(Edit: My first attempt at an answer had the wrong nutrient... thanks to @kevinsky for the correction.)
Phosphorus is the key nutrient that promotes flowering.
Except that plants can't use pure phosphorus (not to mention that pure phosphorus is dangerous!), they need it in the form of phosphate (which actually describes any of a handful of different chemical compounds). Some liquid fish/kelp fertilizers are high in fast-acting phosphorus, but be careful not to overapply.
Plants can't take up phosphorus as well if the pH is too low or too high. Aim for slightly acidic -- which is what many plants prefer anyway.
In my opinion, the best approach would be to use limited quantities of fast-acting soluble phosphate like fish emulsion if you have an immediate deficiency, but to aim for a longer-term prevention of deficiency by (a) maintaining pH and (b) using an organic long-lasting, slow-release phosphate source like rock phosphate or bone meal.