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.

After the flames: How fire affects soil nutrients

Hundreds of thousands of acres of forest, rangeland and cropland have sadly gone up in smoke this summer in Montana. In addition to the devastating effect on personal property and direct loss of crops and livestock, fire can affect soil properties and soil nutrients. The impact is highly dependent on the fire intensity/duration and the proportion of plant material that is burned. Timber and shrubs will burn hotter and longer with greater impact on soil than range- or crop land. Fast moving grass fires have minimal impact on soil nutrients and soil health compared to slow moving, intense fires in moderate to heavy fuels.

In general, fires reduce the pool of nutrients stored in organic matter, release a flush of plant available nutrients in the short term, and redistribute nutrients through the soil profile. The availability of nutrients, especially nitrogen, is increased after low intensity fires, yet, a portion of nitrogen and sulfur is lost to the air. Although these losses are not trivial and are similar to removal by harvest and losses to wind erosion, they are small compared to the average pool of nutrients in the top six-inches of soil.

Nitrogen can additionally be lost through nitrate leaching, as the burned plant matter creates a large pool of nitrate and few active plant roots are left to take up either the nitrate or soil water. This can have long term impact on the productivity of forest and rangeland ecosystems, but can be minimized or remediated on croplands. The other nutrients such as phosphorus, potassium, magnesium, zinc and manganese are more stable and not lost directly through combustion, but rather through blowing ash, and post-fire soil erosion.

Cropland fires rarely burn hot enough to affect soil organic matter. The bigger concern is loss of surface plant residue, which is very important to reduce wind erosion, and protect against the physical sealing impact of raindrops. Ash particles also contribute to reduced water infiltration as they plug soil pores. All these factors increase the risk of water runoff and soil erosion.

Intense forest and shrubland fires can burn soil organic matter, reducing the pool of nutrients in the soil, soil aeration and water infiltration/retention, and the soil’s ability to hold nutrients coming from ash or fertilizer.

In addition, forest and shrubland fires can create a water repellent layer within the top 2 inches of soil that comes from compounds in the burnt litter, coating soil aggregates or minerals. The depth and thickness of this layer can vary greatly, and it can affect infiltration for several months to years. This layer should not form on grassland or stubble fires.

Fire kills bacteria and fungi at the soil surface but microbes rapidly recolonize from deeper soil layers, except in severe fires where the soil is sterilized several inches deep. Microbial activity can actually increase with the flush of nutrients available after a fire. However, new input of plant material is important to sustain their populations.

Post-fire management includes soil testing to determine nutrient availability, and establishing ground cover where possible. Test for nitrogen, phosphorus, and potassium to calculate fertilizer needs. Because drought preceded fire, it’s likely that many fields have nitrogen that wasn’t used this summer, so less might be needed next spring. When soil sampling burned fields, be sure to select representative sites, avoid areas where there may have been a windrow, bale, or other high accumulation of straw or residue. Spreading manure can be very beneficial post-fire but this is rarely available or reasonable at large scales.

The MSU Soil Fertility Extension website has several publications and presentations on soil testing and calculating fertilizer rates. Contact Clain Jones at or 406-994- 6076 if you have any questions.