Soil Acidity is Causing Crop Failure in some Montana Fields: Is yours next?

Farmers in several Montana counties are experiencing nearly complete yield loss in portions of their fields due to soil acidity (low pH). Standard top 6-inch soil pH testing may not definitively identify soil acidity problems. Most fields with low pH problem areas also have larger areas with higher pH that buffer the pH value when soil samples submitted to labs are mixed from 6-8 subsamples per field. Also, the lowest pH is generally in the top 2 to 3 inches, not the top 6 inches, further masking the issue. MSU soil scientists have now identified fields in 15 Montana counties with soil pH levels below 5.5, some as low as 3.8. Because many Montana soils have pH levels greater than 7.0, soil acidification received little attention until recently when yield-limiting acidity was identified in Chouteau County.

At pH levels below 5.0, naturally-occurring soil metals (like aluminum and manganese), become more soluble and can stunt root and shoot growth. Young plants in acidic areas are often yellow with club or “witch’s broom” roots (see photos). Substantial yield losses occur at pH levels below 4.5. The most sensitive cereal crops appear to be barley and durum, followed by spring wheat.

The major cause of acidification appears to be ammonium fertilizers, including urea, applied in excess of crop uptake. No-till concentrates the acidity near the surface where fertilizer is applied. Acidity problems usually start in low lying areas of a field (where yield has historically been high), and acidity symptoms spread outward. To identify if you have an acidification problem, look at your top 6-inch soil test. If the pH is consistently above 7.5, it’s unlikely you have a problem. If it is below 6.0, you likely have areas with pH below 5 and have yield-limiting soil acidity.

On fields where standard soil test pH levels in the top 6 inches are below 7.5, scout for yellow seedlings and club roots. To verify that those symptoms are caused by low pH, analyze just the top 2 inches for pH, either with a field pH stick, pH probe, or lab analysis. Soil in the zone at the edge of poor growth areas

should also be sampled to determine if the pH is close to toxic on the margins, but do not yet exhibit symptoms. The potential is there for problem areas to grow in size. Areas where pH is 5 to 6 should be managed differently to prevent further acidification.

Chouteau County Extension and Chouteau County Conservation District Team up to Measure Soil Acidity across Chouteau County

Determining the extent of this newly-identified problem within Chouteau County will be a challenge. As an initial step, members from the Soil Acidity Working Group utilized the Chouteau County Soil Moisture Survey sites as a base sample survey for evaluating pH levels. While gathering soil moisture information, each of the 20 monitoring sites was measured for soil pH at the 2” and 6” soil depths with a field pH meter.

As an overview of soil acidity as a concern here in the county, the brief survey flagged a few points that should be noted by county producers. First, most of the soil textures at the sites are loams and clay loams which are typically less likely to be influenced by acidity. However, this has generally not been the case here. Based on this survey and samples collected since 2015, it appears that none of the cropland soil types are immune to increasing acidity. Second, both the 2” and 6” soil levels were relatively similar in their pH values at the sites. This may indicate that the acidity problem is or has been moving downward in the soil profile. Third, almost half of the recorded sites noted soil pH’s below 5.5 at the 2” level. This can be a real concern when considering viable seed options for crop species and varieties as well as increased potential for signs of metal toxicity, i.e. aluminum and manganese. Fourth, the site survey appears to have captured an issue that is generally wide spread and producers should be evaluating their soils from a whole new perspective and level of definition.

For additional information on this emerging issue, go to http://landresources.montana.edu/soilfertility and click on Soil Scoops where you will find two documents on soil acidification, or click on Presentations. Please contact Dale Krause, Chouteau County Conservation District (406-622-5627, or 622-3659) or Tyler Lane, MSU Extension Chouteau County (406-622-3751), if you have any questions.

Montana State University, U.S. Department of Agriculture and Montana Counties Cooperating MSU Extension is an equal opportunity/affirmative action provider of educational outreach. USDA and Conservation Districts are equal opportunity service providers and employers.