Phosphorus is one of the at least 18 essential nutrients that plants need to grow. Without phosphorus, a plant cannot complete its life cycle. In plants, the most important functions of phosphorus are energy storage and transfer, regulation of protein synthesis, roots development, seeds and fruits formation. Basically, phosphorus is important for every metabolic reaction in plants. Sufficient phosphorus availability can strengthen structural tissue such as wheat straw and tomato stems. When deficient in phosphorus, plants look stunted in growth, and often show an abnormal dark-green or reddish-purple color that first shows up in older leaves.

Reddish-purple color of canola P deficiency symptom that show up in older leaves first. Photo by Haiying Tao

The abnormal color normally shows up early in the spring and may disappear when weather warms up. Note that reddish-purple color of plant is not always an indicator of phosphorus deficiency. Other stresses, such as cold, insect, herbicide injury can also cause reddish-purple color. Some plant cultivars have been bred for purple or reddish foliage as well.   

Reddish-purple color of corn P deficiency symptom that show up in older leaves first. Photo by Haiying Tao

Plants take up phosphorus via the roots from soil. Native soils contain phosphorus but the amount of phosphorus that is available for plants varies from soil to soil. Application of fertilizers and soil amendments that contain phosphorus, return of grass clippings or other plant residue, like leaves or through compost, can also introduce phosphorus to the soil. To verify if your soil is sufficient in phosphorus for plants, you can send your soil samples to University of Connecticut Soil Nutrient Analysis Laboratory (UConn-SNAL) for testing. The lab provides guidance on how to take soil samples for phosphorus testing as well as collecting and shipping soils to the lab. Fertilizer and limestone recommendations are made based on soil test results as long as the crop is provided. Such information can be found at https://soiltest.uconn.edu/.

The functions of phosphorus in the plants cannot be replaced by any other nutrient. Therefore, if your soil test results indicate that your soil is deficient in phosphorus, you should apply fertilizers based on the recommendations provided by the UConn-SNAL. There are many types of phosphorus fertilizers available on the market, but they vary in their nutrient analysis depending on the source. This information would be found on the label of the containers or bags. It is important to read the label of the fertilizer that you purchased and calculate out the right amount of phosphorus to apply so that enough phosphorus will be available for plants but not overapplied. Excessive phosphorus application to soils can potentially cause phosphorus pollution to the environment. Phosphorus can be lost from the soil via leaching, subsurface runoff, and surface runoff. And the loss of phosphorus can increase dramatically if soil test values are higher than the soil’s capacity to hold on to this element.

Phosphorus is the number one source of inland freshwater pollution in Connecticut. Our native soils are low in phosphorus so any phosphorus entering water bodies comes from human activities – mostly overapplying this element whether through fertilizers, manure, composts, or other sources. Nutrient pollution is one of the most widespread, costly, and challenging problems in our environment. When phosphorus enters waterbodies, it supports fast growth of algae and other aquatic plants, to levels that exceed the capacity of our ecosystems to handle. The large growths of algae (called algal blooms) and other aquatic plants can cause degradation of ecosystem services of waterbodies, such as reduced water quality for recreation, unsafe drinking water quality, illness or killing of fish and other aquatic life.

Algal bloom in wetlands below athletic field in Amherst, MA. Photo by D. Pettinelli

For commercial or home garden crops, the best way to apply phosphorus fertilizers is to band (place) it close to the seeding or seedling row but not in the row. Phosphorus moves very slowly in the soil, which means only the phosphorus that is near roots will be taken up by plants. So, banding near the seeding row or next to transplants can ensure accessibility of phosphorus to the roots and can double the phosphorus use efficiency compared with just broadcasting phosphorus throughout the soil.

Avoid applying fertilizer along with seeds in the seed rows or in planting holes for transplants as the fertilizers may hinder germination and damage seedlings. For perennials, like lawns, perennial beds and shrub plantings, surface broadcast applications are most common. Since phosphorus legislation was passed in Connecticut around 2013, the use of fertilizers containing phosphorus on established lawns has been banned unless a soil test within the last two years recommends this element. Phosphorus can be used when seeding, sodding or over seeding lawns, however. For those unaware of the phosphorus legislation, please read ‘Your Lawn and the New Phosphorus Legislation ( https://soiltest.uconn.edu/documents/Phosphoruslawlawnstmclean.pdf)

If you have questions about phosphorus and lawn applications, feel free to contact the lab at (860) 486-4274.

Haiying Tao Ph. D, UConn Assistant Professor

Soil Fertility and Soil Health