Abstract: To elucidate the common mechanisms and concentration effects of six typical organic water-soluble fertilizers (OWSFs) on soil phosphorus (P) activation mediated by indigenous microorganisms, crop P uptake, and growth enhancement, a pot experiment was conducted using cotton in slightly saline soil. This study investigated two factors: 1) OWSF types (six representatives: alginic acid powder, alginic acid aqueous solution, polyglutamic acid, chitin, fish protein, and molasses fertilizers), and 2) organic carbon application rates (0, manufacturer-recommended dose, 40, 80, 160, and 240 mg·pot⁻¹). The effects of organic carbon application rates within individual and across different OWSF types at the same organic carbon application rate were evaluated on cotton seedling growth (plant height, leaf area, biomass, and P uptake), soil microbial activity (microbial biomass P and phosphatase activity), and soil available P content. The carbon priming effect (changes in microbial biomass P, phosphatase activity, bioavailable P, and plant P uptake per unit carbon input) of the six OWSFs was quantified. The key findings showed that 1) all six OWSFs enhanced soil P activation, cotton P uptake, and seedling growth in a carbon-dose-dependent manner, with dose-response relationships well fitted by linear-plateau models. The priming effects of different OWSF types on cotton seedling growth were associated with critical thresholds of organic carbon application rate. Although the organic carbon application rate thresholds varied among different OWSF types, the majority of the thresholds fell within the range of 30–60 mg·pot⁻¹. 2) At same organic carbon application rates, OWSF types showed no significant differences in promoting cotton growth but exhibited distinct priming effects on microbial biomass P, phosphatase activity, and bioavailable P (P<0.05). This indicated that OWSFs indirectly enhanced plant growth by stimulating microbially mediated soil P activation rather than through direct effects. 3) The efficiency of the priming effect of the six OWSF-carbon sources varied significantly, with the molasses-based OWSF demonstrating the highest priming effect on microbial biomass P among the six types. We further discussed the current definition of OWSFs, proposing that their “fertilizer” function was primarily achieved through water-soluble organic carbon priming microbial activity to activate soil P turnover.