Plant roots are a highly heterogeneous and hierarchical system. Although the root-order method is superior to the root diameter method for revealing differences in the morphology and physiology of fine roots, its complex partitioning limits its application. Whether root order can be determined by partitioning the main root based on its diameter remains uncertain. Four methods were employed for studying the morphological characteristics of seedling roots of two Pinus species in a natural and nitrogen-enriched environment. The intrinsic relationships among categories of roots by root order and diameter were systematically compared to explore the possibility of using the latter to describe root morphology. The normal transformation method proved superior to the other three in that the diameter intervals corresponded most closely (at least 68.3%) to the morphological characteristics. The applied methods clearly distinguished the results from the natural and nitrogen-rich environments. Considering both root diameter and order simplified the classification of fine roots, and improved the estimation of root lifespan and the data integrity of field collection, but failed to partition all roots into uniform diameter intervals.