Spectral Reflectance Characteristics of Laboratory

Communications in Soil Science and Plant Analysis

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Spectral Reflectance Characteristics of LaboratoryGrown Salt Crusts on Silty Clay and Sandy Soils Luiz Guilherme Medeiros Pessoa, Maria Betânia Galvão Dos Santos Freire, Bradford Paul Wilcox, Collen Green Rossi, Anderson Mailson De Oliveira Souza & Josiclêda Domiciano Galvíncio To cite this article: Luiz Guilherme Medeiros Pessoa, Maria Betânia Galvão Dos Santos Freire, Bradford Paul Wilcox, Collen Green Rossi, Anderson Mailson De Oliveira Souza & Josiclêda Domiciano Galvíncio (2015) Spectral Reflectance Characteristics of Laboratory-Grown Salt Crusts on Silty Clay and Sandy Soils, Communications in Soil Science and Plant Analysis, 46:15, 1895-1904, DOI: 10.1080/00103624.2015.1059849 To link to this article: http://dx.doi.org/10.1080/00103624.2015.1059849

Accepted online: 07 Jul 2015.Published online: 07 Jul 2015.

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Date: 10 September 2015, At: 08:48

Communications in Soil Science and Plant Analysis, 46:1895–1904, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 0010-3624 print / 1532-2416 online DOI: 10.1080/00103624.2015.1059849

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Spectral Reflectance Characteristics of Laboratory-Grown Salt Crusts on Silty Clay and Sandy Soils LUIZ GUILHERME MEDEIROS PESSOA,1 MARIA BETÂNIA GALVÃO DOS SANTOS FREIRE,1 BRADFORD PAUL WILCOX,2 COLLEN GREEN ROSSI,3 ANDERSON MAILSON DE OLIVEIRA SOUZA,1 AND JOSICLÊDA DOMICIANO GALVÍNCIO4 1

Department of Agronomy, Universidade Federal Rural de Pernambuco, Recife, Pernambuco, Brazil 2 US Department of the Interior, Bureau of Land Management, National Operations Center, Salt Lake City, Utah, USA 3 Department of Ecosystem Science and Management, Texas A&M University, College Station, Texas, USA 4 Department of Geography, Universidade Federal de Pernambuco, Recife, Pernambuco, Brazil Salinization of soils has led to the loss of cropland and represents a major threat to food production. Hyperspectral imaging may prove to be useful for characterizing the spectral behavior of salt-affected soils but the methodology needs to be better evaluated. In this study, we characterized the spectral behaviors of four types of chloride salt crusts [calcium chloride dehydrate, magnesium chloride dehydrate, potassium chloride, and sodium chloride (CaCl2·2H2O, KCl, and NaCl)] formed in the laboratory. We found that (1) as salt concentration increased, the reflectance intensity decreased for both soil types, and the decreases were especially pronounced for the soils leached with the CaCl2·2H2O and MgCl2·2H2O solutions; (2) soil texture had little if any effect on reflectance; and (3) reflectance intensity decreased in the order CaCl2·2H2O < MgCl2·2H2O < KCl < NaCl. By clarifying the spectral behavior of chloride salt crusts on soils, our work demonstrates hyperspectral imaging may differentiate some types of salts and determine relative salt concentrations. Keywords Degraded soils, hyperspectral, remote sensing, saline soils, salt identification, salt minerals

Received 9 June 2014; accepted 6 April 2015 Address correspondence to Bradford Paul Wilcox, US Department of the Interior, Bureau of Land Management, National Operations Center, Salt Lake City, Utah, USA. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www. tandfonline.com/lcss.

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L. G. M. Pessoa et al.


