Struvite is a phosphorus (P)-rich by-product of wastewater treatment facilities that can be recycled as a P source in agriculture. Because struvite is not water soluble, it is solubilized gradually by organic acids released by soil microorganisms and from growing plant roots when used as fertilizer. To speed up the solubilization process, struvite can be combined with biostimulants such as P-solubilizing bacteria (PSB) or earthworm casts (WC). The objective of this greenhouse study was to compare the fertilizer value of struvite, with and without PSB (Bacillus megaterium) or WC, with that of triple superphosphate in two contrasting soils (a low-P soil and a high-P soil). Oat (Avena sativa L.) was grown for 8 wk under a controlled environment, and dry matter yield at harvest, total N and total P uptake, the soil residual Mehlich-3 P, and phosphomonoesterase activity were measured. The high-P soil was unresponsive to P application, but the low-P soil was responsive. In the low-P soil, there was more Mehlich-3-extracted P when struvite was combined with PSB or WC compared with struvite alone, resulting in greater oat dry matter and more total N and more total P uptake. Combining struvite with biostimulants increased total dry matter and total P uptake by an average of 39% and 33%, respectively. We conclude that greater P release from struvite occurs when it is combined with PSB or WC, particularly in low-P soil, but this needs to be confirmed in field-scale studies.