Tropical hibiscus generally produce more blooms when given regular feedings of fertilizer. The best fertilizer mixes for hibiscus rosa-sinensis varieties are fairly low in phosphorus. That is the “middle number” of the markings on most fertilizer packages, so look for a mix that has proportions something like 20-10-20.
A common saying among gardeners is “Plants don’t eat, they Drink!”. This is important to consider, because plants can only absorb nutrients available in liquid form. Dry fertilizer is useless to plants, until it has been “watered in” and dissolved into damp soil that the roots can access.
Hibiscus in particular require a lot of water, which may sometimes not only dissolve nutrients, but can also wash them right out of the soil in potted plants. For this reason, potted plants often do better with regular small doses of fertilizer.
Early in the growing season, a little higher nitrogen in the mix may promote more stem and leaf growth, producing a larger plant to bloom later in the year. Late in the season, nitrogen may not be needed as much, so a blend similar to 15-10-15 may work well. Hibiscus also need a variety of trace minerals, so consider a supplement if the fertilizer package does not list trace elements.
A good soil with lots of organic matter will generally provide much of the nutrients needed for plant growth, but for best blooming performance, additional fertilizer is usually needed. Healthy, well fertilized plants are also easier to propagate, and more resistant to insect pests and fungus.