Factors Influencing the Calibration of near Infrared Reflectometry Applied to the Assessment of Total Nitrogen in Potato Tissues. I. Particle Size, Milling Speed and Leaf Senescence

 Several factors thought to modify the calibration between nitrogen concentration, [N]_NIR and [N]_Dumas, were examined including particle size using size-classes from the distribution produced by milling through a standard screen. The standard milling procedure produced differing distributions of particle size from different tissues, [N] differed between size classes and the pattern of [N] with particle size differed between tissues. In leaf and stem the smallest particles had the highest [N], in tuber material they had the lowest. Samples for analysis are generally milled in batches and analysed later. These findings show that it is important to ensure that samples are well-mixed and that they are not allowed to stratify. Only well-mixed samples should be used to fill the sample cups on a near infrared (NIR) analyser. A divergence in the relations between [N]_NIR and [N]_Dumas for stem samples implies that the calibration model used for that material might be particularly sensitive to particle size distribution. Within the range examined, milling speed was not an important variable in the preparation of leaf and tuber material. There is little to be gained from treating small quantities of senescent leaves separately from the remainder of a sample. However, if [N] is to be assessed in large numbers of samples of leaves that are mostly senescent then a separate calibration should be derived.