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Journal of Soils and Sediments

, Volume 18, Issue 8, pp 2863–2867 | Cite as

Quantitative and qualitative characterisation of humic products with spectral parameters

  • Ekaterina Filcheva
  • Mariana Hristova
  • Pavlina Nikolova
  • Todorka Popova
  • Konstantin Chakalov
  • Valentin Savov
Humic Substances in the Environment

Abstract

Purpose

The application of different humic products for the treatment of soils and plants has increased in recent years. The characteristics of humic products, such as the content and composition of organic carbon and the maturity, provide valuable information which is essential for an adequate application. Such information is crucial for manufacturers, business consultants and users involved in the production, distribution and implementation of humic products. This article presents the correlation between the quantitative indicators of commercial humic products and their spectral characteristics via measurements in the ultraviolet spectrum at 300 nm, in the visible area at 445 and 665 nm and in the near-infrared spectrum at 850 nm.

Materials and methods

We evaluated humic products (liquid and solid) of different origins. Via wet combustion, the content of total organic carbon in humic products can be determined. The precipitation of humic acids from the starting solution determines the composition of the humic products in terms of humic acids (HAs) and fulvic acids (FAs). The dissolution of HAs determines their concentration by titration, while the specific extinction can be assessed via spectrophotometry via measuring the absorption of HAs spectra at the following wavelengths: 300, 465, 665 and 850 nm. The degree of aromaticity and condensation of humic products determines the optical density of the HAs via the E4/E6 ratio.

Results and discussion

The content of total organic carbon varied widely from 0.55 to 37.5% across all groups. The content of carbon in HAs, as a percentage of the total carbon in fulvic-type humic products, ranged from 1.29 to 16.00%, while in humic-type products, it ranged from 51.43 to 91.92%. The minimum value of the E4/E6 ratio was 2.97, while the maximum value was 6.35. We observed a direct relationship between the dominant type of acids in humic products and the E4/E6 ratio.

Conclusions

The optical density of HAs indicates their quality characteristics. The presented optical characteristics for humic products show that there is a direct relationship, especially between HAs/FAs and E4/E6 ratios. Measurement at 300 nm (E300) in the near-ultraviolet area and at 850 nm (E850) in the near-infrared area can increase the range of the spectral study.

Keywords

E300 E850 Fulvic acids Humic acids Humic products Optical characteristics Ratio  E465/E665 

1 Introduction

The application of different humic products for the treatment of soils and plants has increased in recent years. The content and composition of organic matter and the maturity of humic products provide valuable information, which is essential for their application. Such information is crucial for manufacturers, business consultants and users involved in the production, distribution and implementation of humic products.

Humic products play an essential role in soil. They are a source of essential nutrients, improve the soil structure and water-air properties and increase the soil’s buffer capacity (Guggenberger 2005; Schindler et al. 2007). Dissolved organic carbon (DOC) is the most mobile part of the organic substances and includes carbohydrates, proteins, hydrocarbons and other organic compounds—soluble in water or saline solutions with a neutral reaction. From an ecological point of view, the soluble humic substance fraction of soil organic matter is a considerable source of nutrients and energy (Haynes 2005; Gonet and Dębska 2006; Kondratowicz-Maciejewska 2007; Voroney et al. 2008).

Rapid and easy estimation studies are useful to assess the properties of humic products. Such studies consist of spectrophotometric measurements of the extinction of the HAs at 465 nm (E4) and 665 nm (E6) and of the determination of the E4/E6 ratio (Kondratowicz-Maciejewska 2007; Jing-an et al. 2007; Kalembasa and Becher 2009; Kondratowicz-Maciejewska et al. 2011), which can be used to assess the maturity of HAs (Polak et al. 2011).

