In the Western Lake Erie Basin, the Saginaw Bay, and in other watersheds of Michigan, effects of high levels of dissolved phosphorus have become a topic of concern. In fresh water, high levels of phosphorus often lead to impacts like harmful algal blooms that can taint drinking water and recreational waters with toxins. But where did all this dissolved phosphorus come from?
That is the question researchers have been mulling over to help us reduce our phosphorus losses. One of the theories proposed is that the reduction in acid rain has increased the pH of the soil making it more available for plant uptake. Chad Penn, a soil chemist with the Natural Resources Conservation Service (NRCS) Agriculture Research Service, recently confronted the popular myth that the reduction in acid rain has contributed to the rise in dissolved phosphorus in the region.
In Penn’s 2019 paper, he clears the air by acknowledging the seed of truth that dissolved phosphorus concentration can change with pH and pushes back against the false premise being promoted with this question, “First, let me ask you this: is there any evidence that farmers allowed their soil pH values to slip down to 5 during the 80s and early 90s? Because that is the only possible way that a decrease in acid rain could have contributed to increased dissolved P losses that we currently observe. That is, if the soils of 1990 were of pH 5, and then in the 2000s, they were brought up to a pH of 7 – I have found no evidence that that happened.” You can listen to Penn talk about this more in the podcast below.
I recently had the chance to sit down with Penn to talk about phosphorus loss from fields. He highlighted how fields with high soil phosphorus test levels can not only leak that phosphorus as dissolved phosphorus, but also as phosphorus that is bound to soil lost though erosion. There are several best management practices that can be put into place to capture these phosphorus losses; however, Penn rightly points out that not putting the phosphorus on in the first place is more economical than trying to recover it.
Penn also spoke about his research on ideal phosphorus levels and yield. He points out that if soil test phosphorus shows that there is enough phosphorus in the soil then a pop-up/starter phosphorus application is not necessary. He suggests fertilizer applications that take into account the uniqueness of the fields, soil types, and management practices (tillage, cover crops) because some soils have more of an ability to continue to deliver phosphorus into solution than others. Embracing a more wholistic approach through soil health testing including soil type, pH, organic matter, minerology, and texture will support better nutrient recommendations.