Colloidal forms of Fe(III) minerals can be stabilized in solution by coatings of organic or poly-phosphate (P), which reduce the zeta-potential. This opens up a route toward the development of nanoforms of P fertilizers. However, it is unclear if such P forms are bioavailable. To address this question, spinach (Spinacia oleracea) was grown in nutrient solutions, at equal total P, using three different forms of P (orthophosphate = P; hexametaphosphate = HMP; myo-inositol hexaphosphate = IHP), free or bound to goethite/ferrihydrite colloids. After 10 days, P uptake was determined with a dose-response curve using colloid-free P as a reference treatment. The P concentration generating equal P uptake as in colloidal P treatments was used to calculate the relative bioavailability of colloidal P (RBA). The RBA was about 60% for P-loaded goethite, stabilized with natural organic matter. For HMP/IHP-P-loaded colloids, RBA ranged between 10 and 50%, in line with their higher sorption strength. In conclusion, colloidal organic P or poly-P can stabilize Fe(III) colloids in solution and can contribute to plant-available P. Soil experiments are required to assess their potential as nanofertilizers.