LVe)
0.00–0.36 A25.18 23.15 0.99 7.38 9.20 0.99 29.08 8.08 0.28
-0.58 Bw138.42 36.98 18.18 19.48 38.12 1.22 0.46
-1.30+ Bw231.18 30.28 21.00 21.81 36.66 2.36 0.45
S5 (LAdx-2)
0.00–0.13 A5.87 8.00 0.98 4.06 2.15 0.99 16.97 10.44 0.08
-0.24 A/B 18 .4 6 25.68 11. 23 4.75 21.67 2.20 0.06
-0.38 B/A 11.16 10.98 11. 63 11.78 22.57 7.42 0.03
-1.50 + Bw 52.04 49.45 42.34 44.67 72.78 10.44 0.01
S6 (LVAe-1)
0.00–0.23 A 10.80 19.42 0.90 30.33 22.58 0.99 29.80 1.79 0.04
-0.42 A/B 11.56 21. 21 33.82 25.14 31.24 0.32 0.05
-0.92 Bw15.56 15.32 2 5.19 16.42 37.92 16.81 0.05
-1.56+ Bw215.02 20.63 61.76 56.71 57.61 4.98 0.01
S7 (LAe-2)
0.00–0.19 A 3.86 2.47 0.99 4.44 5.69 0.99 8.30 2.77 0.01
-0.59 Bw17.0 0 3.82 7. 43 10.29 19.14 9.55 0.01
-1.08 Bw246.40 33.63 18 .0 4 29.53 46.77 4.98 0.01
-1.50+ Bw360.46 46.74 31. 0 4 43.37 62.66 2.94 0.01
(1)S1 (LAdx-1), Latossolo Amarelo distrocoeso típico (Xanthic Hapludox); S2 (CXbe), Cambissolo Háplico Tb eutróco latossólico (Eutrochrept); S3
(LAe-1), Latossolo Amarelo eutróco típico (Xanthic Eutrudox); S4 (LVe), Latossolo Vermelho eutróco chernossólico (Rhodic Eutrudox); S5 (LAdx-2),
Latossolo Amarelo distrocoeso típico (Xanthic Hapludox); S6 (LVAe-1), Latossolo Vermelho-Amarelo eutróco típico (Typic Eutrudox); S7 (LAe-2),
Latossolo Amarelo eutróco típico (Xanthic Eutrudox). GtXRD, goethite contents obtained by X-ray diffraction; GtDR S, goethite contents obtained by
diffuse reectance spectroscopy; HmXRD, hematite contents obtained by X-ray diffraction; HmDRS, hematite contents obtained by diffuse reectance
spectroscopy; and r, coefcient of linear correlation between goethite and hematite contents obtained by the two techniques.
Magnetic and spectral signat ures of soils in the Cerrado 929
Pesq. agropec. bras., Brasília, v.52, n.10, p.923-932, out. 2017
DOI: 10.1590/S0100-204X2017001000012
technique, the colors showed hues between 3 and
10 YR. Compared with the colors obtained in the
eld, very different hues were observed in some soils
(S2 and S4), while similar values were exhibited in
others. However, in general, the values and chromas
determined by the DRS technique were similar to
those ver ied in t he eld , althoug h the colors obt ained
differed, as shown by the illustration of colors
(Table 3); S1 was the soil that presented the most
similar colors.
Table 3. Soil(1) c ol o r ind ex e s o b ta i ne d by d i ff u s e re e ct a nc e sp e ct ro sc op y ( D RS ) a n d e ld ob se rv at io n u si ng th e Mu ns e ll Ch ar t.
