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Poinsettia as an ornamental pot plant is widely grown in peat-based substrates with high phosphorus (P) fertilization. The aim of current study was to evaluate the P demand of poinsettia according to its depletion ability during the growth stages by using a mechanistic simulation model. For this purpose, rooted poinsettia cuttings were grown in the medium with 80: 20 (V: V %) peat+ mineral component (generally is called as clay) and treated with different P levels of 0, 10, 35, 100, and 170 mg P [L substrate]-1. The yield and quality performance of the plants were evaluated thoroughly. Also, depletion of P around the root surface and the effect of buffering power on the depletion profile were assessed by means of mechanistic simulation model. The results showed that in peat-based substrates, P was transported to the root surface mainly by mass-flow. The simulation approach as well revealed that the well supplied plants cultivated in the peat-substrates needed a higher concentration gradient (30-50 µM) to drive the necessary flux and that the amount of plant available P (Cs) was limiting at later growth stages. The optimum yield and quality of poinsettia was obtained at the P application rate of 35 mg L-1 substrate, with Cs of 11-12 and 15-16 mg [L substrate]-1 at planting and 53 days after planting. It has been concluded that, to ensure a sufficiently high concentration gradient, P had to be supplemented by frequent fertigation at later growth stages but not at the early growing stage.