Soil Acidification: Management
Most agricultural soils in Montana have near neutral to basic conditions with surface soil pH 6.5 to 8. However, some areas of Montana have acidic soils, especially in the seeding zone. The Soil Scoop Soil Acidification: Problems, Causes & Soil Testing introduces soil pH, why it is a concern, agronomic practices that contribute to soil acidification, and soil testing for pH.
Management: prevention & mitigation
- Use practices and rates to optimize nitrogen (N) use efficiency – no left-over N. Base N rate on spring soil tests and realistic yield potential, split N applications (and don’t apply 2nd time in dry springs), and place N in the root zone.
- Reduce nitrate (NO3-) loss. Use slow-release N sources or N sources with nitrification inhibitors, and plant deep rooted crops to ‘catch’ deep nitrate. Deep rooted crops can also pull base-forming cations (Ca2+, K+, Mg2+) from the subsurface to the surface.
- Use calcium ammonium nitrate (27-0-0) which has less acidifying potential than urea (46-0-0).
- Use pulse crops in rotation – they don’t need N fertilizer.
- Plant aluminum (Al)- or low-pH tolerant crops (Figure 1). See our list of low-pH tolerant species and varieties.
Inversion till to mix acid zone throughout plow layer and bring up calcium carbonate (CaCO3) from deeper layers. A soil with 5% CaCO3, typical in Montana, contains 100 tons of CaCO3 in the top foot. One-time summer tillage doesn’t negate long term benefits of no-till (Norton et al., 2014). This is only a short-term fix if management isn't changed to minimize further soil acidification.
Fig. 1. Crop species vary in tolerance to low pH, or high aluminum levels (McFarland et al. 2015, Ron Long unpub data).
- Increase soil organic matter (SOM) to buffer pH changes and reduce Al, manganese (Mn) and H+ toxicity. Leave crop residue in field to retain base cations or apply manure. Replace fallow with crops or cover crops.
- Seed-place lime to compensate for annual N application or amend whole field with several years’ of potential lime need. Broadcast conventional ag-lime needs to be tilled to affect the seeding zone. Other options are presented below.
- Band P with seed (binds some Al). 60 lb P2O5/acre (0-45-0) produced similar durum grain yields to 5 ton sugar beet lime application on a field with Olsen P = 48 ppm, but had no impact on grain yields on a neighboring field with Olsen P = 53 ppm (Engel, unpub. data).
- Do field strip trials to see what works for your situation.
Liming material reacts with carbon dioxide and water in the soil to yield bicarbonate (HCO3-), which takes H+ and Al3+ (acid-forming cations) out of solution, raising soil pH. The benefits are varied and depend on the soil pH level reached (Table 1).
|6.1 - 6.5||Improve soil structure; reduce crusting; reduce power need for tillage|
|5.6 - 6.0||Increase soil microbial activity; increase rhizobia health for N-fixation and other mycorrhizal assisted crops (legumes and barley); increase plant nutrient availability|
|5.1 - 5.5||Reduce Al, H+, and Mn toxicity; increase availability of P and other nutrients|
|< 5.1||Few crops can produce if not limed|
Different materials have different ‘potency’ to raise soil pH. Calcium carbonate equivalent
(CCE) compares a
liming material to pure CaCO3. Lime Score (LS; Table 2), also called effective neutralizing value (ENV), combines
CCE with factors for moisture and fineness to calculate liming rates. Fineness is determined by the particle size.
Those particles that pass a 100-mesh sieve react within a few weeks, 60 to 100-mesh in 1 to 2 years, and 20-mesh
in 2 to 3 years (Mullins et al. 2009). The particle size distribution of liming material determines how quickly and how long the material increases soil pH.
|Limestone (CaCO3)||90 - 100|
|Dolomite (CaCO3 + MgCO3)||95 - 110|
|Hydrated lime (Ca[OH2])||120 - 135|
|Burn lime or calcium oxide (CaO)||150 - 175|
|Sugar beet lime (free at beet processing plants)||60a|
|a Olsen's Agricultural Laboratory, Inc., McCook, NE|
Take preventive measures and monitor soil pH to determine if mitigation is necessary. All but very fine lime takes several years to penetrate the seeding zone. Apply lime at least the fall prior to a spring planting. For perennial crops, ideally apply enough before seeding for the longevity of the stand. Humid days with little wind are ideal to minimize the amount of lime blown away during surface application. Tillage increases the rate and depth at which lime increases soil pH.
