## 4 Discussion  
In this study, a small Vis spectrometer and a small NIR spectrometer were connected to each other through a three-in-one optical fiber. The two spectrometers jointly collected the spectrum information of 33 different fertilizers in powder state. S-G smoothing and the SNV method were used to preprocess the spectra, and then the CARS method was used to select the characteristic wavelength corresponding to N, P\({}_{2}\)O\({}_{5}\), and K\({}_{2}\)O nutrient elements, respectively. The PLS and ELM models were established for 550-950 nm, 1050-1640 nm,  
Figure 5: Scatter plot of the experimental value and predicted value. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Nutrient**} & \multirow{2}{*}{**Spectral Band**} & \multirow{2}{*}{**Algorithm**} & \multicolumn{2}{c}{**Calibration Set**} & \multicolumn{2}{c}{**Prediction Set**} \\ \cline{3-8}  & & & \(R_{e}^{2}\) & **RMSECV \%** & \(R_{p}^{2}\) & **RMSEP \%** & **RPD** \\ \hline \multirow{4}{*}{N} & \multirow{2}{*}{550–950 nm} & PLS & 0.653 & 5.211 & 0.596 & 5.620 & 1.29 \\  & & ELM & 0.854 & 3.382 & 0.784 & 4.111 & 2.02 \\ \cline{2-8}  & \multirow{2}{*}{1050–1640 nm} & PLS & 0.891 & 2.920 & 0.881 & 3.056 & 2.74 \\  & & ELM & 0.991 & 0.823 & 0.984 & 1.116 & 7.91 \\ \cline{2-8}  & \multirow{2}{*}{full spectrum} & PLS & 0.933 & 2.284 & 0.908 & 2.676 & 3.159 \\  & & ELM & 0.996 & 0.537 & 0.989 & 0.910 & 9.71 \\ \cline{2-8}  & \multirow{2}{*}{characteristic wavelength} & PLS & 0.931 & 2.317 & 0.906 & 2.709 & 3.10 \\  & & ELM & 0.995 & 0.659 & 0.986 & 1.033 & 8.59 \\ \hline \multirow{4}{*}{P\({}_{2}\)O\({}_{5}\)} & \multirow{2}{*}{550–950 nm} & PLS & 0.333 & 10.587 & 0.330 & 10.618 & 0.69 \\  & & ELM & 0.921 & 3.649 & 0.841 & 5.179 & 2.38 \\ \cline{2-8}  & \multirow{2}{*}{1050–1640 nm} & PLS & 0.779 & 6.100 & 0.745 & 6.553 & 1.76 \\  & & ELM & 0.980 & 1.850 & 0.897 & 4.170 & 3.18 \\ \cline{2-8}  & \multirow{2}{*}{full spectrum} & PLS & 0.899 & 4.123 & 0.853 & 4.636 & 2.55 \\  & ELM & 0.968 & 2.324 & 0.945 & 2.933 & 4.45 \\ \cline{2-8}  & \multirow{2}{*}{characteristic wavelength} & PLS & 0.905 & 3.987 & 0.844 & 5.124 & 2.46 \\  & & ELM & 0.974 & 1.439 & 0.963 & 1.724 & 5.09 \\ \hline \multirow{4}{*}{K\({}_{2}\)O} & PLS & 0.595 & 6.383 & 0.522 & 6.932 & 1.16 \\  & ELM & ELM & 0.880 & 3.468 & 0.730 & 5.214 & 2.00 \\ \cline{2-8}  & \multirow{2}{*}{1050–1640 nm} & PLS & 0.825 & 4.189 & 0.804 & 4.434 & 2.01 \\  & ELM & 0.996 & 0.667 & 0.975 & 1.565 & 6.70 \\ \cline{2-8}  & \multirow{2}{*}{full spectrum} & PLS & 0.955 & 2.117 & 0.906 & 3.070 & 3.23 \\  & ELM & 0.990 & 0.999 & 0.981 & 1.393 & 7.29 \\ \cline{2-8}  & \multirow{2}{*}{characteristic wavelength} & PLS & 0.913 & 2.959 & 0.901 & 3.153 & 3.05 \\ \cline{2-8}  & & ELM & 0.994 & 0.709 & 0.980 & 1.237 & 7.17 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The results of N, P\({}_{2}\)O\({}_{5}\), and K\({}_{2}\)O content models based on different band spectra. full spectrum, and characteristic wavelength, respectively. The results of the study show that the near-infrared waveband from 1050 nm to 1640 nm has better prediction effects on fertilizer nutrient content than the visible light waveband from 550 nm to 950 nm. The characteristic wavelength and the full spectrum have almost equal prediction effects, and the characteristic wavelength reduces the spectral variables to the original by about 1/10 of that, effectively improving the quality of information in the spectra and reducing the amount of model calculations. In the end, the RPD values of the optimal prediction models for N, P\({}_{2}\)O\({}_{5}\), and K\({}_{2}\)O were 9.71, 5.09, 7.29, and for \(R_{p}^{2}\) were 0.989, 0.963, and 0.981, respectively. The experimental results show that for the commonly used chemical fertilizers such as compound fertilizers, blended fertilizers and controlled-release fertilizers, the small Vis/NIR spectrometer can accurately predict the nitrogen, phosphorus, and potassium content, and the effect is better in the near-infrared band.