Horizon DRS analysis DOR Analysis by the Munsell chart DRS color Field color
Hue Value Chroma Hue Value Chroma
S1 (LAdx-1)
A7.81YR 4.98 6.77 2.98 10YR 58
A/B 7.59YR 5.02 7.54 3.62 10YR 58
Bw18.24YR 5.38 7.37 2.41 10YR 58
Bw28.43YR 5.08 6.84 2.11 10YR 58
S2 (CXbe)
A9.66YR 4.32 3.46 0.27 2.5Y 4 3
A/B 9.21YR 4.48 3.54 0.62 2.5Y 4 1
Bi19.34YR 4.98 4.04 0.54 2.5Y 5 3
Bi29.54YR 4.95 4.32 0.40 2.5Y 4 1
S3 (LAe-1)
A8.95YR 5.03 5.41 1.13 7.5YR 6 8
A/B 8.56YR 5.35 6.24 1.68 7.5YR 6 8
Bw18.19YR 5.65 6.91 2.21 7.5YR 6 8
Bw27.76YR 5.29 6.90 2.92 7.5YR 6 8
S4 (LVe)
A5.95YR 3.77 5.55 5.96 2.5YR 48
A/B 4.13YR 4.21 7.35 10.25 2.5YR 58
Bw13.82YR 4.32 7.70 11.02 2.5YR 58
S5 (LAdx-2)
A7.89YR 4.5 5.07 2.38 10YR 3 3
A/B 8.31YR 4.19 4.80 1.94 10YR 3 3
B/A 9.24YR 4.93 4.95 0.76 10YR 3 4
Bw 9.03YR 5.34 6.62 1.20 10YR 6 8
S6 (LVAe-1)
A8.69YR 4.63 5.30 1.50 7.5YR 4 4
A/B 8.15YR 5.18 6.01 2.15 7.5YR 6 6
Bw17.03YR 5.16 7.01 4.03 5YR 56
Bw24.61YR 4.83 6.66 7.43 5YR 6 6
S7 (LAe-2)
A9.68YR 4.77 4.52 0.30 10YR 56
Bw19.27YR 5.63 6.11 0.79 10YR 58
Bw27.17YR 5.00 6.73 3.81 10YR 6 8
Bw38.16YR 5.67 7.40 2.40 10TR 6 6
(1)S1 (LAdx-1), Latossolo Amarelo distrocoeso típico (Xanthic Hapludox); S2 (CXbe), Cambissolo Háplico Tb eutróco latossólico (Eutrochrept); S3
(LAe-1), Latossolo Amarelo eutróco típico (Xanthic Eutrudox); S4 (LVe), Latossolo Vermelho eutróco chernossólico (Rhodic Eutrudox); S5 (LAdx-
2), Latossolo Amarelo distrocoeso típico (Xanthic Hapludox); S6 (LVAe-1), Latoswsolo Vermelho-Amarelo eutróco típico (Typic Eutrudox); S7 (LAe-
2), Latossolo Amarelo eutróco típico (Xanthic Eutrudox. (2)DOR, degree of redness.
930 K.L. Fernandes et al.
Pesq. agropec. bras., Brasília, v.52, n.10, p.923-932, out. 2017
DOI: 10.1590/S0100-204X2017001000012
Although determining colors in the eld and in the
laboratory involves different steps, the color obtained
by DRS can be useful in various study conditions.
Carmo et al. (2016), for example, assessed DRS-
estimated colors to distinguish areas regarding the
yield and quality of coffee (Coffea arabica L.) bean, and
Peluco et al. (2015) showed that the DOR, measured by
DRS, can assist in mapping and identifying areas with
different potential of phosphorus adsorption. It should
be noted that Hm and Gt, two mineral pigment agents
of soils (Lepsch, 2011), have different phosphorus
adsorption capacities in the soil.
The DOR was higher in S4, reecting the more
reddish colors of this soil (Table 3), but still remained
above 7 for the Bw2 horizon in S6 and above 3 for the
same horizon in S7. Even t hou gh these soils did not show
a reddish color, variation due to ferrugi nous concretions
reached 10 and 20%, respectively, on the surface of
the horizons. Campos & Demattê (2004) compared
color ratings of 80 soil samples in the eld through
observations of ve pedologists and a colorimeter.
The authors explained that there are differences in the
determination of hue that induce rating errors and also
pointed out that, on average, researchers overestimate
hue in Munsell determinations. This was observed in
the present study: in most of the analyzed soils, hue
was overestimated in the eld, when compared with
hue estimated by DRS. Aquino et al. (2016), when
studying the colors of soils in the Brazilian Amazon,
also found differences in the colors obtained by DRS
and in the eld for some of the Entisols and Ultisols
evaluated.
The obtained results show that DRS is an alternative
for minimizing errors, allowing the classication and
study of soils in a correct manner, since color is an
important attribute in the classication of soils into
suborders and, in some orders, it can also directly
indicate attributes inherent in soil genesis.
Conclusions
1. Diffuse reectance spectroscopy (DRS) is a
technique that can estimate hematite and goethite contents
in soils in the Cerrado in the state of Goiás, Brazil.
2. DRS allows the accurate characterization of hue,
value, and chroma, compared with the Munsell color
system.
3. Spectral signature can differentiate between the
diagnostic B horizons, according to the typical bands
of organic matter, iron oxides, kaolinite, and gibbsite,
indicating the stage of evolution of the studied soils.
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Received on August 25, 2016 and accepted on February 20, 2017
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