As state officials develop new standards for the application of manure on farm fields, it’s becoming clear that one size will not fit all when it comes to limiting the application of phosphorus. An Iowa State University agronomist yesterday (Monday) gave a detailed presentation to Iowa’s Environmental Protection Commission about the use of phosphorus in Iowa farm fields. That commission will approve the limits on phosphorus application on cropland, but Iowa Department of Natural Resources director Jeff Vonk and his staff will draft the rules. Vonk says any state standard for phosphorus would provide guildelines for various types of soil, types of drainage and the elevation of the field. “So each farm, each farm field, really needs to be looked at,” Vonk says. That means there won’t be a uniform, statewide standard for phosphorus application on farm ground since some ground can handle more phosphorus than others. Vonk describes the draft proposal as an “index” — which means a sliding scale giving some farmers the ability to apply lots of liquid manure — which contains phosphorus — while others will be quite limited. “So it’s going to require site-specific analysis by our farmers and by the technical folks that they hire to assure that the amount of phosphorus…that’s going to be applied is actually going to stay there and be used by the plants or accumluated by the soils,” Vonk says. Current state limits restrict the amount of nitrogen that can be applied to farmground. Manure also contains nitrogen. “The big question is ‘What’s going to be the limiting factor (on the amount of manure that can be applied to a field)? Is it going to nitrogen or phosphorus?’ In some places, on steeper soils, I would guess phosphorus would kick in as a limiting factor much more quickly,” Vonk says. A couple of years ago the state legislature passed a law directing the Department of Natural Resources to develop standards for the application of nitrogen and phosphorus on farm ground. Vonk says phosphorus that runs off fields and into lakes and streams promotes the growth of algae and other underwater plants. That plant life robs fish and other acquatic life of oxygen. In addition, water that’s “super-saturated” with phosphorus smells.
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