What does Dorothy telling Toto that, “Toto, I think we're actually in Kansas,” have to do with acidic soils in western North Dakota and Montana?
It turns out - quite a bit.
Acidic soils, traditionally found in southeastern U.S. and the southern Great Plains, are now being discovered from western North Dakota through central Montana.
So Dorothy might think she is in Kansas, but she might really be in Dickinson.
“This is the area of the state where we are running into it (acidic soils),” said John Breker, soil scientist with AGVISE Laboratories. Breker spoke at the recent Diversity, Direction & Dollars Ag Forum in Dickinson. “It may not be new - it is just that we’re becoming more aware of it.”
AGVISE Laboratories has been soil and plant tissue testing for more than 42 years, now in their third building in Northwood. Their first building burned down in 1996, and just like Dorothy experienced, their second building was destroyed by a tornado in 2007. “Every time you change buildings, you have a chance to make it better. And our third building certainly is better,” Breker said.
Soil pH is a measure of soil acidity or alkalinity.
“pH is important, not just in soils, but in all biological systems. Whether we are talking soils, inside plants, or inside the rumen of a cow, pH controls all biological and chemical reactions,” he said. “The majority of soils in North Dakota are alkaline, so that is what we have all been used to. But, as it turns out, producers are finding soils in some areas of their fields with a pH of 5.5 or 6 - and anything below 7 is considered acidic.
“When soils become too acidic, we start running into problems, and we have to start thinking about things like liming and issues like aluminum toxicity,” Breker said.
When soils are below pH 6, producers should consider liming their soil.
Previously, when one composite soil sample was collected in a field, there were soil cores collected with low pH, and soil cores with high pH, and it would all be mixed together and it came out with an average, “say somewhere around 7.0 or 7.5.”
But now that everyone has moved to precision soil sampling (grid or zone soil sampling), soil scientists are starting to find these little low pH pockets inside fields, and sometimes these are the areas that have production issues.
“There might be an area in your field that never produced well, and low soil pH might be one of the reasons why,” Breker said.
AGVISE also tests subsoil pH on all soil samples. “
Some of these subsoils are acidic, so that’s a further problem.
“We run buffer pH on any composite samples that have a pH less than 6,” he said. “When AGVISE runs a buffer pH test, sometimes that will generate a lime requirement. “So if you look at your soil test and you read, 'Oh, I am supposed to put on lime?,' that should raise a red flag and generate some questions,” Breker said.
Soils that traditionally had low pH and needed lime as a soil amendment were highly weathered and located farther to the east where there is more rainfall.
“In these soils, all of the calcium carbonate (lime) has leached out of the soil profile,” he said. Many acidic soils are in the southeast U.S. and the central Corn Belt, where liming is a standard soil fertility practice. From the Great Plains westward, soils are either young glaciated soils in eastern North Dakota, or old, arid soils in western North Dakota, where there may still be calcium carbonate in the topsoil.
When soil pH is below 6, soil scientists consider lime applications to raise soil pH. If a producer decides to spread lime, he needs to spread a lot, at least compared to the usual fertilizer rates.
“Lime application is measured in tons per acre. When we try to change soil pH, we are trying to change a bulk soil property,” Breker said, pointing to a photo of a producer spreading high rates of lime in Iowa. “It is a serious thing. If you just put on only 200 pounds of lime, don't expect it to significantly raise your soil pH.”
Breker displayed a soil test summary from the entire state for 2018.
It showed the percentage of soil tests with pH less than 6. Throughout North Dakota, there are regions where 1 to 5 percent of samples have a pH less than 6.0, but most of those are in the southwestern region.
“This is the hot spot in the state for acidic soils,” he said. In the Bismarck area, there are about 8 percent of samples with a pH less than 6, and in the Dickinson area, that is 12 percent.
“Liming is not an insignificant thing, and it is something that must be on your radar,” he said.
In low pH soils, there is reduced nutrient availability, with less phosphorus, sulfur, calcium, potassium, and magnesium available to the plant.
