# Survey for Soil Sensing with IOT and Traditional Systems
## 2 Tradicitonal Soil Sensing System
### Fundamentals of Soil Reflectance Spectroscopy  
The utilization of reflectance spectroscopy for discerning soil characteristics has gained considerable attention, primarily due to its non-invasive nature [9, 17, 25]. Traditional reflectance spectroscopy is founded on the principles of the Beer-Lambert law. The Beer-Lambert law [26] explains the diminution of light intensity as it passes through a material, with this reduction being directly related to the substance's properties. As light permeates  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Work** & **Year** & \begin{tabular}{c} **Sample** \\ **Preparation** \\ \end{tabular} & \begin{tabular}{c} **Category of** \\ **Properties** \\ \end{tabular} & **Signal Types** & \
\begin{tabular}{c} **Calibration** \\ **Method** \\ \end{tabular} &
\begin{tabular}{c} **Device \&** \\ **Environment** \\ \end{tabular} & **Applications** \\ \hline
[17] & 2006 & \(\circ\) & \(\star\) & \(\circ\) & \(\circ\) & \(\circ\) & \\ \hline
[18] & 2009 & \(\circ\) & \(\star\) & \(\circ\) & \(\circ\) & \(\circ\) & \\ \hline
[19] & 2010 & \(\star\) & \(\star\) & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) \\ \hline
[20] & 2014 & \(\star\) & \(\star\) & \(\circ\) & & \(\circ\) & \(\circ\) \\ \hline
[21] & 2018 & & \(\star\) & \(\circ\) & \(\star\) & & \(\star\) \\ \hline
[22] & 2018 & & \(\circ\) & \(\circ\) & \(\circ\) & & \(\circ\) \\ \hline
[7] & 2022 & \(\circ\) & \(\star\) & \(\circ\) & \(\star\) & \(\circ\) & \(\star\) \\ \hline This Paper & 2023 & \(\star\) & \(\circ\) & \(\star\) & \(\star\) & \(\star\) & \(\star\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Soil property sensing survey comparison \(\circ\): analyzed; \(\star\) deep analyzed.  
a material, it incites the molecule bonds of each component within the substance to vibrate. Given the unique molecular structures and bonds that each chemical species possesses, each one generates a distinct absorption spectrum. Thus, the Beer-Lambert law is widely employed to determine the concentration of chemical components that can absorb and scatter light in chemical analyses [8, 25, 27].  
One common variant of the Beer-Lambert law establishes a relationship between the decrease in light intensity within a material, which comprises a uniformly distributed absorbing substance, the length of the optical path of light through the substance, and the substance's absorbance capacity [28, 29]. The mathematical depiction of the Beer-Lambert law [30] is presented in Equation (1).  
\[A=\log(\frac{I_{0}}{I_{r}})=e\ell c \tag{1}\]  
Here, \(I\) represents the light intensity that is initially emitted, while \(I_{r}\) signifies the light intensity received after it has traversed an optical path of length \(\ell\). \(A\) stands for the absorbance of the substance, which can be calculated from the initial emitted light intensity \(I\) and the detected light intensity \(I_{r}\) after the light has journeyed the optical path of length \(\ell\). The molar attenuation coefficient is represented by \(\epsilon\), and \(c\) denotes the concentration of the species that attenuates the light. Consequently, the frequencies where light absorption occurs result in a diminished reflected radiation signal. This can be represented in reflectance \(R\) and can subsequently be converted to apparent absorbance, as illustrated in Equation (2) [19, 31].  
\[A=\log(\frac{1}{R}) \tag{2}\]