Phosphorus is one of the three most important nutrients that plants require. Nitrogen, potassium and phosphorus are referred to as macronutrients because plants need large quantities of these vital nutrients. The main role of phosphorus in plant development is to aid in root, seed and flower production.
Unlike some other nutrients' roles in plants, phosphorus' roles in plants is well understood. In young plants, it provides nourishment to developing root systems. As the plant matures, it aids in the plant's reproduction by allowing seed and fruit development. Over the course of the plant's life cycle, phosphorus also helps to fuel reactions within the plant on the cellular level.
A lack of an adequate amount of phosphorus in developing plants is not always easily recognised. Some symptoms that may indicate phosphorus deficiency include stunted growth, spindly growth and weak or thin stems. Discolouration of foliage also may indicate a lack of enough phosphorus. A blue-green hue, yellow stems and leaves or a purple tint in a plant whose leaves should be green are often an indication that the plant receives too little phosphorus. Premature ageing of leaves, such as when they wither and fall off, which is known as leaf senescence, is also a telltale sign of phosphorus deficiency.
When more phosphorus is supplied to a plant than it can readily use or absorb into the root zone, the phosphorus becomes fixed in the soil and largely unavailable for future use. Fixed phosphorus is crystalline in structure and not as soluble as phosphorus that is provided as a solution or that exists in the active pool, which is the zone of soil around a plant's roots that contains soluble phosphorus.
The use of phosphorus as a fertiliser has led to widespread concern. Excess phosphorus that is not absorbed by plants can become runoff into neighbouring lakes and streams. The resulting algae bloom that can occur wreaks havoc on aquatic ecosystems by creating an imbalance of nutrients. The ensuing decline in water quality is detrimental to many fish and other marine life. As a result, many fertiliser manufacturers have reduced or eliminated the amount of phosphorus in their fertiliser blends.