When the pH falls below 5.5, aluminum, which is naturally present in soil but not normally a problem, becomes available to the plant, causing aluminum toxicity. “
When soil pH is below 5-5.5, you need to be really concerned. Aluminum is very toxic to plant roots,” Breker said. “It prevents the roots from elongating, so we end up with these very short, stubby roots.”
With short roots, the plant cannot take up nutrients or enough water to be able to grow. “The yield is greatly reduced,” he added.
In the southwestern region, many crops are produced on stored water, so this region is one of the worst possible areas to experience a low soil pH problem. A short root system on a low pH soil would need good, early rain, and that doesn’t often happen in the southwest.
Plant tolerance to aluminum toxicity also varies among crop species.
“Alfalfa is probably the most sensitive plant species we grow in this region,” Breker said. Any legume, whether it is soybean or a pulse crop, will have reduced nitrogen fixation and not perform well on strongly acidic soils. “Small grains, like wheat and barley, are better options on acidic soils, but they too will suffer when soil pH is less than 5.5 and aluminum toxicity is there.
Breker said the low soil pH problems “did not happen overnight.”
These soils have been acidifying for decades. Rainfall is one natural source of soil acidity, and soils that receive more rainfall are usually acidic. In areas in southwest North Dakota, where the soils are the oldest in the state, some 65 million years old, rainfall has been acidifying soils for centuries.
Crop nutrient removal is another cause of acidification.
“As we take the base cations (like calcium and potassium) away every year in grain or in forages, what remains behind are the acidic cations (like hydrogen and aluminum) that accumulate,” Breker said. “As we grow higher yielding crops over time, the acidifying effects of crop removal become even worse.”
The main cause of soil acidification, however, is nitrogen fertilizer application.
Whenever an ammonia-based source is applied, like anhydrous ammonia, urea, or ammonium sulfate, the conversion of ammonium to nitrate by soil bacteria produces acidity.
“This is the dominant driver of acidity, especially in continuous no-till systems, where we broadcast urea on the soil surface, and the resulting acidity becomes concentrated at the soil surface,” he said. Any fertilizer source that contains elemental sulfur will also acidify soils. Whenever there is plant residue accumulation, such as in no-till, residue decomposition creates organic acids that further contribute to acidity.
“Can we get away from nitrogen fertilizer? Not if we still want to be productive, so we have to accept this as a tradeoff and work with other things (like liming),” Breker said.
In conventional tillage, a tillage operation will incorporate higher pH subsoil and naturally occurring carbonate from below, with the acidified topsoil above.
In no-till, the soil is not mixed, so soil surface pH remains low.
Breker does not suggest occasional tillage to increase soil pH because soil erosion remains one of his greatest soil management concerns in the Great Plains. “
We do not want to reset the clock on 20 or 30 years of long-term no-till and progress,” he added.
Montana is also experiencing low soil pH.
“You guys are not the only ones suffering with this problem. They are finding it in Montana too,” he said. (Soil scientists in Montana say low soil pH has now been found in significant acreage in 15 counties.)
Breker shared photographs of a field in north-central Montana.
“The low-lying areas of the field, where we typically find better crop production than hilltops, are now yielding lower from low soil pH,” he said. “On hilltops, soil erosion and exposure of subsoil calcium carbonate has maintained adequate soil pH. The hilltops have become the better producing areas of the field.”
Breker showed a photograph of low-yielding short safflower from southwestern North Dakota.
What could be the problem? It could be an air-drill problem, a seeding-depth issue, or soil moisture differences across the field. In this case, none of those were the problem.
Agronomists came out and collected soil and plant tissue samples across the field. Since it was a no-till field, they took samples from 0-2 inches depth; 2-6 inches depth; and 6-12 inches depth. Some plants were only 2-3 inches tall. “
The soil pH was incredible in this field,” he said. In the good areas, the soil pH was only 5.6, but in the poorer producing areas, the soil pH was down to 4.5. In the 0-2 inch soil depth, the pH was lowest at 4.3.
“The really scary thing was the amount of soluble aluminum in the soil,” Breker said. “Normally, aluminum levels should be zero when pH is above 6.0. This field, in the top 2 inches, had 91 ppm (parts per million).”