NDVI is basically an estimate of the above ground biomass. In SOC modelling you could interpret this as the input of organic material to the soil (if it is not being removed with the harvest). However, if the vegetation is very open and there is a lot of bare soil in between, the soil color may influence significantly the spectral signal and thus the estimate of the above ground biomass. At that point one can decide to use the SAVI. However, the estimate of the canopy background adjustment factor can be cumbersome and need some site-specific calibration.