CHAPTER 4 RESULTS AND DISCUSSION
4.5 Conclusion
accurate predictions if ECEC values are within that range of prediction compared to all other models even with the inferior performance statistical results. This could be explained by the decrease in range of the SSD from that of the SPD when cLHS was used for ECEC.
The PLSR model also showed that spectral pre-processing reduces RMSE, CV as well as bias values. The PLSR calibration for both unprocessed and pre-processed spectral data showed similar results for the models during validation, unlike that of the Cubist and RF model which showed a significant reduction in performance calculations. While the PLSR model seemingly did not perform as well as the Cubist and RF models, Janik et al. (1998) and Wijewardane et al.
(2018) shows that PLSR is able to produce good results with r2 values of 0.88 and 0.86 respectively. Laboratory determination of ECEC relies on the precision and accuracy at which the cations are extracted using an ammonium acetate solution. This method of extraction from which ECEC is calculated will produce coefficients of variations of 5 – 10% (Soil and Plant Analysis Council, 2000). This criterion allows for the Cubist model that used pre-processed spectral data to be used for the prediction of ECEC values that are within the same margins as for laboratory ECEC analysis.
CHAPTER 5 CONCLUSION AND RECCOMENDATIONS
This research study aimed to create MIR soil property specific calibration models using the Cubist, PLSR and RF calibration models for soil pH, P and ECEC for soils in the Western Highveld region of South Africa. Results presented from this study indicated that different calibration algorithms performed differently for each soil property when trained and used for soil property predictions.
The first and second objective of the study was met successfully with the creation of a SPD with 4 567 soil samples and a SSD with 948 samples that was created using cLHS and combined with spectral data and calibration models to produce a soil spectral library. Eighteen calibration models in total were successfully calibrated and validated to evaluate the performance of the models with both unprocessed and pre-processed spectral data to accomplish the third and final objective of the study.
The soil spectral library contains a Cubist calibrated model with pre-processed spectra that was the best performing algorithm for predicting soil pH as well as a Cubist model calibrated with pre- processes spectra that can accurately predict ECEC soil values. A RF model with pre-processing was the best performing model for extractable P but did not provide accurate enough P values during validation to be used as an extractable P prediction model. The study hypothesis was accepted for both pH and ECEC but rejected for P. Both prediction models for pH and ECEC produced accurate predictions that fall within the laboratory accuracy ranges mentioned for pH and ECEC respectively. Results showed that pre-processing is an important step before model calibration and will produce the best calibration and prediction models. Calibration and prediction models produced similar and comparable results to that of other studies mentioned in the paper for all three soil properties and calibration algorithms. The best prediction models for soil pH and ECEC created in this study were well calibrated with low RMSE values compared to the models presented for pH and ECEC from other studies.
The MIR spectroscopic region is very well suited for determining soil pH and ECEC due to the relationships of these soil properties to soil OC and even soil texture absorptions and reflections that are strongly represented in the MIR spectral range. Other forms of soil P like total P, absorbed P and available P also mentioned in the paper that were used in other studies can also be accurately predicted with MIR prediction models.
This study used a relatively small sample size in the soil spectral database to calibrate the respective prediction models for each soil property. This resulted in slightly lower r2 values than expected when calibrated for pH and ECEC. Other studies that used larger sample sizes or large spectral MIR libraries (Dangal et al., 2019) produced better r2 values for their respective prediction models. Increasing the sample size could further improve the prediction model calibrations of this study. It is also recommended that other statistical calibration methods be used for MIR soil spectroscopy including Artificial Neural Network (ANN) and Machine Based Learning (MBL) algorithms.
The conducted study only included soil samples that originated from areas within the North West Province, South Africa. To expect acceptable results when using these calibration models on soil samples that originate from outside the study area, additional samples must be included from different regions of the country and added to the soil spectral library. Additional soil properties are also required for routine soil analysis and new calibration models need to be created. Only after the successful calibration of prediction models with sufficient accuracy can MIR soil spectroscopy be used as a standard soil analysis method.
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ANNEXURE: METHODS USED FOR SOIL PROPERTY DETERMINATION
pH
pH is measured as pH (KCl) and 10 g of the soil sample is mixed with 25 cm3 of KCl solution at 1 mol dm−3 and stirred into suspension. The mixture is then left for 50 minutes and stirred again and left to stand for 10 minutes. The electrode of the pH meter is placed into a buffer solution with known pH and used to calibrate the pH meter. After calibration the electrode is removed from the buffer solution and placed into the soil suspension which contains the soil sample, the pH is then measured and recorded as pH(KCl) (The Non-affiliated Soil Analysis Work Commitee, 1990).
Phosphorus
The soil is extracted with a solution of NH4F and HCl called Bray-1 extracting solution and then treated with ammonium molybdate and the blue colour obtained is then further enhanced with stannous chloride. Colour intensity is measured with a photoelectric colorimeter at 660 nm wavelength (Ali and Narayan, 2009). For each sample 6.67g of soil is added to a 50 cm3 Bray-1 solution and stirred for one minute after which two drops of flocculant is added. The mixture is the filtered immediately into a separate container and treated with ammonium molybdate and stannous chloride. The solution is then measured with a photoelectric colorimeter (The Non- affiliated Soil Analysis Work Commitee, 1990).
Effective Cation Exchange Capacity
Extractable cations are determined using the ammonium acetate (NH4OAc) method according to the Non-affiliated Soil Analysis Work Committee (1990). A 20g soil sample that was air-dried and sieved to < 2 mm is placed into a 125 cm3 beaker with 50 cm3 0.2 𝑚𝑜𝑙 𝑑𝑚−3 𝑁𝐻4𝑂𝐴𝑐 solution, stirred and left for one hour. The solution was filtered using a Buchner funnel using a vacuum and the leftover soil was leached for one hour using aliquots of the NH4OAc solution into 500 cm3 volumetric flasks (the leachate should not exceed 500 cm3) and weighed after the leaching prosses. The leachate was made up to the 500 cm3mark with 0.2 mol dm−3 𝑁𝐻4𝑂𝐴𝑐 solution and mixed. The soil contained in the funnel was leached for one hour using 50 𝑐𝑚3 of 0.2 mol dm−3 K2SO4 solution into clean 500 cm3 volumetric flasks and the leachate were tested for NH4+ using Nessler’s solution. If no NH4+ were detected the 0.2 mol dm−3 K2SO4 solution can be used to