Comparing methods with a new sample set (soil)

I did develop calibrations for soil texture with the three methods I use (LOCAL , PLS and ANN) with more than 2000 samples and I have a new set of samples to test them (samples scanned after the calibration development). The idea is to check which one performs better.

First as usual I look to the spectra and found that some of the new samples seems to have gypsum, so I mark these samples as spectra outliers, but I validate the results anyway, because the idea is to see  those sample marked (red cross) in the XY validation plot.

Now for the Silt parameter, I plot the validation set predicted vs. the reference values :

                                             SILT validation XY plots

The samples which seems to have gypsum seems to be well extrapolated with the PLS calibration, and not with the ANN. The LOCAL calibration did not found similar samples in the database and does not give predictions except for one of the samples.

Gypsum can be found on the three texture options, more in the clay fraction but in some areas  gypsum silt soils can be found and this could be the case. 

This samples are very good to extend the variability of the database and a good way to check how the calibrations work with extreme samples. When the database has a better distribution the ANN and LOCAL will perform better predicting these samples.