Laboratory methods of estimating potentially mineralizable nitrogen in organic potting mixes. II. Development of near infrared reflectance spectroscopy method
Gd. Smith et al., Laboratory methods of estimating potentially mineralizable nitrogen in organic potting mixes. II. Development of near infrared reflectance spectroscopy method, COMM SOIL S, 32(17-18), 2001, pp. 2769-2781
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.