# Soil Moisture Influence on the FTIR Spectrum of Salt-Affected Soils
## 4 Discussion  
The experiment was conducted on two soil types, Folic Fluvisol and Fluvic Arenosol, which are the two dominant soil types in the coastal and estuarine areas in Vietnam, such as the Red River delta. Arenosols are among the most extensive soils in the world, covering about 900 million hectares or seven percent of the land surface [14]. In the humid tropics, Arenosols are either young soils in coarsely textured alluvial, lacustrine or aeolian deposits, or they are very old soils in residual acid rock weathering that lost all primary minerals other than (coarse grained) quartz in the course of an impressive pedogenetic history [56].  
The spectral features of the salts commonly found in soils are different from the spectral signals from soils contaminated with these salts, particularly for mono-salts. Mono-salts that commonly occur in soils include halite (NaCI), thenardite (Na\({}_{2}\)SO\({}_{4}\)), nahcolite (NaHCO\({}_{3}\)), calcium carbonate (CaCO\({}_{3}\)) and gypsum (CaSO\({}_{4}\).2H\({}_{2}\)O). The spectral absorbance of Na\({}_{2}\)SO\({}_{4}\) g is similar to gypsum at 1.978 \(\upmu\)m, making it difficult to distinguish between them [32]. Absorbances at 1.4, 1.9 and 2.25 \(\upmu\)m were attributed to moisture [30; 31; 32; 33; 39; 44]. Dissolved salts dominate the spectrum, e.g., in soils with NaHCO\({}_{3}\) and CaCO\({}_{3}\), NaHCO\({}_{3}\) dominates the spectral signature with absorbances at 1.334, 1.472, and 1.997 \(\upmu\)m [32; 44; 57].  
In our experiments, no specific spectral absorbance region was found for NaCl and Na\({}_{2}\)SO\({}_{4}\) contaminated soil in the wavenumbers ranging from 650 to 4500 cm\({}^{-1}\) (2.2-15.4 \(\upmu\)m), but a clear absorbance for Na\({}_{2}\)CO\({}_{3}\) contaminated soils was found. The strength of the absorbance was related to the salt content in the soil. The depth of spectral absorbances was different when both the soil moisture and salt concentration changed, which was confirmed by [32; 33]. In general, the spectral absorbances of salts are influenced by water molecules [28; 44].  
Besides the influence of moisture and soluble salts, absorbance spectra are also influenced by the soil's physico-chemical characteristics, such as clay and organic matter content [58; 59]. We found that the Fluvisol had stronger absorbances than the Arenosol. The relationship between the spectral absorbance and the added mass of Na\({}_{2}\)CO\({}_{3}\) (in g) is better (R\({}^{2}\) = 0.85) than with the electrical conductivity (R\({}^{2}\) = 0.50), which can be explained by the physico-chemical soil characteristics and the dynamics in the soil solution [59]. There is a need for physico-chemical analysis of initial and altered salt compositions following the addition of dry salt mixtures to the soil, to elucidate the soil's salinity dynamics. Further research will be needed to establish whether the accuracy of salt prediction is influenced by soil colloid characteristics in the spectral range of 2.2 to 15.4 \(\upmu\)m.  
Figure 13: Relationship between soil moisture and spectral absorbance at wavenumber 1630 cm\({}^{-1}\) (wavelength 6.13 \(\upmu\)m).  
Most research has focused on the remote sensing detection of soil salinity in the visible spectrum and near- to shortwave infrared [60; 61; 11; 62]. Current satellite sensors offering data in the spectra beyond the near-infrared spectrum have ground sampling distances ranging from 100 m for Landsat and Aster to 1 km for Modis and Sentinel-3 [45]. Despite low spatial and temporal resolutions of the current satellite sensors, detection of soil salinity in the long-wave and thermal spectrum offers possibilities for soil salinity monitoring [4; 63]. Newly planned thermal infrared satellite constellations will enable a revisit time of a few days and a resolution below 100 m.