Introduction The loss of productive agricultural soils because of human-induced soil salinization represents a major threat to both regional and global food security. The problem is especially acute in semi-arid and arid zones where irrigation is practiced, and its economic toll is enormous. For example, conservative estimates of monetary losses due to soil salinity amount to US$750 million/yr in the Colorado Basin (USA), more than US$300 million/yr in the Punjab province of Pakistan, and more than US$200 million/yr in the Murray-Darling Basin in Australia (Metternicht and Zinck 2003). Estimates of the land area affected by human-induced soil salinity vary—reflecting the challenge of making such estimates—from 45 to 75 million ha. It is likewise uncertain how much land is newly affected by soil salinity each year; estimates range from 200,000 to 500,000 ha (Metternicht and Zinck 2008a; Shoshany, Goldshleger, and Chudnovsky 2013). Although remote sensing technology has been widely used to assess and characterize salt-affected soils at large scales (Farifteh, Farshad, and George 2006; Evans and Caccetta 2000; Sharma and Bhargava 1988), improved methodologies are urgently needed—particularly technologies that enable the type of salt and the concentration to be determined (Farifteh et al. 2007). New developments such as imaging spectrometry offer this potential (Ben-Dor et al. 2008; Shoshany, Goldshleger, and Chudnovsky 2013; Metternicht and Zinck 2003). The methodology has been used to characterize soil salinity with some success in a number of locations. With respect to characterizing soil salinity, several spectral-reflectancebased studies have highlighted the potential of the methodology(Metternicht and Zinck 1997; Ben-Dor et al. 2002), although significant challenges remain (Ben-Dor 2002). Remote sensing of salt-affected soils is influenced by the types of salts contained in them, because the spectral reflectance of these soils varies according to salt mineralogy (Metternicht and Zinck 2008b). The spectral properties of common salt types have been well studied. However, as noted by Metternicht and Zinck (2008b), these properties are quite different from those of salt crusts developed on soil, which are altered by other soil properties. Determining the spectral properties of salt crusts formed on actual soils provides critical information for the effective use of remote-sensing technology (Farifteh et al. 2007). Chloride salts are among the most common found in saline soils, but the spectral properties of different chloride salts have not been well characterized, nor has the influence of salt concentration or soil texture on spectral properties been evaluated. The objective of this study was to determine whether the spectral properties of four different chloride salts [calcium chloride dehydrate, magnesium chloride dehydrate, potassium chloride, and sodium chloride (CaCl2·2H2O, MgCl2·2H2O, KCl, and NaCl)] formed on soils are affected by salt type, salt concentration, and soil texture.

Materials and Methods Collection and Preparation of the Soil Samples Soil samples were collected from two nondegraded soils—a sandy soil and a silty clay soil, both from Serra Talhada County, a semi-arid region in northeastern Brazil. The soil samples were collected from the surface layer (0–20 cm), air dried, crushed, and passed through a 2-mm-mesh sieve. The chemical and physical characterization was carried out using standard practices (United States Salinity Laboratory 1954; MacDowell 1997) (Tables 1 and 2).

Spectral Reflectance of Salt Crusts

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Table 1 Physical attributes of soil samples

Soil type

Sand Silt Clay (%) (%) (%)

Clay flocculation (%)

Sandy Silty clay

87.4 07.7 04.9 19.2 42.0 38.8

42.9 25.4

Clay Soil Bulk dispersion density (%) (g cm−3) 57.1 74.6

1.5 1.2

Soil Particle density (g cm−3)

Total porosity (%)

2.7 2.7

44.7 55.0


Experiment Setup and Procedures A total of seventy-two samples of air-dried soil, each weighing 150 g, were packed in funnels with filter paper for leaching with solutions of the chloride salts CaCl2·2H2O, MgCl2·2H2O, KCl, and NaCl. A total of twelve solutions were prepared, with three concentrations per salt: 0.5, 1, and 2 mol L−1. After being packed in the funnels, each soil sample was leached with a saline solution equivalent to five times the pore volume of its particular soil type. After leaching, the soil samples were air dried, crushed, and sieved (2-mm mesh). Two 100-g samples of each soil type, leached with each of the four salts at each of the three concentrations, were then placed in 13.5-cm-diameter, acrylic Petri dishes and subjected to further leaching with a quantity of saline solution equivalent to the pore volume of each soil type. Subsequently, Table 2 Chemical attributes of soil samples Soil Type Attribute

Sandy

Silty clay

Soil exchangeable cations pH water (1:2,5) Ca2+ (cmolc dm−3) Mg2+ (cmolc dm−3) Na+ (cmolc dm−3) K+ (cmolc dm−3) Cation exchange capacity (cmolc dm−3) Exchangeable sodium Percentage (%)

7.5 5.0 1.5 0.1 0.3 8.6 0.9

07.10 08.5 03.2 00.30 00.5 15.9 0 1.9

Saturation paste extract pH Electrical conductivity (dS m−1) Ca2+ (mmolc L−1) Mg2+ (mmolc L−1) Na+ (mmolc L−1) K+ (mmolc L−1) Sodium adsorption ratio (mmolc L−1)°,5

7.3 0.8 0.50 0.6 2.0 4.9 2.6

7.40 0.9 5.1 3.70 2.5 0.6 1.2

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the leached soils were put into a forced-air-circulation oven at 65 °C for 72 h to promote the formation of soil crusts.


