MSU Extension Ag Alert: Soil Acidity, An emerging issue that requires scouting

MSU researchers encourage crop producers and crop advisers to be on the lookout for decreasing soil pH leading to low production and sometimes crop failure. Farmers in several Montana counties are experiencing nearly complete yield loss in portions of their fields due to soil acidity (low pH). This is an emerging issue in the state, where low soil pH has traditionally not been a concern. MSU soil scientists, Extension Agents, crop advisers, and producers have now identified fields in 20 Montana counties with soil pH levels below 5.5, some as low as 3.8. MSU will be hosting a field day at the Post Farm (west of Bozeman) on July 13, where Clain Jones, Extension soil fertility specialist, will share research-based information on the topic in the afternoon.

Bulked soil sampling (containing multiple subsamples) in the top 0 to 6-inch depth across large field landscapes may not be helpful in identifying fields with soil acidity problems. Many Montana fields have wide spatial variances in soil pH. Often soil pH in low lying areas will be considerably lower than in summit hillslope positions only a 100 yards away. Also, many Montana fields exhibit pH differences of up to 3 units (e.g. 5 to 8) between the surface and 18 inches down. Because the lowest pH is generally in the top 2 to 3 inches of soil, this low pH may be masked by collecting soil samples in a standard 0 to 6-inch depth increment.

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 (similar to nitrogen deficiency, yet less uniform) or even pink with club or “witch’s broom” roots(similar to nematode damage). 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. Legumes can develop nitrogen deficiency in low pH areas before they exhibit aluminum toxicity because nitrogen fixation is impacted below about pH 6 (see photograph below).

Acidity problems usually start in low lying areas of a field, where yield has historically been high, and acidity symptoms spread outward. Producers are encouraged to look at pH values in top 6- inch soil tests. If the pH is consistently above 7.5, it’s unlikely the field has a problem. If pH is below 6.0, the producer should consider sampling different topographic areas of their fields. If pH is between 6 and 7.5, a judgment by the crop adviser and/or producer will need to be made if additional soil sampling or scouting is worthwhile. Surface soil pH can vary more than 2 pH units over short distances (< 100 yards). For example, the soil pH in low lying areas may be less than 5, and then abruptly change up a small hill/slope. Soil sampling is recommended even if no symptoms are observed because once low pH symptoms are observed, yield has likely been lost.

On fields where standard bulked soil test pH levels are below 6.0 scout for yellow seedlings and club roots. To verify that those symptoms are caused by low pH, the top 3 inches of soil can be analyzed for pH, either with a field pH stick, probe, color strips, or lab analysis. The 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 crops do not yet exhibit symptoms. The potential is there for problem areas to grow in size. Areas, where pH is less than 6, should be managed differently to prevent further acidification.

Based on regional research, 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. A cooperative research study led by Rick Engel (LRES) and including Dr. Jones, and people from the Central Ag Research Center, the Montana Salinity Control Association, Chouteau County Extension, Chouteau County Conservation District, and producers are in progress to develop prevention, mitigation, and adaptation options for Montana croplands.

For additional information on cropland soil acidification, go to this site or contact Clain Jones, 406-994-6076.

To view pictures visit the MSU Extension Ag Alerts page.

Time to Survey for Alfalfa Weevil

Written by Kevin Wanner and Emily Glunk

Alfalfa weevil is the key insect pest of alfalfa, causing variable levels of economic damage across Montana each growing season. After mating the female weevils lay their eggs in alfalfa stems, and newly emerged larvae crawl up to the developing terminal buds where they chew small “pin” holes in the leaves. The larvae develop through four instar stages (Figure 1); the larger 3rd and 4th instar larvae feed openly on unfurled leaves and cause the largest economic loss. Severe feeding damage will give the field a “frosted” appearance. Mature larvae develop into the next generation of adults that leave the alfalfa field to find overwintering sites. In Montana there is one generation per year. The majority of crop damage occurs prior to the first cutting as a result of feeding by larger larvae. Management decisions are based on surveying the number of weevils to determine if their population will exceed the economic threshold, the point that warrants action to be taken.

Alfalfa weevil sampling should begin in the spring when the stand is about 8 to 10 inches tall. Weevil populations can be estimated using sweep nets (net with a 15 inch diameter, can be purchased online) or by shaking alfalfa plants in a bucket. An average of 20 alfalfa weevil larvae per sweep meets the economic threshold for action. Ten sweeps are taken at each of 3-5 five sites in a field (30-50 sweeps per field) and the total number of weevil larvae counted to determine the average per sweep. An alternative is to cut 10 stems from each of 3-5 different sites in a field (30-50 stems per field) and shake the stems in a bucket to collect the larvae. An average of 1.5 – 2.0 larvae per stem meets the economic threshold for action. To get an accurate average more samples are required for larger fields. A minimum of three samples are recommended for fields up to 20 acres, four samples for fields up to 30 acres and five samples for larger fields. Based on historical weather data, sampling for alfalfa weevil in Montana typically begins between May 24 and June 16, depending on the location and the seasonal weather.

Typical dates that alfalfa weevil monitoring begins in Montana:

Sidney – May 24.    Glasgow – May 29.   Lewistown – June 13.   Kalispell – June 7.   Dillon – June 10.   Bozeman – June 8.   Red Lodge June 16.

When the economic threshold has been met (more than an average of 20 larvae per sweep or 1.5-2.0 larvae per stem) action is required to preserve yield. If stand growth is sufficient early harvesting is the most effective and economic action. If early harvesting is not an option then an insecticide can be used to reduce weevil populations below economically damaging levels. Additional management information including insecticide options is listed online in the High plains IPM guide:

Additional video resources: