Prediction of phosphorus sorption index and availability by NIRS and MIRS 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 (NIRS and MIRS) 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 (PTF). Chemometric methods failed to predict available P (Presin). However, a P sorption index, determined as the P remaining in solution (Prem), was estimated with acceptable accuracy with both NIRS and MIRS (R2cv = 0.70 - 0.73; R2v = 0.50 - 0.53). The PTF showed that the PSI was well explained by iron oxide, gibbsite and sand contents, all of these compounds being well predicted by NIRS or MIRS (R2v> 0.70). These results indicate that NIRS and MIRS can be helpful for a rapid estimate of PSI of highly weathered ferralitic soils.