This research aimed to classify 16 horticultural composts collected across Canada into management group according to their phosphorus (P) fractions and carbon (C) content using isometric log ratio (ilr) and to interpret the clusters against the total P content and C-to-P ratio indices. The ilr approach was found to be more discriminant for grouping the composts compared with the conventional statistical analysis. The C-to-P ratio index was representative of cluster 1 only. This cluster included organic amendments with C-to-P ratios higher than 100 and high capacity to increase soil organic matter content without excessive P dosage. Total P separated clusters 2 and 3 despite the amalgamation of P forms into total P. Cluster 2 showed high total P (>10 g P kg^-1) and low C-to-P ratios (24–38), whereas cluster 3 showed variable C-to-P ratios (18–78) and total P <9 g kg^-1 after excluding one compost close to cluster 2. Clusters 2 and 3 were considered as potential sources of plant-available P. The ilr approach suggests that composts made of municipal biosolids and poultry manures in cluster 2 have the highest potential as plant-available P source compared with those made of other livestock manures or food processing wastes.