Phosphorus and other nutrient pollution is an increasingly important topic that is being addressed in different ways. Phosphorus was largely responsible for the algae blooms seen in the 1960's and 1970's in Lake Erie and is responsible for the blooms we are seeing today. It is also a factor in saltwater and brackish water bodies where nitrogen is more of a limiting nutrient.
There are two types of phosphorus that can cause problems in water bodies: particulate phosphorus (PP) and dissolved reactive phosphorus (DRP). Both of these can come from non-point source pollution (to which agriculture belongs), and both contribute to eutrophication. However, they are not equal. Put simply, PP is phosphorus that is attached to a soil particle, while DRP is soluble phosphorus that has been dissolved in water. In this article, I will describe both types in more detail, how they are released in the environment, and their effect in a water body.
PARTICULATE PHOSPHORUS
PP is the most common form of phosphorus found in water bodies and can come from multiple sources. It is generated by sewage plants, erosion, industrial waste and by natural processes. It can be present in both an organic (attached to biological particles) and inorganic (not attached), though for the purposes of this article this distinction is not important. In terms of agriculture, the largest source of PP is erosion (Minnesota Pollution Control Agency, 2007). Phosphorus generally bonds very tightly with soil particles and therefore does not easily dissolve (Uusitalo et al, 2001). It can then be lost when soil is washed away, making it the most vulnerable during heavy rain events and snow melt.
Since it is tightly bonded with a soil particle, it is also not as bio-available as DRP once it has entered a water body. I recently wrote an article on legacy loads in terrestrial and aquatic environments, and PP makes up the portion of the phosphorus that "sticks around" in sediments. Under calm and normal conditions, a low percentage of PP is available to plant life in an aquatic environment, but there are a number of activities that can cause it to become more bio-available (Reynolds and Davies, 2001). The process by which this happens is through conversion to an orthophosphate, which is very bio-available (Reynolds and Davies, 2001). Agitation, biological processes, and chemical changes in the water will all release the phosphate, but the most dangerous process is during a eutrophication event. If an anoxic (low oxygen) zone develops near the bottom, the increased acidity of the water accelerates phosphate conversion a lot, thereby enhancing the conditions that caused the eutrophication in the first place (EPA, 2012).
DISSOLVED REACTIVE PHOSPHORUS
DRP is less common than PP but can come from some of the same sources, as well as some others. It is also generated by sewage plants, industrial waste and natural processes, but is also generated by some water treatment strategies, fertilizer use, and detergents.
Historically, DRP was not produced en masse in agriculture because, as stated above, phosphorus bonds readily with soil particles (Mallarino and Haq, 2015). However, the advent and overuse of commercial phosphate fertilizers has brought agriculture into the equation. Many fertilizers use orthophosphate or another stable type of phosphorus because they are easily absorbed and used by the plant. Even unstable varieties, such as polyphosphates, will convert to orthophosphates in water (EPA, 2012). Now, whether that plant is in a field or a water body does not matter. Therefore, if the phosphorus is lost (by dissolving in solution and running off) it can be easily used by the algae, plankton and plant-life wherever it ends up. It is really quite simple: the only reason DRP is an issue in agriculture is because we have used so much that it cannot bond fast enough to soil particles to avoid being washed away. This lines up perfectly with our knowledge that dry fertilizers are inefficient and therefore need to be used in massive quantities to provide any benefit.
How can a farmer address phosphorus loss? DRP is clearly the more urgent type to deal with, but we cannot ignore PP either. Both can be understood from an economic perspective as well: any lost fertilizer is wasted capital. Building up phosphorus in the soil can be a good thing, but only so long as we can prevent it from eroding away. The simple solution is to just use less fertilizer. If we have gotten this far while losing so much to waste, why not cut back? Since it is so reactive, DRP can be addressed quickly and with relatively fast results. If we do not saturate the soil with phosphorus it will bond with soil particles and stay in the soil. There is a reason DRP loss was not an agricultural issue pre-commercial fertilizer. The same processes that release PP from sediments will also act in the soil, releasing it for plant use over time. We can then address erosion. This problem has been around as long as agriculture, but that doesn't mean it is a given. Properly aerated soil (say, with lime) that is loose throughout its profile will disperse and hold water throughout the profile. Smart tillage practices and timing can also help or hinder with erosion.
In my opinion, dealing with phosphorus loss is not a complicated issue. It is, however, a controversial one. On one side, government and the public looking for solutions to their increasingly foul waters, and on another companies and interest groups looking to sell more fertilizer. As with all agricultural issues, the farmer who thinks critically for himself and applies that thinking to his operation will be able to deal with whatever comes his way.
Mallarino, A.P., Haq, M.U. (2015). Phosphorus Loss with Runoff After Applying Fertilizer of Manure As Affected by the Timing of Rainfall. North Central Extension-Industry Soil Fertility Conference. Retrieved from http://tinyurl. com/ycc2e4dz
Minnesota Pollution Control Agency, (207). Phosphorus: Sources, Forms, Impact on Water Quality. Retrieved from http://tinyurl.com/hyydul8
Reynolds, C.S., Davies, P.S. (2001). Sources and Bioavailability of Phosphorus Fractions in Freshwaters: a British Perspective. Biological Reviews of Cambridge Philosophical Society, 1. Retrieved from http://tinyurl.com/ycorohjg
United States Environmental Protection Agency, (2012). Phosphorus. Retrieved from http://tinyurl.com/ybk87zbw
Uusitalo, R., Turtola, E., Kaupilla, T., Lilja, T. (2001). Particulate Phosphorus and Sediment in Surface Runoff and Drainflow from Clayey Soils. Journal of Environmental Quality, 2. Retrieved from http://tinyurl.com/yab679p7
This is an excerpt from the Early Fall Growers Solution (2018) written by Zach Smith.
Signup for our newsletter to stay in the loop
