# Soil Moisture Influence on the FTIR Spectrum of Salt-Affected Soils
## 2 Materials and Methods
### Laboratory Experimental Design
#### 2.2.3 Soil Spectral Measurements and Analysis  
Field soil samples and laboratory soil samples were measured spectroscopically with a diamond-faced probe. These samples were pressed directly on the instrument Agilent Cary 630 FTIR of Agilent Technologies. Data processing was achieved with the MicroLab PC spectrometer software and Agilent Resolution spectrometer software.  
The soil moisture monitoring and spectroscopy measurements were conducted during five sampling days:  
* Sampling day 1: The soil samples were saturated with water. The soil moisture and FTIR spectrum were measured;
* Sampling days 2-4: The soil samples were left to air-dry. The soil moisture and FTIR spectrum were measured. The duration from 1 st to 2 nd day, 2 nd to 3 rd and 3 rd to 4 th was one day and night;
* Sampling days 5-6: The soil samples were dried at 105 \({}^{\circ}\)C until constant weight. The soil moisture, FTIR spectrum and electrical conductivity were measured. The duration from the 4 th to 5 th day was 2 days and nights.  
The database is hosted on a website with a general introduction on the determination of salinity level of surface soil for agricultural cultivation in some areas near the coastal estuaries of Hai Phong-Thai Binh. Alluvial and sandy soils were measured in the laboratory and some spectral samples were measured in the field. The spectral data provide information on the FTIR spectroscopy measured in the laboratory samples. Transmittance measurements, i.e., the percentage of incident light which is transmitted, were converted to absorbance using Equation (1). Conversion of wavenumbers into wavelength, commensurate with satellite sensor specifications, was accomplished using Equation (2).  
\[A=2-\log_{10}(\%T) \tag{1}\]  
\[WL=10^{7}\frac{1}{WN} \tag{2}\]  
where %T is transmittance in percentage, \(A\) is absorbance, \(WN\) is wavenumber (cm\({}^{-1}\)) and \(WL\) is wavelength (nm).  
The database includes 280 FTIR spectral samples measured in the laboratory of a Fluvisol (TL2) and Arenosol (KT2) contaminated with: NaCl, Na\({}_{2}\)SO\({}_{4}\), Na\({}_{2}\)CO\({}_{3}\) or a mixture of: NaCl:Na\({}_{2}\)SO\({}_{4}\) in 1:1 proportion; NaCl:Na\({}_{2}\)SO\({}_{4}\) in the proportion of 1:5; NaCl:Na\({}_{2}\)SO\({}_{4}\) in the proportion 1:10; NaCl:Na\({}_{2}\)SO\({}_{4}\):Na\({}_{2}\)CO\({}_{3}\) in the proportion of 1:1:1 with four increasing salinity levels. The samples were monitored and measured at five different moisture levels. The electrical conductivity (EC\({}_{1:5}\)) was determined in a 1:5 (\(w/v\)) soil to water extract after the salts were added. The experiments were repeated three times, the results presented are the averages; the coefficient of variation was lower than 1.3%. A two-way analysis of variance was conducted in R [52] to compare the main effects of soil, salinity level and sampling time and the interaction effect of soil, salinity level and sampling time on the moisture content. Tests for normality were performed using the R packages nortest [53] and rstatic [54], while ggpubr [55] was used for preparing factorial plots.