It helps your plants produce the goods!
To make sure your plants get the proper amount of phosphorus, don't just pour the fertilizer into the pot. Mix it deep into the soil so the roots can reach it. Fertilizer sticks that can be placed near the roots or below newly planted seeds will also work.
If you're looking for good sources of phosphorus, check the ingredients of any plant food you buy. The "P" number of the "N-P-K" formula will tell you the percentage of phosphorus, by weight, in the mix. You should also look for ingredients like bonemeal, colloidal phosphate, or rock phosphate.
You may also see superphosphates, a more soluble form of phosphorus. Be careful with these: Overfeeding with superphosphates can actually create phosphorus deficiencies because they wash away too easily (the perils of a "quick fix").