Laboratory methods of estimating potentially mineralizable nitrogen in organic potting mixes. II. Development of near infrared reflectance spectroscopy method

Near infrared reflectance spectroscopy (NIRS) was tested as a method of pre dicting potentially mineralizable N (PMN) in organic potting mixes. Initial ly, calibration models were developed using the near infrared spectra of 10 0 potting mix formulations and the amount of N recovered by ryegrass herbag e after 168 d of growth in the formulations, focusing on which combination of a range of regression, derivatization and scatter correction techniques gave the best calibration model. In the second part of the study, a validat ion exercise was performed, where the 100 formulations were split into sepa rate calibration and validation sets. In the initial calibration exercise, the most effective combination of regr ession technique and spectral pretreatments was modified partial least squa res in conjunction with a second order derivative and a standard normal var iate and detrend scatter correction technique, which had an R-2 of 0.99. Ho wever, in the validation study the coefficient of determination was much lo wer (r(2) = 0.73), and results were not in line with expectations. The expl anation offered for the poor validation results relates to the constriction of the set of calibration samples. Nevertheless, NIRS appears to have grea t potential for predicting the performance of organic potting mixes, and de spite several practical drawbacks is worthy of investigation by other worke rs.