Soil Acidification: Measure & Manage

Acidity assessment

• Look for unexplained poor health generally in low or mid-slope areas. Plants affected by aluminum toxicity have club shaped roots (Figure 1). For more photos go to our Nutrient Deficiency and Toxicity webpage.
  

  Figure 1. Club shaped roots on durum wheat caused by aluminum toxicity.

• Test a soil/water slurry at the 2- and 4-inch depth with a pH meter stick/probe (for digital reading, rather than color strips) where low pH is suspected. If soil pH < 5 at either depth then soil acidity is likely a concern. Soil pH tests to calculate lime rates should be on well-mixed 6-inch deep samples. See Soil Acidification: Problems, Causes & Soil Testing for more information.
• Look at standard soil tests for composited soil from a field in the top 6 inches. If pH < 6 it is likely the field will have spots close to or below pH 5. If pH > 6, don't assume there are no areas with low pH.
• Soil test top 3 inches from affected regions for KCl extractable aluminum (Al). If Al > 5 ppm then soil acidity is likely a concern.
• Compare manganese (Mn) tissue analysis from 'good' and 'bad' crop areas. Tissue Mn > 500 ppm is likely a problem (Ohki 1984) . Aluminum does not translocate very well from roots to shoots, so shoot tissue analysis for Al is not worthwhile. Also, since acid tolerant crop varieties have lower leaf Al concentrations than acid sensitive ones, it is difficult to provide critical concentration levels for a crop (Foy 1996).
• Dr. Amber Moore's WERA-103 presentation on the ins and outs of soil pH and buffer pH testing

Manage low pH

Tools and steps for remediation, adaptation and prevention are detailed in other materials listed on our Resources page.

Lime

Based on the MSU studies, you can use one of the following two methods to determine lime rates:

• Ask a lab to use the modified Mehlich buffer test to determine buffer pH and liming needs. Watch The Ins and Outs of Soil pH and Buffer pH Testing for more information.
• See Soil Acidification: Remediation with Sugar Beet Lime, Fertilizer eFact No. 80 for liming rates with spent sugar beet lime. Or use 0 to 6-inch soil pH values and this equation (Engel unpub. data) to estimate lime requirements. Lime rate (CaCO₃ ton/acre) = 1.5 x (desired pH increase)
  
  • