UREA on Citrus Plants

My gardening SIFU said that citrus plants need UREA to flower and fruit. I have been vexing over why my limes (big green lime, yellow lemon and limau kasturi) are not producing flowers and fruit. So decided to try UREA. Bought a pack 2 weeks ago and applied some on all of my citrus plants. This morning, in one of my pot of limau kasturi, I saw flower buds! Is it due to UREA? Not too sure because the other ones are still barren. I will need to re apply once again to see if the others will give me some flowers.

My yellow lemon is still barren though its leaves are beautifully green!