Spectral platform for soil samples of the Democratic Republic of Congo


This is a spectral platform for predictions of soil properties for soil samples from the Democratic Republic of Congo and includes more than 1300 individual soil samples. The samples have been collected from agricultural fields, coffee plantations, from an arboretum, from mixed, monodominant (Gilbertiodendron dewevrei), bamboo, lowland, montane primary and secondary forests and from termite mounts in the provinces Tshuapa, Tshopo, South Kivu, Bas-Uele, Haut-Katanga and Kongo Central map . The wet chemistry analsyis has been done at ETH Zurich, Ghent University, Université Catolique de Louvain, IITA Kalambo, ICRAF and Université de Lubumbashi.

The spectral analysis and the modeling is done in R (R core Team, 2018), and is mainly based on the package simplerspec (Baumann, unpublished). It embeds several packages for data preprocessing, splitting and modeling, such as the caret package interface (Kuhn et al., 2016) for model development and predictions and pls package (Mevik et al., 2016) for Partial Least Squares Regressions (PLSR). For the final models, the spectra were resampled to a new wavenumber range from 500 cm -1 to 3996 cm -1 with a spectral resolution of 2 cm -1 and preprocessed using a Savitzky-Golay filter (Savitzky and Golay, 1964) with a first derivative based on third order polynomial approximation and a window size of 21 cm -1 . For model tuning, a 5 times repeated 10-fold cross-validation was performed and the maximal number of PLS components was set to 10. The model evaluation was done by predicting the hold-out data and computing the residuals between the predicted and observed values, the Root Mean Square Error (RMSE), R 2 and the Ratio of Performance to Deviation (RPD).

PLSR models for total carbon, total nitrogen, pH and soil texture

Created by Laura Summerauer, ETH Zurich (2019)

Contributors: Philipp Baumann (ETH Zurich), Dr. Matti Barthel (ETH Zurich), Dr. Johan Six (ETH Zurich), Dr. Marijn Bauters (Ghent University), Dr. Kristof Van Oost (Université Catholique de Louvain), Dr. Pascal Boeckx (Ghent University), Dr. Elizabeth Kearsley (Ghent University), Dr. Eric Van Ranst (Ghent University), Dieudonné Chiragaga (IITA Kalambo), Dr. Bernard Vanlauwe (IITA), Dr. Basile Mujinya Bazirake (Université de Lubumbashi), Andrew Sila (ICRAF), Dr. Keith Shepherd (ICRAF), Nathanaël Dériaz (ETH Zurich), Simon Baumgartner (ETH Zurich), Nora Gallarotti (ETH Zurich), Oscar Vercleyen (Ghent University), Kasper Minten (Ghent University), Marco Bravin (Université Catolique de Louvain), Emilien Aldana Jague (Université Catolique de Louvain), Dr. Sebastian Dötterl (ETH Zurich) www.congo-biogeochem.com

Soil map: Jones, A., Breuning-Madsen, H., Brossard, M., Dampha, A., Deckers, J., Dewitte, O., Gallali, T., Hallett, S., Jones, R., Kilasara, M., Le Roux, P., Micheli, E., Montanarella, L., Spaargaren, O., Thiombiano, L., Van Ranst, E., Yemefack, M. , Zougmoré R., (eds.), 2013, Soil Atlas of Africa. European Commission, Publications Office of the European Union, Luxembourg. 176 pp.

1. Recommended soil processing after sampling

- Oven drying: 48 hours, 60°C
- Milling to fine powder (e.g. ball mill: frequency 30 Hz, 50 seconds)

2. Sample labelling

Unique and quickly identifiable sample_id's are crucial but often underestimated. Do not use any spacing, capital letters or special characters within the labels (e.g. +/()='* ). R cannot read those characters and will therefore end in error messages. Use only '_' to separate words or letters.


3. FT-IR spectroscopy using ALPHA Bruker spectrometer

Standard operating procedures for FT-IR spectrometer, Sustainable Agroecosystems, ETH Zurich

4. R modeling
R package simplerspec by Philipp Baumann

5. Literature, tutorials and other
simplerspec teaching material by Philipp Baumann