Be Careful With Phosphorus

— Written By Bill Lord and last updated by Margaret Green

The middle number in the bag of 10-10-10 fertilizer you buy for your garden indicates the percentage of phosphorus in the fertilizer. Phosphorus has the chemical symbol of P, thus the bag of 10-10-10 has 10% N, P, and K, meaning nitrogen, phosphorus, and potassium. A 50 pound bag of 10-10-10 contains 5 pounds of phosphorus. Phosphorus is an essential element, vital for human life as it is a constituent of ATP, RNA, and DNA, some of the basic building blocks of life. Phosphorus has an interesting history in North Carolina, since we have one of the largest phosphate mines in the world in the eastern part of the state, and we have at least 2 river systems that have been declared ‘nutrient sensitive’ due to problems in the rivers in part due to phosphorus pollution.

Potash Corp. is located in Aurora, N.C., where they operate a large phosphate mine (see: According to their web page: Phosphate is “Made from ancient marine fossils, phosphate rock is combined with acids for use in fertilizer, feed and industrial products. By aiding in photosynthesis and cell division, phosphate fertilizer speeds crop maturity and increases yields. As a feed supplement, phosphate is necessary for skeletal development and aids in animal muscle repair.”   Many North Carolina elementary school students have made a trip to Aurora to hunt for marine fossils in the piles of phosphate ore the Potash Corporation dumps in downtown Aurora for students to dig through. The state port in Morehead city has been in the news recently due to problems with shoaling of the shipping channel in and out of the port. One of the largest shippers and customers of the state port is Potash Corp. The storage facilities at the state port used for phosphate storage are in plain view on the drive from Morehead City to Beaufort on US 70 highway.

In a previous article we discussed the role of nitrogen in agriculture and the environment, noting that nitrogen has a negative charge like the soil and is very prone to leach out of the soil into our water supplies. Phosphorus is very different from nitrogen in that it does not occur in its elemental form in nature and is usually bound up in other complex molecules. When phosphorus fertilizer or animal waste containing phosphorus is applied to the soil the available phosphorus is quickly bound to the soil. Soil-bound phosphorus is slowly released in the soil and is used by growing crops. Phosphorus is tightly bound to soil that contains iron, aluminum and clay. Soils in Franklin County and the North Carolina Piedmont have abundant red clay, and the red color comes from iron compounds (think rust). Less visible are high levels of aluminum in our soils and low pH levels of our native soils which make them acidic, facilitating chemical reactions that further tie up phosphorus in our soils.

Farmers can be frustrated by phosphorus. In cold, wet springs crop plants like corn can show phosphorus deficiencies such as yellow or purple stripes on seedling plants and slow growth. While soils may contain adequate phosphorus the cold soils inhibit plant root growth and the roots cannot reach available phosphorus. Since phosphorus rarely leaches from the soil it will build up over time, particularly in clay soils or sandy acidic soils such as those in the North Carolina coastal plain. Using an excess of caution, many farmers apply phosphorus every year to their crops even though their soils may be loaded with large amounts of phosphorus from previous applications of fertilizer or animal manure. One N.C. State University soil science professor used to say that there was enough phosphorus in the soils of the North Carolina coastal plain to last for 200 years even in no further phosphorus was applied by farmers.

Franklin County is bisected by the Tar River and the Tar has been designated as a nutrient sensitive river and has had regulations put in place to attempt to control the amounts of nitrogen and phosphorus entering the river. If phosphorus is held so tightly by the soil, how is it getting into the river? The answer is plain to see in the muddy waters of the Tar. When soil is washed into our streams and rivers the soil carries with it all the compounds and molecules that are attached to the soil particles, including phosphorus. The Pamilco Tar River Foundation once estimated that 178,000 tons of soil are washed into the Tar River each year, and with that soil comes phosphorus. The problem with phosphorus entering waterways is that water is very sensitive to phosphorus pollution, much more so that nitrogen. Even small amounts of excess phosphorus can lead to algae blooms in the river and estuaries and lead to fish kills and other water quality problems. Another source of phosphorus entering the river system is discharges from municipal and industrial waste water discharges. However, waste water dischargers are heavily regulated and monitored and have greatly reduced phosphorus and nitrogen releases to the river in recent decades.

The best way for farmers and homeowners to avoid causing problems with phosphorus is to be careful with it. Other states are taking action to limit homeowner access to phosphorus. In the states of Wisconsin and New Jersey homeowners cannot buy phosphorus containing fertilizers. Thus the former bag of 10-10-10 will now read 10-0-10 in those states. Many counties in Florida have banned the sale and application of fertilizer to home lawns in the summer months to avoid problems with runoff of nitrogen and phosphorus. These actions are being taken because of documented problems with algae blooms in waterways and water supply sources.

In North Carolina the best strategy to avoid causing problems with phosphorus is to take a soil test from your yard, garden, or farm field before deciding what analysis of fertilizer to use. Many soils in North Carolina have large reserves of phosphorus and will not need more applied, thus saving money for the applicator and protecting the environment. A second way to protect our rivers and estuaries from phosphorus pollution is to avoid soil erosion. Farmers have been assisted by the Natural Resources Conservation Service for many years in installing soil conservation practices. Homeowners should practice good soil conservation practices too to keep soil in place in yards and gardens to avoid water pollution. Remember, our streams and rivers should run with clear water, even after heavy rains. When was the last time the Tar River ran clean after a heavy rain? We have work to do.

Feel free to visit your local Franklin County Cooperative Extension Office at 103 South Bickett Blvd., Louisburg, NC. You can reach us at 919-496-3344. To stay up-to-date on events and activities, don’t forget to visit us on Facebook.