What is Phosphorus Fertilizer?

All plants require phosphorus to grow. Phosphorus fertilizer can be any fertilizer that contains phosphorus, or a fertilizer that has an especially high phosphorus content. Many US cities have banned or limited the use of high phosphorus fertilizer on residential lawns and gardens because its overuse is seriously damaging to lakes and rivers.

Most prepared synthetic fertilizers sold in garden stores are made up of a mixture of nitrogen (N), phosphorus (P), and potassium (K). The amount of each element in NPK fertilizers is indicated by numbers that refer to the percentages of the elements contained in the mix by weight, so a 4-3-3 NPK fertilizer is 4 percent nitrogen, 3 percent phosphorus, and 3 percent potassium. Different balances of elements are required by different plants, so NPK fertilizers are often marketed with labels like grass fertilizer, tomato fertilizer, or rhododendron fertilizer. In fact, it is the balance of nutrients that is important, not the specific plant type written on the bag.

Synthetic phosphorus fertilizer is generally made by chemically processing rock phosphate from the ground. Organic phosphorus fertilizer can be found in composted animal manure or in ground-up animal remains left over from food production and usually sold as bone meal. Unprocessed rock phosphate is also used for organic fertilizer, but it can take several years to break down enough for plants to be able to access the nutrients.

Phosphorus is essential to plant growth because it plays a role in photosynthesis, cell division, and the plant's ability to use sugars and starches. Depending on soil conditions like pH, temperature, and other available nutrients, plants are able to make use of synthetic or organic phosphorus to varying degrees. Apparent phosphorus deficiencies such as reddening of the leaves can indicate other deficiencies that are preventing the plant from using the phosphorus. Often, there is adequate phosphorus in the soil, but the plants are unable to use it because the soil pH is too high or the nitrogen content is too low.

Many gardeners believe that phosphorus makes roots grow. This is true to an extent, but without other nutrients, large doses of high phosphorus fertilizer do not have a significant effect on roots. Most of the phosphorus used residentially ends up getting leached out of the soil by rain and irrigation before the plants can use it. Overuse of phosphorus fertilizer is a serious threat to water quality because it enters waterways through the storm drains. Phosphorus can cause blooms of blue green algae and other unwanted aquatic plants that deplete the oxygen in water and compete with other wildlife.