Rx for aging soils?

EDITOR'S NOTE: The following article was excerpted from a story written by Andy Napgezek that first appeared in the Feb/March 1999 edition of the NPM Field Notes, a newsletter of the University of Wisconsin Extension Nutrient and Pest Management Program. The late Professor William Pierre, long-time head of the ISU Department of Agronomy, led similar research in the 1970s, but the long-term negative effect of nitrogen on soil quality needs more study in Iowa.

Thirty-eight years of data collected from a plot at the University of Wisconsin agricultural research station at Arlington are yielding alarming results: acidification from excess nitrogen inputs is wearing out the soil.

The Wisconsin research began in 1962. The plot has been planted to tobacco and soybeans, as well as 15 years of continuous corn, mirroring many of the state's fields rotated among a variety of crops. Initial work dealt mainly with crop response to alkalinity because crops such as alfalfa suffer from acidic soil conditions. Researchers found that acidity reduces the soil's cation exchange capacity-its ability to hold onto calcium, magnesium and potassium needed for crop growth. The change is irreversible.

Soil becomes more acidic when nitrogen sources, whether from urea, legume plowdown or commercial fertilizer, are not completely taken up by the crop. This excess nitrogen becomes nitric acid, which then reduces the soil's cation exchange capacity. Because the soil is unable to hold calcium, magnesium and potassium, these nutrients can move into the groundwater. Unlike nitrogen, these nutrients are harmless, but to replace them can cost up to 20 percent of what farmers normally spend on nitrogen fertilizers.

Unneutralized nitric acid can age soil very quickly, according to Phillip Barak, UW soil chemist and plant nutritionist who has been working on the project. He estimates that the soil at the Arlington test site has, in 30 years of "normal" agricultural acid inputs, aged the equivalent of 5,000 years with natural source acid inputs. "Keep in mind these soils have only been in existence for 10,000 years," he said.

One preventive measure is to use nitrogen more efficiently. Producers should credit all on-farm sources of this nutrient, and adjust their commercial inputs accordingly. Excess nitrogen not only acidifies, it can leach into groundwater or run off into surface water.

This second remedial measure is agricultural liming. Through this practice, farmers can neutralize the damaging acid and protect their fields. "Ag liming has been known for 3,000 years," Barak said. "Use it. It's like TUMS® for the soil." By closely monitoring pH levels and appropriately applying agricultural lime, farmers can greatly retard what Barak refers to as "accelerated soil weathering."

Barak describes the fine soils of Wisconsin as "tender," and claims that the very qualities that make them fertile also make them vulnerable. They are easily dissolved by acidity. He warns that, should excess nitrogen inputs continue unneutralized, northern soils might soon become like the sandy, less productive soils of the southeast region of the United States.



Return to Spring 2000 Leopold Letter index