Abstract
Visible and near infrared (vis–NIR) diffuse reflectance spectroscopy has made invaluable contributions to the accurate estimation of soil properties having direct and indirect spectral responses in NIR spectroscopy with measurements made in laboratory, in situ or using on-line (while the sensor is moving) platforms. Measurement accuracies vary with measurement type, for example, accuracy is higher for laboratory than on-line modes. On-line measurement accuracy deteriorates further for secondary (having indirect spectral response) soil properties. Therefore, the aim of this study is to improve on-line measurement accuracy of secondary properties by fusion of laboratory and on-line scanned spectra. Six arable fields were scanned using an on-line sensing platform coupled with a vis–NIR spectrophotometer (CompactSpec by Tec5 Technology for spectroscopy, Germany), with a spectral range of 305–1700 nm. A total of 138 soil samples were collected and used to develop five calibration models: (i) standard, using 100 laboratory scanned samples; (ii) hybrid-1, using 75 laboratory and 25 on-line samples; (iii) hybrid-2, using 50 laboratory and 50 on-line samples; (iv) hybrid-3, using 25 laboratory and 75 on-line samples, and (v) real-time using 100 on-line samples. Partial least squares regression (PLSR) models were developed for soil pH, available potassium (K), magnesium (Mg), calcium (Ca), and sodium (Na) and quality of models were validated using an independent prediction dataset (38 samples). Validation results showed that the standard models with laboratory scanned spectra provided poor to moderate accuracy for on-line prediction, and the hybrid-3 and real-time models provided the best prediction results, although hybrid-2 model with 50% on-line spectra provided equally good results for all properties except for pH and Na. These results suggest that either the real-time model with exclusively on-line spectra or the hybrid model with fusion up to 50% (except for pH and Na) and 75% on-line scanned spectra allows significant improvement of on-line prediction accuracy for secondary soil properties using vis–NIR spectroscopy.
Highlights
Accurate and high-resolution data on soil properties are essential for optimal soil management site- with the aim of maximizing land production at minimum environmental footprints
One of the best proximal soil sensors is visible and near infrared diffuse reflectance spectroscopy, which is a simple, non-destructive and rapid technique, needs no sample preparation for field applications, and can be used for off-line [5,6,7,8] and on-line [9,10,11,12] measurement modes
Vis–NIR has attracted the attention of soil scientists since it was first proposed as potential means of measuring primary soil properties, which have direct spectral response in the NIR spectroscopy [14]
Summary
Accurate and high-resolution data on soil properties are essential for optimal soil management site- with the aim of maximizing land production at minimum environmental footprints. The unique feature of the on-line mode is that it offers high sampling resolution (≈ 2000 samples per ha) data, compared to the laboratory and in situ modes, provided with sufficient prediction accuracy for several precision agricultural applications [13]. Vis–NIR has attracted the attention of soil scientists since it was first proposed as potential means of measuring primary soil properties, which have direct spectral response (e.g., organic carbon and moisture content) in the NIR spectroscopy [14]. The vis–NIR technique allows estimation of soil properties without direct spectral responses in NIR range, designated as secondary soil properties. This is attributed to the covariation of absorption features with one or more primary soil properties lower prediction accuracy ranges are to be expected [16]. Earlier studies showed that the extractable sodium (Naex) and potassium (Kex) are among the most difficult properties to be accurately determined using NIR spectroscopy [13,15,17,18,19,20]
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