The evaluation of HAs in the E4/E6 ratio is frequently being used to assess the degree of condensation and the aromaticity of the carbon structure, the carbon content and the molecular weight of humic products (Blondeau 1986; Song et al. 2014). The E4/E6 ratio is mainly related to the molecular weight and total acidity and correlates with the concentrations of oxygen, carbon and CO2; however, it is not directly related to fused aromatic rings (Chen et al. 1977). Some authors used the well-known correlation between the destructive analyses that prove structural formula of the type (CxHyOzNn) and spectral methods and evaluated the degree of condensation and aromaticity via the E4/E6 ratio. Such methods consist of ultraviolet-visible spectroscopy (UV-VIS), infrared spectroscopy with the Fourier transform method (FTIR), Carbon-13 nuclear magnetic resonance (13C NMR) and fluorescence spectroscopy, which reveal the essential characteristics of the humic products (Canellas and Façanha 2004; Enev et al. 2014). Other studies have reported that HAs isolated from lignite show typical streaks due to the aromatic C-O structure (Pospíšilová and Fasurová 2009). The order in which these HAs reduce condensation and aromaticity in soils and substrates is as follows: HAs from lignite > HAs in Haplic Chernozem > HAs in Eutric Gleysols > HAs in Haplic Luvisols > HAs in Gleyic Stagnosols > HAs in Eutric Cambisols. The authors assume that the HAs isolated from lignite, and those of Haplic Chernozem have the highest degree of aromaticity (Pospíšilová and Fasurová 2009). These studies also used spectroscopic analysis and the HAs/FAs ratio in the visible region to assess the degree of humification in organic matter during composting (index of maturity of compost). Spectroscopy in the UV and NIR ranges is a useful approach for monitoring the composting process, but the projections of near-infrared spectroscopy (NIRS) yielded better results than the precision of UV calibration (Albrecht et al. 2011). Other researchers have shown a weak correlation between the HAs/FAs ratio with the UV index (Domeizel et al. 2004). Studies on the optical properties of HAs, which prove the relationship between the degree of condensation and aromaticity, overlap according to different theories. One of these theories describes the concept of a donor-acceptor bond, which occurs in all natural hydroxy- or polyhydroxy-aromatic polymers that form suitable acceptors under partial oxidation; lignin, polyphenols, tannins and melanins are some examples (Del Vecchio and Blough 2004).

In the present study, as in our previous ones, we are dealing with humic products of different origins and ways of production from leonardite, lignite biotransformed with Trichoderma sp. (Plantagra), plant materials after pyrolysis and composting. The basis of our research is represented by standard quantitative methods for determining the contents of organic matter as well as HAs and FAs within organic matter. Our previous studies proved that there is a good correlation between the composition of the humic products as a mixture of humic acids (HAs) and fulvic acids (FAs) and their optical characteristics of spectral measurements at wavelengths of 300, 445, 665 and 850 nm (Filcheva et al. 2017).

This article presents the correlation between the quantitative indicators of commercial humic products and their spectral characteristics by measurements in the visible area at 300 nm, in the visible area at 445 and 665 nm and in the near-infrared area at 850 nm.

2 Materials and methods

This study describes humic products with different origins. We evaluated 14 humic commercial products used as soil improvers and leaf feeders. For all humic products, we determined the concentration of organic carbon via the modified Turin’s method (Kononova 1966; Filcheva and Tsadilas 2002; Filcheva 2015). The method consists of dichromate digestion at 125 °C for 45 min in the presence of Ag2SO4 and (NH4)2SO4FeSO46H2O titration, using phenyl anthranilic acid as an indicator. The humic products can be partitioned into two groups: liquid and solid (Table 1). The methodology is also applied on this basis. First, we prepared a starting solution by dissolving the solid products in water and bringing the liquid products to the same volume. The carbon content in the HAs was determined by precipitation (with the addition of H2SO4) from the starting solution. Then, the HAs were dissolved in hot NaOH, and their concentrations were determined via titration. The concentration of FAs was determined after precipitation of HAs and subtracted from the total carbon content. The ratio HAs/FAs indicates a predomination of the fraction in the composition of the commercial humic products. Values below 1 indicate that the dominant fraction is that of the FAs, while values above 1 indicate that the HAs represent the dominant fraction.
Table 1

