Acid linked to soil aging

Thirty-seven years of data collected from a plot at UW–Madison’s Arlington agricultural research station is yielding alarming results: acidification from excess fertilizer is wearing out the soil.

Phillip Barak, UW–Madison associate professor of soil chemistry and plant nutrition, and his colleagues have found the cation exchange capacity, or the soil’s ability to hold onto small bits of calcium, magnesium and potassium, decreases because of soil acidity.

“This change is irreversible,” he says.

According to Fred Madison, UW soil science professor, this is important news for farmers.

“Now you’ve got a whole path of destruction,” he says.

Producers may have overlooked the danger that excess nitrogen poses to groundwater, but evidence that it may also permanently damage the soil is beginning to garner attention, according to Madison.

Barak says health experts have long known the role that nitrogen plays in blue baby syndrome, a condition in which high levels of nitrogen inhibit the blood’s ability to transport oxygen. The soil becomes more acidic when nitrogen sources, whether from urea, legume plowdown – such as alfalfa and red clover – or commercial fertilizer are not completely used up by the crop.

This excess nitrogen becomes nitric acid, which destroys the soil’s vital ability to retain the calcium, magnesium and potassium necessary for crop growth. Instead, these nutrients leach out of the soil and into the groundwater.

Although they are harmless, they do cost money to replace – a price that Barak estimates at 20 percent of what farmers normally spend on nitrogen fertilizers.

According to Barak, the United States has a 50 percent applied nitrogen efficiency rate. This means only half of the nitrogen applied by farmers is actually taken up by the plants, leaving the other half to become nitric acid, he said.

“That’s like a tanker truck of nitric acid being dumped on a field,” says Barak.

These tons of unneutralized nitric acid age the soil very quickly, according to Barak. The soil at his Arlington test site has, in 30 years of “normal” agricultural acid inputs, aged the equivalent of 5,000 years with natural source acid inputs.

Barak says the aging is remarkable considering the age of the soils. “Keep in mind these soils have only been in existence for 10,000 years,” he says. The fine soils of Wisconsin are “tender,” and the very qualities that make them fertile also make them vulnerable. They are easily dissolved by acidity, he says.

According to Barak, if excess nitrogen inputs continue, unneutralized, northern soils might soon become like the sandy, less productive soils of the southeast region of the United States. “With the long term, over-application of nitrogen, we run the risk of irreparably damaging the soil,” says Madison. But Barak says the news is not all bad. He cites two ways farmers can better care for their valuable soil.

The first, and preventative, measure is to use nitrogen more efficiently. Producers should account for all sources of nitrogen, and adjust their commercial inputs accordingly. Excess nitrogen not only acidifies, it can leach into groundwater or run off into surface water.

The second, and remedial, measure is applying agricultural lime. Liming can neutralize the damaging acid and protect the fields. “Agliming has been known for 3,000 years,” says Barak. “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.”