Any gardener who has designed a fertilization program for homegrown plants is likely aware of the wide range of materials available to gardeners for use as a fertilizer. Each fertilizer has specific advantages and disadvantages, so it can sometimes be prohibitively confusing to choose a fertilizer that suits the needs of your plants. Phosphate fertilizers, for example, carry with them specific benefits and risks. Understanding these risks is key to avoiding problems related to improper use of phosphate fertilizers.
One of the principal ways in which fertilizers differ from one another is in the materials they use to deliver nutrients to plant soil. A phosphate or phosphorus fertilizer is one that delivers higher concentrations of phosphorus than of other plant nutrients such as nitrogen and potassium. Some common organic sources of phosphorus used in fertilizers include superphosphate, concentrated superphosphate, monoammonium phosphate, diammonium phosphate (DAP), ammonium polyphosphate (APP) and rock phosphate.
Overfertilization is a major and common problem for phosphate fertilizers. Many gardeners mistakenly add phosphate fertilizers to soils that they believe to be deficient in phosphorus when nitrogen deficiencies are a much more common cause of nutrient deficiency symptoms in plants. In these cases, an overabundance of phosphorus in the soil not only fails to resolve the nutrient deficiency problem but can actually make matters worse by causing leaf chlorosis and harming beneficial microorganisms living in soil.
Another major disadvantage of phosphate fertilizers is the high probability of water pollution. Phosphorus that makes its way into soil via phosphate fertilizers and binds tightly to soil particles is unlikely to move out of the soil. But if too much phosphate fertilizer is applied to soil, excess phosphorus can easily find its way into water systems via storm drains and plumbing. Grass clippings and leaves from plants that have been grown in soils with an overabundance of phosphorus will release their phosphorus into water, leading to algae and bacteria outbreaks and water contamination issues.
In summing up her research on problems associated with phosphate fertilizers, Linda Chalker-Scott of Washington State University remarks that "excessive use of phosphorus in landscapes is a resource-wasteful, ecosystem-damaging practice." The operative word of her statement is "excessive." It is important to understand the problems that can sometimes result from the use of phosphate fertilizers, but using phosphate fertilizers properly can be a beneficial and safe way to improve soil fertility. Only apply phosphate fertilizers when they are needed as indicated by soil fertility tests and always follow the manufacturer's labeled application instructions exactly.