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By MATTHEW WEAVER Capital Press This article, reprinted by permission, first appeared in the Capital Press, March 31, 2020
Growers should immediately sample their soil to get a sense of how low the pH is and the effect it will have on their crops. “It’s knowing how big of a problem this is,” said Clain Jones, soil fertility professor and extension specialist at Montana State University. “I think the most urgent thing, if it hasn’t been done already, is to know the depth of the low pH, to know whether or not tillage could work and also to know the extent of it.”
Jones spoke March 27 during a Washington State University Farmers Network web seminar on soil acidity and liming.
Jones began studying agricultural lime in 2017. Crop yields decrease as soil acidity increases, he said.
“The longer you wait to mitigate your soil pH, the more lime it’s going to take, and likely also the more yield loss you might experience,” he said.
Soil pH can drop as nitrogen fertilizer rates increase, especially when using ammonium-based nitrogen such as UAN, urea and anhydrous. [Editor's note: these ammonium-based fertilizers are prohibited materials for certified organic production.]
Jones said the pH dropped faster in coarser-textured soil, which has less buffering than finer soil.
Growth issues begin when soil pH levels drop below 5.2. The lower the pH, the more acid soil is. A pH of 7 is neutral.
He said the impact of liming a field is long-lasting.
“Let’s say we were to lime a silt loam soil to a fairly high level, 7.2 or so. It would take thousands and thousands of pounds of nitrogen per acre over time, probably 20 to 40 years, before we end up with a pH that we would consider to be a problem,” Jones said.
Jones applied varying amounts of sugar beet lime, from no lime to roughly 6 tons per acre, to see how much it raised the pH in the top 4 inches of soil.
It takes less lime to bring soil pH up from 4.5 to 5.5 than it does to bring pH up from 5.5 to 6.5, Jones said.
The problem of soil acidity crept up on Montana farmers, Jones said.
The biggest challenge is that agricultural lime is fairly expensive, Jones said, but it pays for itself many times over.
He compared the $100-per-acre cost of applying lime to the thousands of dollars of income that is gained.
“The return is quite a bit higher than the cost,” he said, pointing out that bankers need to understand that it is really a capital investment.
Liming a field “should benefit you at least a decade, maybe two decades, and there should be a very large return on investment,” he said.
But with the high cost of lime, Jones advises farmers to apply it only to the areas that need it.
He used the example of a grower who knew his fields well enough to identify the areas that had no growth, likely due to low pH. He recommended using aerial imagery.
Some farmers might see an economic benefit in the first year, but for the most part the effects of liming will take two to five years, Jones said.
Sugar beet lime is the least expensive lime form but more is required to have the same benefit as pure agricultural lime, Jones said.
It takes many years for surface-applied lime to impact acidified no-till soils, he said.
Jones said a one-time, non-aggressive tillage to incorporate the lime did not reduce soil organic matter. Doing it once every 10 or 20 years will probably not have much impact on soil organic matter and carbon, he said.
Also, he said phosphorus fertilizer can have a beneficial effect on yield similar to lime, countering acidity to a degree.
However, it doesn’t work in every case, Jones said. The lower the pH, the higher the potential for a benefit, he said.
He recommends managing nitrogen rates, picking crop rotations that require less nitrogen, leaving more stubble on the surface and considering adding phosphorus.
Phosphorus won’t pay off as fast as lime, Jones said, but it may work well for a farmer who is leasing land or doesn’t have a spreader to apply lime. Most direct seeders have a way to add phosphorus.
“This would be an option to get you through a few years until you can figure out a liming practice,” he said.
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Matthew Weaver can be reached at Mweaver@capitalpress.com
Posted: to Organic Production on Wed, May 20, 2020
Updated: Wed, May 20, 2020