3.4.4 BD prediction
Although the prediction accuracy during calibration for BD was observed as excellent (R2c = 0.99; MBEc = -0.01; RMSEc = 0.05; RPDc = 5.94) using the SVR model, the results during validation were categorized as non-reliable. The highest prediction accuracy (R2p = 0.52; MBEp = 0.03; RMSEp = 0.22; RPDp = 1.13) for BD was recorded with PLSR (Figure 4d). The BD is one of the fundamental soil physical properties and is often used as an indicator of soil quality, site productivity and soil compaction (HÃ¥kansson, 1990; Suuster et al. , 2011). Previously, only a few studies were carried out mainly focusing on the mid-infrared region (MIR) for predicting soil BD (Minasny et al. , 2008). They observed unsuccessful prediction as BD is related to a structural pore-space condition that cannot be captured by MIR spectra. Saekiet al. (2003) predicted BD using the NIR spectroscopy with a good prediction accuracy using the PLSR technique (R2 = 0.96). Moreira et al. (2009) reported good and satisfactory prediction with R2c= 0.34 obtained using modified PLSR technique (mPLSR) and NIR spectral information. According to Minasny et al. (2008) under certain conditions soil BD is strongly related to the organic matter content of the soil, of which major mass (approximately 58%) exists as carbon (C). Poor prediction accuracy using VIS-NIR spectroscopy in our study could be attributed indirectly with low prediction accuracy for some soil parameters (e.g. total C and N) which shows relatively a small variation in the soil as suggested by van Groenigen et al. (2003).