With a strange verdict by a jury in San Francisco, it became open season on glyphosate, a common weedkiller in use for generations. One new claim is that it leads to higher phosphorous levels.

Yet glyphosate only contains trace nutrients, nothing like what fertilizer has. Overuse of phosphorus-based fertilizer in some areas have led to a saturation of the soil’s capacity to hold the nutrient, which increases the likelihood that any additional phosphorus applied to the land will run off into waterways and cause of harmful algal blooms and deoxygenation leading to fish death.

Roundup as a source of phosphorous has received little attention in the past because it is in defiance of the principles of chemistry. That's where the a new version of bioaccumulation gets invoked. The authors of a recent paper believe it is building up over time, just like fringe biologists believe about trace chemicals in our bodies. They charge that the herbicide’s relatively small phosphorus content starts to add up, reaching levels comparable to other sources, like detergents, that have attracted regulators’ attention in the past.
To bolster their claim, they used data from the US Geological Survey and the UN Food and Agriculture Organization to calculate the relative contributions of glyphosate and fertilizers to the total amount of phosphorus being applied to agricultural land in the United States and elsewhere. They then gathered existing studies that supported thir claim about the mechanisms by which glyphosate contributes to phosphorus levels in the soil and phosphorus outflows to waterways.

Citation: Hébert MP, Fugère V, and Gonzalez A., 'The overlooked impact of rising glyphosate use on phosphorus loading in agricultural watersheds', Dec. 5, 2018. Frontiers in Ecology and the Environment. 17: 48–56.