The split is based on P₂O₅ content: superphosphates with 35% or more P₂O₅ by weight classify under 310311 (high-grade), while those below that threshold fall under 310319. Laboratory analysis or a manufacturer's certificate of analysis documenting the P₂O₅ percentage is typically needed to support the correct subheading declaration.

Heading 3103 covers fertilizers whose fertilizing value derives principally from phosphorus alone. Once a product contains two or three of the primary fertilizing elements (N, P, K) in commercially significant proportions, it shifts to heading 3105, which specifically covers two- and three-element fertilizers. The nutrient composition declared on the product label is the starting point for this determination.