Hyperspectral Analysis The hyperspectral sensor FieldSpec Spectroradiometer with fiber optic cable was used to collect the spectral data. This instrument covers the spectral range 450–2,500 nm with a spectral resolution of 1 nm and the range 1100–2500 nm with a spectral resolution of 2 nm. The spectral readings were carried out by placing the soil samples onto standard white plates having 100 percent reflectance. The ratio of the spectral radiant flux reflected by a sample to the radiant flux reflected by the reference material generates the bidirectional spectral reflectance factor, from which a fitted reflectance curve is obtained (Nicodemus et al. 1977). Six readings were performed on each sample; the resultant curves were then averaged to obtain a single curve for each sample. A mean curve was drawn after three repetitions, generating a single curve for each saline treatment per salt. Statistical Analysis For determining soil salinity status, key spectral ranges in the visible (550–770 nm), nearinfrared (900–1030 and 1270–1520 nm), and middle infrared (1940–2150, 2150–2310,

Figure 1. Comparison of spectral curves for different soil textures following leaching with salts of CaCl2·2H2O, MgCl2·2H2O, KCl, and NaCl.


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and 2330–2400 nm) regions were identified in accordance with recommendations by Csillag, Pasztor, and Biehl (1993). Spectral properties were used to compare (1) different salt concentrations in each soil type leached with the same saline solution; (2) different salt types at the same concentration in both soils; and (3) crusts developed on soils of different texture. The absorption features, shapes, and intensities of reflectance of each spectral curve were also analyzed.


Results and Discussion Soil texture had virtually no influence on spectral reflectance in the case of the CaCl2·2H2O and MgCl2·2H2O crusts, and only a slight influence in the case of the KCl and NaCl crusts (Figure 1). We found that for both soil types, spectral reflectance declined as the salt concentration increased (Figure 2, Table 3). This decrease was more pronounced for the CaCl2·2H2O and MgCl2·2H2O crusts than for the NaCl and KCl crusts. The absorption features that correspond to the presence of water molecules (1400 and 1900 nm), while observed in the spectral patterns of all the crusts, were more intense for the CaCl2·2H2O and MgCl2·2H2O crusts—probably because of the greater hygroscopicity of the calcium and magnesium salts (Lindberg and Snyder 1972). All the crusts showed low reflectance in the visible region and near-infrared region, the result of strong absorption due to the presence of iron at wavelengths shorter than 540 nm (Ben-Dor 2002). Wang et al. (2013)

Figure 2. Comparison of the influence of salt concentration on spectral curves of the sandy soil crusts leached with saline solutions of (a) CaCl2·2H2O, (b) MgCl2·2H2O, (c) NaCl, and (d) KCl.

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Table 3 Average reflectance levels of soils leached with three concentrations of saline solution, measured in selected spectral bands Selected spectral band (nm)


Salt concentration (mol L−1)

0.5 1.0 2.0 Mean Max Min Std 0.5 1.0 2.0 Mean Max Min Std 0.5 1.0 2.0 Mean Max Min Std 0.5 1.0 2.0 Mean Max Min Std

0.5 1.0 2.0 Mean

550– 770

900– 1030

Sandy soil CaCl2·2H2O 0.16 0.28 0.10 0.19 0.07 0.13 0.11 0.20 0.18 0.30 0.07 0.12 0.04 0.07 MgCl2·2H2O 0.13 0.24 0.13 0.23 0.09 0.15 0.12 0.21 0.17 0.30 0.08 0.14 0.03 0.06 NaCl 0.29 0.44 0.29 0.42 0.29 0.41 0.29 0.42 0.31 0.44 0.27 0.38 0.01 0.02 KCl 0.27 0.41 0.26 0.39 0.25 0.37 0.26 0.39 0.27 0.42 0.24 0.37 0.01 0.02 Silty clay soil CaCl2·2H2O 0.15 0.28 0.11 0.20 0.07 0.12 0.11 0.20

1270– 1520

1940– 2150

2150– 2310

2330– 2400

0.35 0.23 0.13 0.24 0.37 0.12 0.10

0.28 0.15 0.05 0.16 0.30 0.05 0.10

0.34 0.20 0.08 0.21 0.36 0.08 0.11

0.29 0.16 0.06 0.17 0.31 0.06 0.10

0.29 0.26 0.15 0.23 0.37 0.14 0.08

0.20 0.16 0.07 0.14 0.29 0.06 0.08

0.26 0.22 0.10 0.19 0.36 0.09 0.09

0.20 0.17 0.07 0.15 0.30 0.06 0.08

0.55 0.53 0.51 0.53 0.56 0.48 0.03

0.61 0.58 0.56 0.59 0.61 0.53 0.03

0.58 0.56 0.54 0.56 0.59 0.51 0.03

0.56 0.53 0.51 0.54 0.56 0.48 0.03

0.53 0.50 0.47 0.50 0.54 0.47 0.03

0.60 0.56 0.53 0.56 0.60 0.52 0.03

0.57 0.53 0.50 0.54 0.57 0.50 0.03

0.54 0.51 0.48 0.51 0.55 0.47 0.03

0.37 0.22 0.10 0.23

0.31 0.13 0.03 0.16

0.35 0.18 0.05 0.20

0.30 0.14 0.03 0.16 (Continued )


