Like people, plants need a balanced diet of certain nutrients or they do not thrive. Phosphorus, or P, in the form of phosphate, is taken up by plants from the soil. Along with nitrogen, or N, and potassium, also called K, phosphorus is classified as a plant macronutrient because of the relatively large amounts of the nutrient plants need. All three nutrients are important to plant growth and are generally added to soils in fertilisers.
Unless a phosphorus-deficient plant is next to healthy plants, the deficiency might not be easy to recognise. According to Arizona State University, a phosphorous-deficient plant usually has stunted growth and is thin-stemmed and spindly. Its foliage often turns a dark green or, on older leaves, reddish-purple or bronze. In severe cases, phosphorus deficiency causes yellowing, quick ageing and early drop off of leaves. Phosphorus deficiency causes sugar to accumulate in plants, which, in turn, increases anthocyanin plant pigments, producing the foliage's reddish-purple colour.
Common causes of phosphorus deficiency are cold, wet soils, such as those of early spring, acid or very alkaline soils and compacted soils that prevent plant roots from drawing up nutrients.
According to the University of Montana, sufficient phosphorus in plants stimulates early plant growth and hastens healthy maturity. It enhances photosynthesis, nitrogen fixation, flowering, fruiting and seed development. Phosphorus also increases root development.
Excess phosphorus in soil is usually identified through the discovery that plants are unable to take up other elements from soil, particularly iron and zinc. Iron deficiencies are characterised by yellowing between leaf veins, called chlorosis. Tissue bleaching is a symptom of zinc deficiency. High soil phosphorus levels also threaten water quality for humans. According to Texas Cooperative Extension, excess phosphorus is caused by overuse of inorganic fertiliser or the use of composts and manures high in phosphorus.