# Soil Moisture Influence on the FTIR Spectrum of Salt-Affected Soils
## 1 Introduction  
Salinity is one of the most serious environmental stresses in the 21 st Century, since it largely affects agricultural production through decreasing cultivated areas, productivity and the quality of agricultural products [1, 2, 3]. An estimated 20% of the total cultivated area and 33% of irrigated agricultural land worldwide are affected by high salinity levels [3, 4]. In addition, more than 50% of the arable land is projected to be at risk of salinization by the year 2050, which necessitates breeding salt-tolerant crops to address food security [5, 6, 7]. Salt accumulates in the soil due to natural processes and occurs in drylands due to a combination of high evapotranspiration rates and erratic rainfall, and the natural presence of soluble salts. In coastal areas, soil salinity occurs due to seawater intrusion into the surface water, groundwater and wetlands, and subsequently enters agricultural fields, asobserved in southeast Vietnam [8; 9]. Seawater intrusion can be caused by tides, waves during storms and storm surges, which are projected to increase due to sea levels rising in a changing climate. The accumulation of salt in coastal environments due to natural processes is called primary salinity, and occurs over a time scale of about 100,000 years or longer [10]. Soil salinity can also occur in a secondary form and can be human-induced in irrigated areas. Primary salt-affected soils are estimated globally at about 955-1070 Mha during 1986-2016, while secondary salinization affects around 77 Mha, with 58% of these in irrigated areas [4; 11]. Secondary salinization occurs especially in arid areas, which have high evaporation rates such as in the drier provinces of southeast Vietnam [8; 9; 12]. In coastal areas, the salinity of cultivated land is affected by aquaculture and, in particular, by shrimp cultivation, where seawater is deliberately pumped into the fields. Currently, this activity is expanding rapidly in Vietnam and therefore, soil salinity has increased accordingly [8]. Sea level rise is a serious climate threat and leads to severe salinity intrusion and tidal inundation in coastal areas [8; 9; 12]. Torrential rainfall contributes to flooding by rivers in many coastal and estuarine areas [13], and adds to spreading salinity in low-lying areas. Salt-affected soils have a high content of soluble salts that interfere with plant growth [14], and are referred to as saline, saline-sodic and sodic, depending on the total salt concentration, the amount of sodium present and the soil reaction [15]. Sodium salts are capable of alkaline hydrolysis, particularly Na\({}_{2}\)CO\({}_{3}\), and have also been termed 'alkali' [16]. Soil salinity is assessed by its influence on crop yield and quality, and measured as the electrical conductivity (EC) of the saturation extract (ECe) in the root zone [16]. Soil salinity can be classified into five levels depending on the influence on the plants [16; 17]: non-saline: 0-2 dS/m; slightly saline: 2-4 dS/m; moderately saline: 4-8 dS/m; strongly saline: 8-16 dS/m; and very strongly saline: \(>\)16 dS/m. Other studies have classified saline soils with values exceeding 4 dS/m (approximately 40 mM NaCl) at 25 \({}^{\circ}\)C and having an exchangeable sodium percentage of 15% [6; 18; 19]. The mineralogy of carbonate, sulfate and chloride salts can be determined by the presence or absence of absorbance features in the electromagnetic spectrum. The V-NIR-SWIR absorbance bands in the spectrum of salt minerals are largely associated with the vibration of the anion groups HOH, OH, CO and SO\({}_{4}\)[11; 20; 21]. The soil salinity state has been identified using laboratory spectroscopy in the visible (550-770 nm) and NIR (900-1030 nm, 1270-1520 nm, 1940-2150 nm, 2150-2300 nm and 2330-2400 nm) parts of the spectrum [22; 23]. In arid soils, the reflectance spectra of saline soils produced distinct spectral absorbances [24; 25; 26], where crushing and salt residues were associated with high reflectance in the visible and near-infrared spectrum [24; 27; 28].