Right fertilization is the key to a bountiful harvest from pomelo. The late Severino Belviz who was an expert in growing pomelos and rehabilitating neglected trees had a fertilizing protocol for 10-year-old trees so they will produce 200 to 400 fruits per tree.
No.1 – Mix one bag of Urea (50 kg), one bag Solophos ((0-18-0) and one bag of Muriate of Potash (0-0-60). Mix the three thoroughly, then apply 2.5 kg – 3.75 kg per tree immediately after harvest.
No,2 – Apply 2.5 – 3.75 kg Triple 14 (complete or 14.14.14) before flowering.
No.3 – When the fruits are chicken egg-size, apply 2.5 to 3.75 kg of a mixture of 2 parts Nitrogen, 1 part phosphorus and 4 parts potassium.
No.4 – Two months before harvest, 5-6 month old fruits, apply 2-3 kg Muriate of Potash (0-0-60) per tree.
Try this out. Maybe, you should test the fertility of your soil and adjust the rates of application accordingly. Make your own experiment. Just make sure you record your treatments and those that work should guide you.