Soil Acidification: Management

Most agricultural soils in Montana have near-neutral to basic  surface soils with pH 6.5 to 8, yet fields with crop losses due to soil acidification have been found in 24 Montana counties. 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. Base N rate on spring soil tests and realistic yield potential and split N applications (don't apply 2^nd dose in dry years). Reduce nitrate (NO₃^-) loss. Plant deep rooted crops to 'catch' deep nitrate and pull base-forming cations (calcium, Ca²⁺; potassium, K⁺; magnesium, Mg²⁺) from the subsurface to the surface. Use slow-release N sources or N sources with nitrification inhibitors.
• 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 and often reduce N needs on following crops.
• Plant perennial crops.
• Plant aluminum (Al)- or low-pH tolerant crops (Figure 1) or varieties. See our Soil Acidification website.
• 

  Fig. 1. Crop species vary in tolerance to low pH, or high aluminum levels (McFarland et al. 2015, Ron Long unpub data).

  Inversion till to  bring up calcium carbonate (CaCO₃) from deeper layers, only if followed by agronomic practices that do not further reduce soil pH. Otherwise acidic soil literally becomes a deeper problem. One-time summer or fall tillage doesn’t negate long term benefits of no-till (Blanco-Canqui & Wortmann 2020, Engel 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, apply manure, replace fallow.
• Lime low pH soils (< pH 5.5) for about a 15-year benefit or seed-place about 200 to 400 lb prilled lime/ac to compensate for acid produced by a typical annual N application.
• Band P with seed (see Fertilizer eFact No. 79).
• Do field strip trials with lime or P to see what works for a given field before spending money on entire field.

Liming

Liming material reacts with water in the soil to yield bicarbonate (HCO^₃-), 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).

Source

Different materials have different ‘potency’ to raise soil pH. Calcium sources such as gypsum (calcium sulfate) that don’t have carbonate, hydroxide, or oxide do NOT neutralize soil acidity (increase 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 moisture and fineness to calculate liming rates. Fineness is determined by the particle size. 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). Liming with spent sugar beet lime is explained in Fertilizer eFact No. 80.

Timing

Monitor soil pH to determine if mitigation is necessary. All but very fine lime reacts slowly over several years and must be incorporated. Apply lime at least the fall prior to a spring planting. For perennial crops, apply enough before seeding for the longevity of the stand. Humid days with little wind are ideal for surface application.

Rate

The following information is needed to calculate a liming rate:

• Lime score (Table 2 or on lime material label)
• Current and desired soil pH. See Soil Acidification: Problems, Causes & Soil Testingand our Soil Sampling website for soil pH testing instructions. Desired pH:
  • > 5 to reduce Al, H⁺, Mn toxicity
  • > 5.5 to have some buffer
  • > 6 to be good for 10+ years

The lime rate can be estimated as:

Lime rate (ton/acre) = 1.5*(desired pH increase)

Lime rate is given in units of CaCO₃ (100% CCE) and must be adjusted by the lime score (LS) of the product being used
(Tables 2 and 3). Then calculate the most economical available source.

The above equation was based on two clay loam soils. Silty and sandy soils require less lime, while soils higher in clay require more. We are still in somewhat trial and error stage of lime rates for different soil types.

On-farm strip trials are highly suggested to evaluate liming. A ‘semi-truck’-load carries about 23 ton sugarbeet lime, which is 14 ton CaCO₃ (see Table 2; 23 ton x 0.60). To raise a soil pH from 4.5 to pH 6 requires 1.5 ton CaCO₃ x 1.5 pH increase = 2.25 ton CaCO₃/ac. Therefore, a single truck-load could treat a little over 6 acres. Preventive measures start looking appealing.

Lime placement options

• Surface broadcast and incorporate with 4 to 6" tillage, or 2 sweeps. Without tillage, 1.5 ton aglime on silty clay loam only increased soil pH to 1.5" depth after 6 years (Mellbye 1992); sugarbeet lime on loam increased soil pH only to 2" depth after 2 years (Fertilizer eFact No. 80). Adding higher rates of lime does not offset the need for incorporation.
• With irrigation water
• Surface spray ultra fine lime – increased soil pH at 1” depth within 6 months (McFarland 2016)
• In seed row – currently prilled 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 in Montana, given the high cost of lime application, variable rate makes sense. Aerial images (e.g., Google Earth) along with ground truthing, are an inexpensive way to map areas that have low productivity and may need liming.

For more information

MSU Extension Soil Fertility: Cropland Soil Acidification webpage
http://landresources.montana.edu/soilfertility/acidif/index.html

Soil Acidification: Problems, Causes & Soil Testing  http://landresources.montana.edu/soilfertility/soilscoop.html

MSU Fertilizer eFacts no. 78, 79, 80

References

Blanco-Canqui, H., and C. Wortmann. 2020. Does occasional tillage undo the ecosystem services gained with no-till? A review. Soil & Tillage. 104534 doi:10.1016/j.still.2019.104534

Engel, R. Professor Emeritus, 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.

Posted August 2020