Growing medium amino acids and mineral nutrients stimulate rhizosphere activities and plant growth. A greenhouse experiment was performed to compare amino acid and mineral nutrient profiles of seafood waste compost (SFWC) and municipal solid waste compost (MSWC) and vermicompost (VC). Their efficacies were also tested on onion (Allium cepa L. “Sweet Utah”). The control was Promix-BX™ alone. The MSWC, SFWC, and VC were composed of total of 36.4, 48.3, and 67.5 mg amino acids/100 g dry weight, respectively. Glutamic acid, aspartic acid, and glycine were the highest while methionine, histamine, and cysteine were the least in all the amendments. The VC had the highest Ca content but the least P and K contents. SFWC had the highest content of P and K and most of the determined micronutrients. The MSWC had significantly (P<0.05) the highest N and leaf chlorophyll contents followed by the VC. The MSWC significantly (P<0.05) increased anthocyanin content while the control recorded the least. The maximum quantum yield of photosystem II (Fv/Fm) and the potential photosynthetic capacity (Fv/Fo) were least in the VC treated plants. Dry matter was not affected by the type of amendment. Overall, plant growth was improved by the MSWC. Future research should investigate effect on secondary metabolites.