Most multinutrient fertilizers contain phosphorus, the quantity of which is indicated in the second of three numbers printed on the fertilizer packaging. Phosphorus plays many roles, affecting the growth of your plants, the fertility of your soil and the health of your local ecosystem.
Phosphorus is a primary macronutrient, meaning that plants require it in relatively large quantities in order to survive. According to the Penn State Cooperative Extension, DNA contains phosphorus, making the element essential for plant reproduction and the development of seeds and fruits. Plants also use phosphorus during energy transfer processes needed for healthy growth.
Clint Waltz, a turfgrass specialist with the University of Georgia Extension, explains that when phosphorus is applied to the soil as fertilizer, it attaches to soil particles, making it available for plants to use. Phosphorus is soluble and moves easily with water. Plants absorb phosphorus through the tips of their roots, and it travels first to the locations where it is most needed in the plant, especially new leaf growth.
Observing phosphorus-deficient plants demonstrates the importance of phosphorus to plant health. The most distinctive symptoms of phosphorus deficiency are leaves that have a purplish tint, especially around the leaf veins. Phosphorus-deficient plants show stunted growth and poor root development. Because of the way that phosphorus moves through the plant, targeting areas where it is most needed, the first signs that you may need to provide phosphorus fertilizer will occur in older plant growth.
Some types of phosphorus fertilizer may also affect the pH of your soil, which can have further implications on plant health. As Waltz explains, monoammonium phosphate and diammonium phosphate fertilizers have a slight acidifying effect on soils, which can cause excesses or deficiencies in other mineral nutrients if the soil is already strongly acidic. Superphosphate fertilizers are called neutral fertilizers because they do not have any effect on soil pH.
Phosphorus fertilizers also play a negative and unintended role in the health of local ecosystems. Phosphates are a major pollutant in surface waters, where they do exactly what they do when applied to terrestrial plants: They encourage growth. The algae and pond weeds that thrive in ponds and lakes polluted by phosphorus choke out all life, blocking sunlight to organisms that live beneath the surface, and increased microbial activity depletes the water of oxygen. According to the Colorado State University Extension, because phosphorus adheres to soil particles, most pollution occurs through eroded soil and leaf and lawn clippings left on street surfaces, where the phosphorus they contain has no soil to stick to and runs off into stormwater drains.