Effects of Agricultural Nutrient Additives on Surface Water Quality

Abstract

The water in Dunn County is green. We know nutrient runoff is a major contributor to the algal blooms that can be found in Lakes Menomin and Tainter, but how can we most effectively reduce the amount of these nutrients getting into our lakes and rivers?

Part of the solution might be found in the farmers’ use of nutrient additives. Our research indicates that switching from chemical fertilizer to manure could significantly decrease the amount of total phosphorus in Wisconsin’s surface waters.

Using water quality and farming data from four states, including Wisconsin, we estimate that a one percent increase in farm acres using manure instead of fertilizer leads to a 0.019 mg/L decrease in total phosphorus levels.

Given the controversy over the environmental effects of manure spreading, this result may seem counterintuitive. How is it that switching from fertilizer to manure improves water quality?

The answer may lie in the beneficial attributes of manure. Because animal waste consists of organic matter, applications can improve a variety of soil characteristics, including increased water infiltration rates and water-holding capacity that help reduce runoff. One study, for example, showed how cropland treated with poultry litter took eight times longer than untreated cropland to begin runoff after rainfall. Using manure to replace highly soluble chemical fertilizer may also explain why farmland treated with manure experiences decreased rates of runoff.

But we only get these benefits if manure is applied correctly. Nutrients from animal waste can similarly enter surface water if manure is managed poorly. Examples might include spills, winter spreading, and excessive application. (Manure may also negatively affect well water quality. However, our study only looks at surface water quality.)

Further, our result is stronger evidence for the effectiveness of manure spreading, as our data include farmers with poor manure management practices. If our aggregate analysis includes over-appliers and winter-spreaders, and our result still indicates a positive effect, it stands to reason that manure usage’s effect on surface water quality would be even greater if all farmers had spread at correct rates.

Thus, our analysis suggests that substituting manure for chemical fertilizer can significantly improve water quality. Policies that encourage such a change could reduce the eutrophication of rivers and lakes, thereby leading to environmental and economic benefits. Our research simulates these economic gains for some home owners in Dunn County. If a policy caused five percent of farm acres contributing to nutrient loading in the county to switch from fertilizer to manure, lakefront property values would increase by $1,587. Larger changes of 10 and 15 percent could increase values even more – by $7,935 and $12,696, respectively.

However, considerations for deciding whether to adopt manure are complex. While it may be better for water quality, there are legitimate concerns about animal waste’s variable nutrient content, weed seeds, and its costs of transport and application. Further work is needed on policies that will make manure usage more cost-effective and appealing to farmers. Moreover, efforts are needed to encourage correct rates of nutrient application that are critical for reducing water pollution. Cleaning up our water is no easy task. But for now, knowing manure may be more effective than chemical fertilizer for reducing runoff is a step in the right direction.