The following information is needed to calculate a liming rate:
- Lime score (Table 2 or on lime material label)
- Current soil pH and desired pH (> 5 to reduce Al, H+, Mn toxicity; > 5.5 to have some buffer; > 6 to be good for 10+ years)
- Soil texture (sand<sandy loam<silt loam<clay loam)
- Desired crop
The lime rate can come from either buffer pH soil test lab results (SMS, Sikora, SMP, Mehlich, modified Mehlich tests are suitable for our soils) and desired crop and target pH, or use MSU's preliminary results (Table 3).
|Initial pH||To pH 6.0||To pH 6.5|
Lime rate may also be estimated using the following adapted from research in eastern Washington (McFarland 2016).
Lime rate (ton/acre) = 1.86*(desired pH increase)
Lime rate is given in units of CaCO3 (100% CCE) and has to be adjusted by the lime score (LS) of the product being used
(Table 4). Then calculate the most economical available source. Lime rates above 2 ton/acre are not economical if only top-dressed without incorporation (Anderson et al. 2013).
|Calculation step||Product A||Sugar beet lime|
|1. Look up LS||89||
2. Adjust for LS
lb = (6,000/LS x 100)
|6,741 = 3.4 ton||10,000 = 5 ton|
|3. Cost per ton||$75||$35 (free material + transport)|
|4. Cost per acre||$253||$175|
Lime Placement Options
- On surface with 4–6” tillage or 2 sweeps of minimum tillage,, e.g. single pass with beavertail spike followed by harrow
- Surface broadcast – doesn’t move deep, 1.5 ton lime applied to silty clay loam increased soil pH to 1.5” depth after 6 years, but no deeper (Mellbye 1992)
- With irrigation water
- Ultra fine lime surface sprayed – increased soil pH at 1” depth within 6 months (McFarland 2016)
- Place pelleted lime in seed row – products and results are under investigation, currently pelleted lime is more expensive than conventional ag-lime
- Inject fluid (liquid) lime into seeding zone - quick acting but more expensive
The economics of variable rate applications are not yet known, but it makes sense to not apply lime where it is not needed.
For more information
MSU Extension Soil Fertility: Cropland Soil Acidification webpage
Eastern Oregon Liming Guide. 2013. Oregon State University
Extension Bulletin, EM 9060. https://catalog.extension.oregonstate.edu/em9060
Soil Acidification: Problems, Causes & Soil Testing http://landresources.montana.edu/soilfertility/soilscoop.html
Soil Acidity in Oregon: Understanding and Using Concepts for Crop Production. 2013. Oregon State University Extension Bulletin, EM9061 https://catalog.extension.oregonstate.edu/em9061
The Soil Scoop at http://landresources.montana.edu/soilfertility/soilscoop.html
Washington State University – assorted lime fact sheets http://smallgrains.wsu.edu/soil-and-water-resources/publications/
MSU Extension publications at http://landresources.montana.edu/soilfertility/publications.html
Soil Sampling Strategies
Interpretation of Soil Test Reports for Agriculture
MSU Nutrient Management Module at http://landresources.montana.edu/NM/
Soil Sampling and Laboratory Selection
Anderson, N.P., et al. 2013. Applying Lime to Raise Soil pH for
Crop Production (Western Oregon). Oregon State University Extension Bulletin, EM 9057
Engel, R. Professor, Land Resources and Environmental Sciences, Montana State University
McFarland, C., et al. 2015. Soil pH and Implications for Management: An Introduction. Washington State University
Extension Bulletin FS170E
McFarland, C. 2016. Liming No-till Soils and Determining Lime Requirement in the Palouse Region. Washington State University M.S. Thesis.
Mellbye, M. 1992. Surface Limed Soil—Six Years Later. OSU Extension Update (Linn County), Vol. XI, No. 9, p. 6. http://hdl.handle.net/1957/38002
Mullins, G.L., et al. 2009. Sources of Lime for Acid Soils in Virginia. Virginia Extension Publication 452-510.
Norton, U., et al. 2014. One-time summer tillage does not negate longterm benefits of no-till. Crops & Soils. May-June:24-25.
Posted September 2019