Soil acidity: is the culprit nitrogen or zero till?

Levels of pH are gradually dropping where nitrogen is applied

Fields that become acidic almost overnight aren’t science fiction. Corn land around Bismarck, North Dakota, has become acidic in just a few years.

It’s due to high rates of nitrogen fertilizer in combination with zero till practices, said local independent agricultural consultant Bob Amstrup.

“We are creating an artificially low pH problem because of the way we farm,” he said.

“Some of my clients have fields here in the central part of the state that have developed a very low pH in recent years. They have numbers down around 5.8 in a couple fields. It’s not overly alarming yet, but it indicates a trend toward acidity.”

Bismarck is in south-central North Dakota, approximately straight south from the Saskatchewan-Manitoba border.

Amstrup said pH numbers are generally high by nature east toward the Red River Valley, so acidity shouldn’t be an issue, even with zero till and high rates of nitrogen fertilizer.

However, it’s a different picture west of there, on the true prairie soil, which is less tolerant of artificial inputs and must be managed more carefully.

“One thing I’m sure of. Since the advent of no till in this area, we’ve seen pH numbers gradually drop over the last 20 years approximately,” said Amstrup.

Such a change in only two decades is drastic considering that the soils are millions of years old, he added.

“We’re leaving all that crop residue on the surface. As it breaks down, the residues have acidic compounds. That’s true of every crop we grow,” he said.

“The pH number measures acidity, and our pH numbers are dropping. You can almost say we’re creating an artificial acidic problem.”

He said the problem is easily cured by working the soil black with a chisel plow. The pH would shoot back up to the previous normal level within a couple of years.

“But of course, if you did that, everything you’ve gained from years of no till gets thrown out the window,” he said.

“Your organic matter is destroyed. Erosion becomes an issue again. Moisture retention becomes an issue again. All your efforts get thrown out the window. You have to start over from scratch. So I’ve recommended to some of my clients that if they continue no tilling corn with high rates of N, then they’ll have to consider lime. That usually involves a massive volume of product and a considerable cost.”

Other solutions include less corn in the rotation or lower target yields with lower nitrogen rates.

Amstrup said a producer planning to expand his corn acreage may want to put lime in the farm’s long-term budget.

“But the larger issue I have in the back of my mind is the wisdom of taking a plant like maize that originates in central Mexico and breeding it for an area as far north as Bismarck,” he said.

For more information, contact Amstrup at 701-221-2589 or email [email protected]

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  1. Was it ever considered that tillage over the last century has created a soil with an artificially high pH and that reduced tillage and soil conservation practices have resulted in a return to normal pH levels. Also consider sample depth when in relation to pH. I’m willing to speculate that if the soil were sampled in 2 inch increments that at most the top 4 inches have deviated from the “normal” and most likely the top 2 are the biggest driver of change with soil below 6 inches remaining unchanged.

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