Humic products of different origin with the main determined parameters

Number

Type of humic product

TOC

g/l

HAs

g/l

FAs

g/l

HAs/FAs

E4/E6

E300

cm−1

E850

cm−1

1S

Agros (Leonardite)

147

84.9 a

57.76b

62.1 a

42.24

1.37

4.50

0.73

0.03

2 L

Biohumas

101

1.30 a

1.29

99.7 a

98.71

0.01

6.35

2.34

0.04

3 L

Bioprantagra

246

199 a

80.89

47.0 a

19.11

4.23

4.14

2.34

0.03

4S

Humates potassium

247

220 a

89.07

27.0 a

10.93

8.15

4.16

0.64

0.02

5S

Humates natrium

357

212 a

59.38

145 a

40.62

1.46

3.43

0.63

0.02

6 L

Humic products

5.00

0.80 a

16.00

42.0 a

84.00

0.19

5.72

2.42

0.04

7 L

Humic acid BB

76.6

3.80 a

4.96

72.8 a

95.04

0.05

5.65

2.15

0.05

8 L

HAs

7.00

3.60 a

51.43

3.40 a

48.57

1.06

4.23

0.74

0.02

9S

HAs (Leonardite)

280

234 a

83.57

46.0 a

16.43

5.09

4.37

2.25

0.05

10 L

HAs

6.20

5.40 a

87.10

0.80 a

12.90

6.75

3.99

1.90

0.03

11S

Humic products

303

211 a

69.64

92.0 a

30.36

2.29

3.98

1.40

0.04

12S

Humintech

297

273 a

91.92

24.0 a

8.08

11.38

3.58

2.50

0.08

13S

Lumbri 21

140

125 a

89.29

15.0 a

10.71

8.33

2.97

2.50

0.07

14 L

Lumbri bio

185

2.20 a

1.19

183 a

98.81

0.01

5.45

2.48

0.05

S solid humic products TOC [g/kg], L liquid humic products TOC [g/l]

aValues in Italic g/l%

b% of total organic carbon

Absorbance was only measured for HAs as the chromophore groups are contained in the alkaline solution of HAs, while FAs are soluble in acid or water solutions. Spectrophotometric analysis at different wavelengths estimates the extinction due to the absorption of light from the humic substances in the solutions. Wavelengths 300, 465, 665 and 850 nm recorded the extinction of the HAs adsorption spectra (cm−1). The extinction depends on the dissolved organic carbon concentration; however, occasionally, the samples must be diluted hence the reading comes to the measuring scale. An index of the optical density of HAs, their degree of aromaticity and condensation of humic substances is the ratio E4/E6. In the HAs spectra, the wavelengths of 300 and 850 nm are indications of maximum and minimum extinction of each studied product and individual concentration (cm−1). The E300 and E850 parameters extend the scope of this study to the near-ultraviolet and infrared areas. The accuracy of the method of carbon determination is approximately 5% (Flicheva 2015).

3 Results and discussion

Table 1 shows the types of humic products, the content of total organic carbon, the content of carbon in HAs and FAs, the ratio HAs/FAs and the ratio E4/E6, E300 and E850. The presented humic products are sorted in alphabetic order. All commercial humic products are marked as solid humic products (S) and liquid humic products (L). The investigated products are divided into two groups by the ratio HAs/FAs. The first group includes HAs/FAs ratios below 1, indicating that FAs dominate (2 L, 6 L, 7 L, 14 L). The second group includes ratios above 1, indicating that HAs dominate (1S, 3 L, 4S, 5S, 6 L, 7 L, 8 L, 9S, 10 L, 11S, 12S, 13S, 14 L). The content of total organic carbon widely varied from 5.50 to 375 g/l in both groups. The content of carbon in HAs, as a percentage of the total carbon in fulvic type products, accounted for 1.29 to 16.00%, while in the humic types, it represented 51.43 to 91.92%. The ratio E4/E6 had a minimum value of 2.97 (13S) and a maximum value of 6.35 (6 L) (Table 1). Absorption at 465 nm reflects the organic material at the beginning of humification, while absorption at 665 nm indicates a high degree of condensation of aromatic constituents (Albrecht et al. 2011).

