Citric acid exudation by plant roots is often linked to the mobilisation of recalcitrant soil phosphorus (P) for plant nutrition. In this case study, we have explored the effect of citric acid on the abundance, size and composition of water-dispersible soil colloids (WDC) to understand the mineral source of desorbed P and the chemical nature of P-carrying mobilized colloids. After incubation with citric acid, WDC were isolated using a soil particle-size fractionation method consisting of sedimentation, centrifugation and syringe filtration. The size range and composition of WDC was assessed using field-flow fractionation (FFF), combined with inductively coupled plasma mass spectrometry (ICP-MS) and UV spectrometry, for in vitro P desorption assay samples under the influence of increasing doses of citric acid. Three sharp and well-defined FFF particle size fractions of WDC containing P (12–23, 23–36 and 36–300 nm), with elution times matching carbon (C) peaks and offset from Fe, Al and Si fractions. The concentration of soluble or WDC-associated P, C, Fe, Al and Si increased in response to increasing citric acid doses. Silica colloids were only detected using syringe filtration below 5 µm. The Si, Fe and Al-containing fine colloid fractions (<600 nm) were positively correlated with P (de)sorption parameters measured by diffusive gradient in thin films in previous work. The P desorbed by citric acid originated predominantly from the disaggregation of Fe and Al oxides and silicate clays. The citric acid effect on mobilizing organic P carrying WDC fractions may increase soil organic P cycling and availability to plants.