| Methods for rapid, cost effective and reliable estimation of soil organic carbon and other soil properties are needed for quantitative assessment of land management and carbon sequestration in West Africa. The main objective of this study was to evaluate loss-on ignition (LOI) and visible near infrared spectroscopy (VNIRS) as alternative methods to traditional chemical methods. Two hundred thirty samples were collected from selected agro-ecological zones of Mali, and scanned for estimation of OC, sand, Fe, clay, Al, and phosphates. Results show that the weight loss percent measured by LOI, with an initial temperature of 105°C and a final temperature of 350°C, was highly correlated with the combustion method (adj. R2 = 0.89). Among the three compared statistical packages (ParLeSRTM2.1, SAS, and classification regression trees-CART) used in the present study, ParLeSRTM2.1 seems to be the most user friendly, i.e. simple to manipulate providing the lowest RMSE = 0.1485, the lowest bias = -0.0790 and the highest R2=0.91. Testing the ability of VNIRS to determine an increase of OC in the soil using spiked samples with carbon as glucose, a poor prediction was obtained. Furthermore, results indicate that VNIRS techniques are suitable for the measurement of other soil properties. Results indicate that spectrometric measurements of soil reflectance can be used to assess organic carbon with an adj. R2 = 0.91 and sand (0.90). The technique gave also a good result for clay (adj. R2 = 0.87) and a slightly lower value for iron (0.83). On the other hand, the calibration for aluminum and phosphate (ammonium oxalate extraction) was very poor with adj. R2 of 0.30 for aluminum and 0.18 for phosphate. Validation results for OC indicated an accurate estimate of Mali-2004 samples in the range of 2.5 to 9 g and unreliable for Senegal-2006 samples. This indicates that for regional calibration a larger number of samples must be collected over the area of interest. VNIRS and LOI have potential as alternative methods to existing laboratory methods for assessing soil OC and other properties. |
