The data and findings discussed here come from the cited paper and do not represent test results from VISPEK equipment.
How the soil is dried before being fed into the spectrometer can also affect the final prediction. This study controlled the soil source and spike conditions, and specifically compared different drying temperatures, making it easier to see the impact of the sample preparation process on the nitrogen model, and also provided a basis for understanding the applicable scope of the experimental results.
Summary
This study adds sodium nitrate to homologous soil, compares near-infrared reflection models under different drying conditions, and evaluates predictions by combining wave selection and multiple regression methods. The better performing routes were from controlled samples and artificially spiked dry soils, supporting a uniform sample preparation protocol and not directly proving that natural wet soils or field nitrate nitrogen can be measured equally accurately.
(AI-generated conceptual illustration for context; it is not data from the paper, a photograph of the research equipment, or a VISPEK product diagram.)
Why It Matters
Drying conditions significantly altered model performance, illustrating the need for soil instrument development to include sampling protocols as part of calibration.
Methods and Reference Measurements
The sandy loess from Huaian, Jiangsu was ground to 60 mesh, and sodium nitrate solution was added to form 11 concentrations of 0.5–2.5 g/kg and 0.2 increments, with 12 repetitions per level. Five groups were dried at 50°C for 36 hours, 65°C for 24 hours, 80°C for 18 hours, 95°C for 12 hours, or 25°C for 24 days. After black and white correction of the 900–1700 nm reflectometer, a flat surface coverage window was taken, three scans were averaged, 400 points, 10 nm optical resolution, the optical path was not reported; using the prepared concentration as the label, preprocessing, RFFS/CARS/SPA wave selection and comparison of PLSR/SVM/ANN.
Wavelengths and Acquisition Parameters
| Band/parameter | Original meaning and usage instructions |
|---|---|
| 900–1700 nm | Reflectometer range in this study, optical resolution 10 nm |
Key Findings
The SPA-ANN dried at 80°C is the best, with calibration R²=0.998, predicted R²=0.989, and RMSEP=0.257 g/kg; it is an artificially spiked dry soil experiment.
Limitations and Interpretation
Artificial spike concentrations and homologous matrices may cause strong correlation; it is not appropriate to quantify the exclusive NIR absorption of nitrate.
Image copyright belongs to the paper’s original authors. License: CC BY 4.0.
Source Paper
Zhou L, Yao J, Xu H, Zhang Y, Nie P. (2023). Research on the Effects of Drying Temperature for the Detection of Soil Nitrogen by Near-Infrared Spectroscopy. Molecules (Basel, Switzerland). https://doi.org/10.3390/molecules28186507
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VISPEK products shown for illustration; these were not the devices used in the cited study.