The next most important element after nitrogen, phosphorus is needed in smaller quantities (approximately about one tenth of the amount). Phosphorus, or phosphate, is mainly responsible for good root growth, so a deficiency causes slight stunting of the plant. It can be diagnosed by a distinct blue colour, which affects the older leaves first. Sometimes the leaves darken and develope a blue and green tinge. In addition, the plants root system will be under developed.
Phosphorus is vital to the growth and health of plants. It assists in converting the sun's energy and other chemicals, such as nitrogen, into usable food for plants. A phosphorus deficiency will lead to stunted, sickly looking plants that produce a lower quality fruit or flower.
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.