Arbuscular mycorrhizal (AM) symbiosis is the most common plant strategy that increases phosphorus (P) acquisition, involving approximately 80% of terrestrial plants. The AM fungal symbionts provide a very effective pathway (the AM pathway) for uptake, scavenging P from large soil volumes and overcoming depletion in the rhizosphere that occurs when direct (epidermal) root uptake is faster than replacement from the bulk soil. Recent physiological and molecular research has shown that the AM pathway makes very large contributions to total plant P even in plants that show no growth increases when AM, compared with non-mycorrhizal (NM) counterparts. The AM contribution remains unless radioactive tracers are used to track delivery via the AM pathway. Importantly, this finding demonstrates that the direct pathway delivers less P to AM plants than to NM counterparts and implies fungus-to-plant signaling. The mechanisms by which direct uptake is reduced are unknown, but the hidden contribution of AM uptake means that AM fungi cannot be regarded as parasites, because there is mutualistic exchange of P for organic C regardless of plant growth responses. Furthermore, the dominance of the AM pathway helps to explain the persistence of AM symbiosis over evolutionary time, even in plants that apparently show no benefits.