Depending on your age and your interests, a mention of the color purple brings to mind rain and the untimely passing of Prince, a book that had a real impact on your life, or your sexual orientation (the last more related to lavender than purple).
In this day and age, almost no one associates the color purple with phosphorus deficiencies in plants. Back in the day, however, purple leaves were a sure sign of insufficient phosphorus for our crops or exotic tropical plants in the home. Phosphorus (P) is considered to be a macronutrient in plants, required in concentrations almost as high as nitrogen. If you buy synthetic fertilizer, P is the middle of the three numbers listed with nitrogen being the first (N) and potassium (K) the last.
Whereas nitrogen comes from the air (77% of our atmosphere is nitrogen gas), phosphorus comes from rocks. Nitrogen is there for our taking in an effectively infinite supply — providing we spend the energy to convert it from gas to a mineral form. Phosphorus on the other hand will last only as long as the calcium phosphate mineral supply does. There was a recent scare that we were past peak phosphorus, i.e., that our mineral supplies were running short. That scare has been put on hold for the time being. But the majority of the remaining mineral sources of P are in deposits in Morocco, followed by Saudi Arabia and China. These newfound sources in troubled areas have scientists calling out the importance of recycling P: Using P-rich residuals such as animal manures, municipal biosolids and food scrap composts as P fertilizers instead of rock phosphates.

Feast Or Famine

Our history with phosphorus has been one of feast or famine. Way back in the time of purple leaves, when the creator of Purple Rain was still in diapers, soil scientists were realizing that only a small fraction of the total P added to soils was making it up to the plant. Soils were places where P could go to hide. Even if you added enough for the plant, the soil would sequester the P and your plants would end up purple. In acid soils, P would bind to iron and aluminum oxides and stick like glue. In higher pH soils, calcium grabbed onto the P. Only at the perfect pH of 6.5 did plants have a chance to outcompete the soil surfaces.
Extension agents reacted to this by telling farmers to just add more. If you just kept adding P to the soil (i.e., feast), you would be sure that your plants would be green and tall. There were soil tests to determine if your plants would need P — and these were designed with a hungry plant in mind. The two most famous tests, Olsen and Bray, were attempts to mimic how P-hungry plants would behave in acid or calcareous soils. When I started in graduate school this was the paradigm.

P On The Move

By the time I finished graduate school, the paradigm was beginning to crumble. It turns out that not all soils have enough iron and aluminum on the acid side, or calcium on the basic side, to hold onto all of the P that had been added to them. Studies had just started to show that the impossible was happening: P was starting to move through soils to groundwater. Nitrogen was the nutrient that leached — never phosphorus. But here were sites where P movement was not only detectable, it was happening with enough regularity to pose an environmental concern. If too much P ended up in phosphorus-limited waters, the same algal blooms that were typically attributed to N would occur.
I was in graduate school in Maryland where excess P from the sandy soils on the Eastern Shore had started to leach P into the Chesapeake Bay and compromise water quality. The pendulum started to shift back to famine in the scientific community. A USDA research group was formed to share research to address this new surfeit of P. New soil tests were developed to predict how much of the too much P that had been added to soils had the potential to do environmental harm. Instead of trying to mimic hungry plants, these tests were much less aggressive. Water soluble P, iron strip P, and the phosphorus saturation index are examples of these new tests. Each is designed to estimate how much of the excess P that’s been added to soil has the potential to move down the soil profile. The SERA 17 website has some excellent publications on how to test for excess P in soils.
Concerns about excess P have now made their way into the regulatory community. Rules are setting limits, saying that no extra P can be added to soils if they already have enough or too much. These are in place in the Great Lakes region, Florida, the Chesapeake Bay watershed, and are pending in New England. This has enormous potential to impact residuals based soil amendments.
In the biosolids community, P-based application rates have replaced N-based recommendations. Because there is a history of over application of P fertilizers (as per recommendations), this can effectively make land application rates so low as to be impractical. In certain cases, where regulators are saying “no more P,” it can also rule out use of composts that have measurable P concentrations. In cases where added P has been banned, that means no biosolids, no compost and no manure.
These regulations are a well-intended response to the environmental threat of excess P. But they are also short sighted and not always grounded in good science. At the same time we are faced with the P famine brought on by these new rules, we are facing an actual global P famine brought on by the depletion of P reserves in nations that are reliable business partners. That makes for a bit of a conundrum. If we recycle P, we have to use the P-rich residuals. But if we are prohibited from using extra P, that means we can’t use these materials for their N. We can’t have one without the other — residuals based N always come with P. Even struvite, the crystalline fertilizer from wastewater, has equal amounts of N and P.
What is critical to talk about, test for, and describe is that the P in these materials is typically much less available to plants and to waters than P added as synthetic mineral fertilizers. Certain biosolids have enough iron, aluminum and/or calcium to bind the P for multiple growing seasons. Even in low mineral composts, the P is present as part of the organic matrix and is only slowly available. While adding these materials to soil may not provide immediately available P for plants, it will increase soil reserves and improve long-term access to phosphorus.
It is also a lot better than misusing or disposing of this phosphorus and the organic matrix that it comes in. If excess P is truly a concern and a reason to ban use of synthetic P fertilizers, we can safely continue to use P in residuals by adding more binding power to them or to soils. Water treatment residuals, an underutilized material, have been shown as an effective tool to limit P solubility. These can be added to composts and biosolids as insurance that the added P won’t do environmental harm.
The appropriate way to insure that our current famine for use of P fertilizers does not end up banning the use of organics for soils is to test appropriately and blend products that recycle nutrients in a responsible way. I prefer purple in my pants, not my plants. With reasoned policies, we can keep the purple rain on our Spotify lists and out of our waters.
Sally Brown is a Research Associate Professor at the University of Washington in Seattle and a member of BioCycle’s Editorial Board.