Phosphorus -- a blessing and a curse
Phosphorus is a blessing and a curse. On the one hand, the potential for world- wide reserves to run out of this fertilizer has been the subject of numerous articles forecasting doom and starvation. On the other hand, too much of this fertilizer has damaged water quality when heavily fertilized soils erode into lakes and streams. For biosolids, the forecast has generally been a combination of confusion and doom. Biosolids are typically applied to meet the nitrogen needs of a crop. Crops also need P, but a lot less of it than N. According to one of the papers in this month’s library, the right ratio for plants is 8 N to 1 P (Schroder et al.). The biosolids used in that study had a ratio more like 1:1. Typical biosolids contain 5% - 7% N and 1% - 2% P -- still more P than plants use. This ratio is further complicated by what P does in biosolids and in soils. If you add 10 units of P, the plant will be able to see anywhere between 0 and 10 units of P depending on a combination of soil properties and biosolids properties. In soils, phosphorus binds strongly to iron and aluminum at mid to low soil pH. At high soil pH, phosphorus will bind to calcium. Optimum pH for plant available P is about 6.5. Phosphorus will cycle in soils between being bound to soil minerals and being part of organic matter. Traditional soil tests were developed to see if there were enough P for good crop growth. New soil tests have been developed to see if there is too much P (see the first article in the library). Concerns about P in biosolids have prompted extensive research on the behavior of P in biosolids and biosolids/soil systems. In fact, three previous resource libraries have focused on this topic (March 2007 and 2011 and August 2006). In those, you will find most of the classic work that shows that, in general, P in biosolids is much less available than P in synthetic fertilizers. Just how much P is available depends on a combination of biosolids treatments and soil factors. In all cases, the most significant threat to water quality is erosion of soil particles into fresh water bodies. A much less significant threat is leaching through the soil profile. Because of environmental concerns, biosolids is now regulated in many states based on P application rate and the P status of the soil. A recent article from the USDA news service previewed a paper about P availability in long-term biosolids amended soils (http://www.ars.usda.gov/is/AR/archive/jan13/biosolids0113.htm). The paper has not been published and is going through a second review partially as a result of concerns expressed by the biosolids community when the research was reported in a popularized USDA newletter. The library this month is focusing on P as a way to put that news article in context. It starts with a review paper focusing on the behavior of P in soils and the best ways to test for available P. The next paper, one of the first I read on this topic is by Mike McLaughlin who is now a soil chemist in Australia. For me, it is one of the clearest reviews of factors that influence the availability of P in biosolids and biosolids amended soils. The third paper is another oldie but goodie. McCoy et al. was one of the first papers to note that biosolids P was less available than fertilizer P. From there, I review a paper from Oklahoma on long-term availability of metals and biosolids for dryland wheat. Here the important thing to notice for both the P and the metals is how high the P and the Pb in the biosolids were; you would be hard pressed to find current day biosolids material so rich in P, and you wouldn’t find biosolids that high in Pb. Finally, if you are still concerned about P, the library ends with a paper on using water treatment residuals to reduce available P. For those wastewater agencies with a connection to their local drinking water plants, we have a possibility of collaboration, as water residuals can significantly influence P availability in soils and biosolids when the materials are blended together.
Dr. Sally Brown