Commercial fertilizers are denoted by three numbers indicating the percentage by weight of N (nitrogen), P (phosphorus) and K (potassium) in the fertilizer. These are the three main macro-nutrients needed by plants. Phosphorus is believed to promote the development of roots, flowers and fruits by enabling plants to store and transfer energy. However, its use in lawn and turf is hotly contested by those concerned with its environmental impact on lakes and ponds.
If soil lacks phosphorus, plants grow slowly; if phosphorus is added, they may grow normally. The added phosphorus does not stimulate growth, it restores it. Soils most likely to need phosphorous are sandy or acidic soils and those that are used heavily to grow agricultural crops. Reddening of leaves, a classic symptom of phosphorus deficiency, may also be caused by hot sun, cold temperatures, drought and insects.
Commercial fertilizers contain forms of phosphorus extracted from rock phosphorus by acids or by heating in furnaces. Phosphorus from powered rock phosphorus was formerly used, but is less available to plants. Inorganic fertilizers contain agents that dissolve phosphorus in water, making it available to plants.
Water-soluble phosphate is that percentage of phosphorus in a fertilizer that dissolves in water. The phosphate that does not dissolve in water is placed in a solution of ammonium citrate; the amount that dissolves there indicates its level of citrate-solubility. The total of water solubility plus citrate-solubility is the percentage of phosphorus available to plants and is listed on the label.
Organic growers can best obtain phosphorus from feedlot compost, dairy manure or vermicompost, the manure of compost worms. Researchers at Colorado State University say manure or compost has less environmental impact than commercial inorganic sources of phosphorus. Applying mulch on landscapes may provide garden and landscape plants with adequate amounts of slow-release phosphorus as a product of natural decomposition.
To accept the maximum phosphorus, soil pH should be between 6 and 7. Horticulturalists at the University of Minnesota report that plants use the same amount of phosphorus whether it is applied dry or in a liquid form; either inorganic or organic sources of phosphorus may be used.
Too much phosphorus causes leaves to turn yellow, called chlorosis. Leaf chlorosis is a classic symptom of iron deficiency. When phosphorus competes with iron, the roots take up less iron. The result is that an over-application of phosphorus may cause leaf chlorosis.
Horticulturalist Chalker-Scott of Washington State University says ammonium nitrate fertilizer is in most cases adequate for transplanting. If adding nitrogen does not relieve a perceived nutrient deficiency, she says gardeners should apply phosphorus to the leaves to see if that relieves the symptom, a precaution to prevent the addition of excessive nitrogen to the soil.
Plants can use only so much phosphorus. Excess phosphorus binds with soil particles; runoff from irrigation and rain washes it from lawns, gardens and fields into lakes and ponds where it causes excessive growth of algae resulting in smelly, green water and dead fish.
Minnesota and Maine prohibit the sale and use of lawn and turf fertilizers containing phosphorus. Several Michigan and Florida counties and more than 100 New Jersey municipalities have similar prohibitions.