Soils with low pH remain a problem for Montana producers.
“Acidic soils are a growing issue in Montana in 2019,” said Clain Jones, Montana State University Extension soil fertility specialist.
Some 23 counties are now reporting acidic soils in their fields.
At Sidney ARS and MSU Eastern Ag Research Center’s dryland field days, Jeff Chilson, MSU Extension agent in Roosevelt County, said that acidic soils could result from both natural and management-induced mechanisms.
Producers may be able to do something about the management-induced mechanisms in order to be able to manage their soil pH.
They don’t need to pay for the expensive soil samples to see if there is a problem.
“Multiple soil tests can be expensive, but you can purchase a digital soil pH probe or strip test at a store to see if you have a problem,” Chilson said. Soil probes might be about two-tenths of a pH level off of an accurate soil sample reading.
“Soil probes and pH strips are not replacements for soil testing but can be used to help identify problematic areas.”
To verify that there really is a problem, if you no-till, “you would need to take soil samples at 0-3 inches, 3-6 inches, and 6-12 inches, and send it to a lab, with instructions not to mix the samples,” he said.
The reason for the three different sets of soil samples is in no-tilling, soil scientists are finding there is soil stratification.
“It is important to understand what could be happening in your field,” Chilson said. “Without accurate sampling, low pH or acid soils could be missed, leading some producers to wonder why their yields are dropping off in certain areas of the field.”
Applying nitrogen fertilizer to soils increased during the 70s, in part related to decreased costs as improved synthetic manufacturing of N fertilizer became available.
“About 30 years ago, many farmers went to ammonium-based fertilizer, because it was even cheaper and more available,” he said.
When ammonium-based fertilizer is applied, the ammonium gets into the soil. When mixed with water, it forms nitrates and releases hydrogen atoms, which leads to acidic soils.
“When we measure the pH in the soil, we are looking at how many hydrogen ions are in the soil,” he said.
Distilled water is considered neutral, a pH of 7. In alkali soils, pH is above 7, and a soil pH below 7 is considered acidic.
“Below a pH of 6, you may have problems with germination in your peas and lentils, and below 5, you may have problems with germination in your wheat and other small grains,” he said.
Several years ago, another Extension agent Chilson knew told him he had a field in his county where the wheat never came up. The field across the road, on the other hand, had a good wheat stand.
“What plants did come up in the field were very stunted. They initially thought it was disease or insects. In fact, they considered everything they could think of,” he said.
At the time, acid soils were not considered to be a problem in Montana.
But after ruling out other issues, the Extension agent soil sampled the field and found the pH to be below 5.
In clay and silt soils, the pH could be low in the first 0-3 inches “where we plant our peas and wheat.”
In those soils, if a producer can get the plant started and the roots can reach far enough down to that basic level (neutral pH soil), the plants will grow out of it.
“But the acid frees up aluminum and there is so much aluminum in those soils that it kills the plant,” he said.
Up on the Froid Research Farm, in 2012-2013, in the continuous wheat on wheat rotation, soil health specialists found low pH in those Roosevelt County fields. They found low pH in some Valley County fields, as well. Low pH has not been found in Richland County soils yet.
“If you notice poor germination, poor stands or poor yields, you should monitor for low soil pH,” Chilson said.
Low pH can cause other problems, including: aluminum toxicity, nutrient deficiencies, poor N fixation in legumes, increased fungal disease and changes in herbicide effectiveness and persistence.
Soil sampling with the probe
When using a digital soil probe, Chilson said to calibrate it first, and make sure to keep it clean at all times.
Moisten the soil sample first unless it has rained before the sample is taken and it is already moist. Chilson cautions users to read the manufacturers labels thoroughly, as probes may have different operating procedures.
“When moistening the soil, use distilled water so it is as close to a neutral substance as possible,” he said.
Wait for the digital reading to stop flashing, and that will be the reading.
Chilson took a digital reading of a soil sample taken from 0-3 inches outside at the Sidney research farm.
“There is more clay and silt in the soil in Sidney,” Chilson said. “I have a pH reading of 6.7 pH, not terrible, but lower than I would expect in the top 3 inches.”
At the Froid farm, the soil reading taken at the edge of a field on the north side of the creek was a pH of 5.4.
“If you are growing peas or lentils, you will have trouble (with germination and stand), but you could grow a newer aluminum-tolerant variety of wheat,” he said.
A sample taken 50 feet into the field came out with a pH of 4.8.
“The germination rate was not great (in that field this spring,” Chilson said in June 2019. “We are noticing problems as a result of that.”
For producers who are checking their soils for pH, Chilson recommended noting the GPS when the sample is taken.
Take samples at 0-3, 3-6 and 6-12 inches separately.
“Acidic soils are still a growing issue in Montana in 2019,” Jones said, adding it might not be the entire field but only a portion of it that has a low pH. “Producers out scouting their fields could find bare spots or plants with yellowing leaves, and stubby club roots, which could indicate a low pH level.”
What can be done about low pH in soils?
- Don’t overload soils with fertilizer and make sure to soil test. Soil health specialists know about different and better N formulations, so check with your Extension agent.
“Have realistic yield goals and only put down the N you need during planting,” he said.
- Planting a perennial grass for five years has been shown to raise pH.
Stay away from perennial legumes, such as alfalfa, in low pH situations, because they can take up a lot of calcium, magnesium and other basic buffers in the soil.
- When soil sampling, consider testing by zone.
“Look at your soil composition and your topography and test by zone, and then do variable rate application in those zones,” Chilson said. Variable rate is a more accurate way of application and N can be increased in better-yielding portions of the field.
- Realize soil pH can change drastically throughout a field.
- Weather can also affect the pH, and sandy soils are more prone to acidification.
- Liming the soil can help keep yields intact.
In a study conducted at MSU Central Ag Research Center, scientists studied planting different crops and varieties under limed and not limed conditions at two locations in Chouteau County and at the center. For results, contact the center.
In addition, scientists planted one variety of durum under the same conditions using five different rates of phosphorus fertilizer (0-45-0) and compared entries for yield, test weight and kernel weight.
In the first year of the two-year study, phosphorus was found to mitigate aluminum toxicity in durum.
Rick Engel, MSU professor of soils in the Department of Land Resources and Environmental Sciences, noticed increased yields in durum from liming.
“With durum, Rick Engel has seen a doubling of yield in one field last year from liming. Sometimes fields yield a little better without lime; I think that happens when the plants basically save their water because they’re stressed but then do have water at grain fill,” Jones said.