There is a direct relationship between the groups with prevailing HAs or FAs in the humic products and the E4/E6 ratio. When FAs were dominant, the E4/E6 ratio ranged from 5.45 to 6.35, i.e. for the humic products Limbi bio (14 L), with a high quality, and Biohumas (2 L), with the lowest degree of condensation. The E4/E6 ratio ranged from 2.95 to 4.50 when HAs were dominant. The highest degree of condensation was found in the preparation of Lumbi 21 (13S), while the humic product Agros (1S) showed the lowest degree of condensation. Results published by other authors also prove that E4/E6 ratios of HAs of around 4.04 indicate high quality. In the case of FAs domination, the E4/E6 ratio reaches values of up to 6.17; when HAs dominate, the value is 4.34 (Eshwar et al. 2017). This ratio is independent of the concentrations of HAs and FAs, but varies in humic substances extracted from different soil types (Tahiri et al. 2016). A systematic study on the optimisation of the isolation of HAs in forest soils found E4/E6 ratios in the range from 5.84 to 6.60 (Amran et al. 2017).

Based on our previous studies, we present data on the extinction of HAs at different wavelengths. For example, we introduce a wavelength of 300 nm established as the maximum wavelength (Filcheva et al. 2017). Our research is in agreement with other studies which reported that spectra of HAs and FAs are similar, but the extinction values decreased with increasing wavelengths (Eshwar et al. 2017). The maximum extinction (E300) values at 300 nm covered an extensive range of 0.63 (5S) to 2.50 (12S) in the second group (HAs). In the other group (FAs), the E300 values did not vary significantly and ranged from 2.1 (7 L) to 2.5 (14 L). The values of the minimum extinction at 850 nm (E850) showed a similar variance. Based on these results, E300 and E850 are more stable indicators for humic products when FAs dominate.

Our study found no significant correlation between E300 and E850 (R2 = 0.5717). Similarly, a comparative study took into account the relationship between the UV absorbance of nine different dissolved humic products; the wavelength was 272 nm and aromaticity was confirmed by 13C NMR. The percentage of aromatic carbon, determined by NMR, highly correlated (r = 0.94) with UV absorption. The authors therefore assumed that UV absorbance at 272 nm could be used as a quantitative assessment of the content of aromatic carbon in dissolved HAs (Traina et al. 1990). Other studies have shown that the E2/E3 ratio (254 and 365 nm) is more accurate in terms of determining the aromatic structure of HAs (Sparks et al. 1996; Weishaar et al. 2003; Peacock et al. 2014). In a different study, the authors measured in the visible and near-infrared area and showed decreased reflectance with increasing soil organic carbon contents. They assumed that this is partially due to the higher light absorption in addition to the adsorption by C bonds (Debaene et al. 2017).

4 Conclusions

The optical density of HAs indicates their quality characteristics. The presented optical characteristics for humic products show that there is a direct relationship, especially between HAs/FAs and E4/E6 ratios. Measurement at 300 nm (E300) in the near-ultraviolet region and analysis at 850 nm (E850) in the near-infrared region can increase the range of the spectral study.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Ekaterina Filcheva
    • 1
  • Mariana Hristova
    • 1
  • Pavlina Nikolova
    • 1
  • Todorka Popova
    • 2
  • Konstantin Chakalov
    • 2
  • Valentin Savov
    • 3
  1. 1.N. Poushkarov Institute of Soil ScienceAgrotechnology and Plant ProtectionSofiaBulgaria
  2. 2.Balkan Plant Science Ltd.SofiaBulgaria
  3. 3.Sofia UniversitySofiaBulgaria

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