# Survey for Soil Sensing with IOT and Traditional Systems
## 2 Tradicitonal Soil Sensing System
### Soil Sample Pretreatment
#### 2.2.2 Pre-Processing Steps for Soil Samples  
In conventional in-lab soil attribute sensing experiments, pre-processing of soil samples is a standard procedure, as evidenced by various studies [19, 32, 33, 34, 36]. Considering that spectra obtained from horizontal cross-sections of 5 cm soil slices yield slightly less precise predictions [19], such pre-processing is vital. This is because these preparatory steps ensure consistent quality across all soil samples, facilitating the development of a more generalized model based on these standardized samples.  
Figure 1 depicts the procedural flow for soil sample pre-processing. The initial stage of soil pre-processing involves drying. For instance, in the study by [33], soil samples were stored for a day to equilibrate, followed by air-drying for a week. Similarly, in [34], soil samples collected from 20 cm below the soil surface were air-dried for approximately a week. In the research by [36], soil samples were dried at 80 \({}^{\circ}\)C for 8 h after being thoroughly mixed with varying concentration solutions. In [37], the soil samples are thinly spread, with a thickness of about 2 cm, and left to dry in a well-ventilated indoor area with ample light. After drying, the samples are transferred into beakers and further dehydrated using an electric blast dryer. The objective of dehydration is to eliminate the impact of soil moisture during the spectroscopy analysis process.  
Next, sieving is the subsequent step in soil preprocessing. In the study by [33], dried soil samples are passed through a 2 mm nylon sieve to exclude stones, insects, large debris, pebbles, and other extraneous materials. Similarly, Ref. [34] filter the dried samples using a 2 mm sieve. In the research conducted by [32], hand cleaning is employed to eliminate stones and excessive residues. The primary purpose of this step is to discard materials with larger particle sizes, as they can introduce significant noise to the spectroscopy analysis.  
The final preparatory step before spectroscopy analysis involves grinding and screening. In the study by [33], soil samples were ground using a mechanical agate grinder and then sieved through a 100 mesh screen (with a diameter of 0.150 mm). Similarly, in the research presented by [34], all soil samples underwent grinding. The study by [32] opted to grind their samples and subsequently sieve them through a 1 mm screen. Soil samples in [36] were first ground, then sieved using an 80 mesh (160 \(\upmu\)m) screen, and later compressed into 10 mm \(\times\) 10 mm samples with a thickness of 2 mm under 10 MPa pressure using a bench press machine. In the study by [35], the approach involved grinding the soil samples into a powdered form and subsequently passing them through a 100-mesh sieve. The main objective of this procedure is to minimize the impact of diffuse reflectance from soil samples that are not uniformly distributed. After these steps are carried out diligently, the soil samples are prepared for in-lab spectroscopy analysis to identify the concentration levels of specific soil attributes.  
Figure 1: Pre-processing steps for the raw soil samples.