How can you help your clients with this growing issue?
Farmers in several Montana counties are experiencing nearly complete yield loss in portions of their fields due to low pH. To best help your clients with this major problem, you first need to know if they have acid soils. Standard top 6-inch (or 12-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 pH 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. A recent soil sampling of the 20 Chouteau County Soil Moisture Survey locations in Chouteau County by Tyler Lane (Chouteau County Extension Agent) and Dale Krause (Chouteau County Conservation District) found that 8 of the 20 locations had 2” soil pH < 5.5, indicating this is a widespread problem in Chouteau County and likely other areas in Montana.
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 by Tyler Lane and Rick Engel below). 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.
Another complicating factor is that low pH slows degradation of certain herbicides such as imidazolinones (e.g. Beyond and Pursuit) and triazolopyrimidines (e.g. Powerflex), possibly resulting in crop herbicide damage even when standard rotation intervals are used. So unexpected herbicide damage might be the first indication of low pH, even before aluminum toxicity develops.
The major cause of acidification appears to be nitrification of ammonium fertilizers, including urea, especially when 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, and acidity symptoms spread outward. To identify if your clients’ fields have an acidification problem, first look at their top 6-inch soil test. If the pH is consistently above 7.5, it’s unlikely there’s a problem. If it is below 6.0, they likely have areas with pH below 5 and have yield-limiting soil acidity. Between pH 6 and 8, pockets of acidity cannot be ruled out since one subsample with pH 4.5 will be buffered by 5 – 7 subsamples of pH 7.5 resulting in a composited pH in the low 7s.
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 the 0-3” and 3-6” depths. 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. If spring crops aren’t out of the ground yet, but you’re soil sampling, consider taking an extra sample from locations you, or the producer, recall had unexplained poor growth in 2016 or 2017. For additional information on this emerging issue, go to http://landresources.montana.edu/soilfertility and click on Soil Scoops where there are two documents on soil acidification, or click on Presentations. Please contact Clain Jones, MSU Extension Soil Fertility Specialist (firstname.lastname@example.org, 994-6076), MT Salinity Control Association (406-278-3071), or Dale Krause, Chouteau County CD, (email@example.com, 406-622-5627 or 406-622-3659) if you have any questions or you identify soils with pH < 5.5.