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Table 3 (Continued)


Selected spectral band (nm) Salt concentration (mol L−1)

550– 770

Max Min Std

0.16 0.30 0.06 0.11 0.03 0.07 MgCl2·2H2O 0.14 0.28 0.11 0.22 0.10 0.15 0.12 0.22 0.16 0.29 0.07 0.12 0.03 0.06 NaCl 0.23 0.41 0.28 0.41 0.29 0.42 0.26 0.41 0.30 0.43 0.15 0.28 0.05 0.04 KCl 0.24 0.40 0.22 0.36 0.25 0.37 0.24 0.38 0.27 0.41 0.21 0.35 0.02 0.02

0.5 1.0 2.0 Mean Max Min Std 0.5 1.0 2.0 Mean Max Min Std 0.5 1.0 2.0 Mean Max Min Std

900– 1030

1270– 1520

1940– 2150

2150– 2310

2330– 2400

0.39 0.10 0.12

0.33 0.03 0.12

0.37 0.05 0.13

0.32 0.03 0.12

0.36 0.27 0.13 0.26 0.37 0.11 0.10

0.29 0.18 0.05 0.18 0.30 0.04 0.10

0.34 0.24 0.08 0.22 0.35 0.07 0.11

0.28 0.19 0.06 0.18 0.30 0.05 0.10

0.52 0.51 0.51 0.51 0.52 0.37 0.05

0.56 0.53 0.53 0.54 0.56 0.29 0.09

0.54 0.51 0.51 0.52 0.54 0.34 0.06

0.51 0.49 0.49 0.49 0.51 0.28 0.07

0.52 0.48 0.47 0.49 0.53 0.44 0.03

0.57 0.53 0.50 0.54 0.59 0.47 0.04

0.54 0.50 0.48 0.51 0.56 0.45 0.03

0.52 0.47 0.45 0.48 0.54 0.43 0.03

report that soil concentration had no influence on spectra reflectance for sodium salts (NaCl, Na2SO4, and Na2CO3) and on that basis concluded that salt concentrations have little effect on spectral reflectance. Similarly we found that for sodium chloride, concentration had little effect; however, for other salts (MgCl2·2H2O and CaCl2·2H2O) there were strong differences with concentration. Moreira, Teixeira, and Galvão (2014) found that for MgCl2 reflectance declined with concentration; however, for NaCl they found that reflectance increased with concentration. Their results for NaCl differ from our results as well as those of Wang et al. (2013). The absorption bands for the soils leached with the NaCl and KCl solutions showed smaller concavities (narrower and less deep) than those for the soils leached with CaCl2·2H2O and MgCl2·2H2O. These findings agree with those of others, showing that

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the depth of absorption features increases with increasing salt concentration (Howari, Goodell, and Miyamoto 2002; Farifteh et al. 2008). Differences in the spectral properties of the different salt crusts are displayed in Figure 3. For both soils, the crusts formed after leaching with NaCl and KCl had significantly more intense reflectances than those formed after leaching with CaCl2·2H2O and MgCl2·2H2O—differences attributable to the lower hygroscopicity of the NaCl and KCl salts.

Figure 3. Comparison of the influence of salt type on spectral curves at three different concentrations, for leaching of the two soil types.


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Conclusions


In summary, in this study we have examined the influence of salt type, salt concentration, and soil texture on the spectral reflectance of saline soils. To our knowledge, only a few other studies have evaluated the spectral properties of soil crusts formed in a laboratory setting on soils. The advantage, of course, is that the salt type and concentration of the soil crust can be controlled. We have found that (1) there are distinct differences according to salt type, with the hygroscopic salts (MgCl2·2H2O and CaCl2·2H2O) having a greater reflectance than the nonhygroscopic salts (NaCl and KCl); (2) salt concentration has a strong effect on spectral reflectance, particularly for the hygroscopic salts; and (3) soil texture has relatively little effect on the spectral reflectance of soil crusts. These findings make an important contribution to our knowledge base with respect to remote sensing, improving our ability to use this technology for the characterization of soil salinity.

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