Myth vs. Fact on farming, nitrogen, buffers and water quality

A ditch buffer strip in southern Minnesota.

A ditch buffer strip in southern Minnesota.

Myth: Farmers apply too much nitrogen on crop land.

Fact: University of Minnesota Best Management Practices for Nitrogen Use identifies an acceptable range of nitrogen fertilizer (N) at 130-180 pounds per acre for corn after corn and an acceptable range of 100-140 pounds per acre for corn after soybeans. An example of actual application rates, Minnesota farmers applied an average of 140 pounds per acre of nitrogen fertilizer on corn acres in 2010. Minnesota Department of Agriculture (MDA) estimates nitrogen application rates averaged between 120-140 pounds per acre in the years 1992-2013. While individual operations will vary, University of Minnesota data from 2010 broadly suggests that Minnesota corn farmer’s average N application rates are within best management practices recommendations. Other survey data may show different results and indicates the variability of fertilizer application rate data available.

Myth: Farmers continue to use more and more nitrogen to increase yield.

Fact: Data for Minnesota indicates that nitrogen (N) application rates on corn have only increased 4 percent from 1992-2013 and that farmers continue to gain greater production efficiency (bushels produced increased 40 percent in the same time frame) from each pound of nitrogen applied. Efficiency increases are the result of improved plant nitrogen use efficiency and agronomic practice. Incidentally, a 2009 study reported that Iowa farmers in the Raccoon River Basin (currently under scrutiny via a lawsuit) were reported to show no significant increase in N use from 1994-2009.

Myth: Broad use of cover crops will eliminate nitrate loss to ground or surface water.

Fact: Research shows that nitrogen loading in tile or via leaching was reduced by only 10 percent over a five-year period where cover crops were utilized. Coincidentally, research from Iowa State University indicated that a rye cover crop (most commonly used cover crop in Minnesota) reduced nitrate loss by 31 percent. When used in areas where they can be most effective, cover crops can help farmers reduce nitrate loss. However, cover crops will not eliminate nitrate loss to ground or surface water.

Myth: Most of the nitrogen farmers apply to fields ends up in the ground or surface water.

Fact: Research shows that only 6 percent of the total nitrogen inputs are lost from Minnesota cropland during an average climactic year. Losses were predicted to be 58 percent less than average during dry years and up to 62 percent more than average during wet years. Weather remains a major factor in nitrogen loss from farm fields.

Myth: Addition of buffers on all Minnesota waterways, lakes and ditches will protect and improve drinking water quality in Minnesota watersheds.

Fact:  Buffers are primarily designed to intercept surface waters flowing from the land surface to a waterway.  A waterway with a backslope on the bank between field and waterway is likely to have minimal overland flow into the waterway. While the root system of the buffer can intercept up to 50 percent of soluble pollutants in the shallow ground water that might pass through it, the percent of shallow groundwater interacting with the root zone is estimated to be quite small. Buffers are not intended to directly address deeper groundwater quality.

Myth: Buffers will greatly reduce the amount of nitrogen and phosphorus entering Minnesota streams and rivers.

Fact: The effectiveness of buffers to remove sediment, sediment bound contaminants, or soluble pollutants is extremely variable dependent on a number of factors. Individual field, soil type, topography and climatic conditions all impact buffer effectiveness. Buffer effectiveness to trap sediment compared to unbuffered terrain ranges from 41 percent to 100 percent dependent on variation of each of these factors.  An extremely small quantity of nitrogen moves with surface water flow. In consideration of that small quantity, 15-foot buffers were reported to reduce total surface flow nitrogen loss in two different studies by 17 percent or 50 percent. Doubling the width of the buffer to 30 feet reduced the total surface flow nitrogen loss to 51 percent or 67 percent. In the same studies, a 15-foot buffer reduced total surface flow phosphorus loss 41 percent or 57 percent, respectively, and doubling to 30 feet reduced surface flow phosphorus loss to 53 percent or 74 percent, respectively.

Myth: A 50-foot buffer on all Minnesota ditches, just like those required on lakes, would give the best protection of Minnesota waters.

Fact: Multiple options exist that could provide improved remediation of sediment and sediment bound contaminant loss from overland flow. For example, research in sloped topography has shown that narrow (less than 3 feet) grass hedges planted on the contour can reduce runoff by 52 percent and soil loss by 53 percent under no-till residue management. Tillage practice, infiltration rate, concentration of flow (area of the buffer that overland flow encounters) as well as velocity of overland flow all greatly impact buffer effectiveness.

Myth:  Minnesota soils are depleted from poor agricultural practice and that is why farmers have to add so much fertilizer.

Fact: Minnesota has rich soils but they are quite variable across the state. Also, natural atmospheric deposition into Minnesota soils is reported to account for up to 15 percent of available soil nitrogen (N) each year. A third source of soil nitrogen, mineralization, is reported to release an annual amount of inorganic N comparable to inorganic N from fertilizer and manure application combined on average in Minnesota soils. Our soils are far from depleted and the use of commercial fertilizer serves to enhance our soil’s natural productivity and allows Minnesota corn farmers to provide low cost food inputs for what we eat every day.

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