Prediction of Phosphorus Sorption Index and Availability by NIR and MIR in Soils in Madagascar

The development of techniques for the rapid, inexpensive, and accurate determination of the phosphorus (P) availability and sorption index (PSI) in soils is important for P management in highly weathered tropical soils. The applicability of near- and mid-infrared reflectance spectroscopy (NIR and MIR) as tools for estimating P availability and PSI was assessed over a wide range of highly weathered soils in Madagascar. The predictions were based on chemometric methods using multivariate calibration models with partial least squares (PLS) regressions, and pedotransfer functions (PTFs). Chemometric methods failed to predict available P (P-resin). However, a P sorption index, determined as the P remaining in solution (P-rem), was estimated with acceptable accuracy with both NIR and MIR (R-cv(2) = 0.70 - 0.73; R-v(2) = 0.65 - 0.77; SEP(c) = 5.5 - 4.6 mg kg(-1)). The PTFs showed that the PSI was well explained by iron oxide, gibbsite, and sand contents, all of these compounds being well predicted by NIR or MIR (R-v(2) > 0.70). These results indicate that NIR and MIR can be helpful for a rapid estimate of PSI of highly weathered ferralitic soils.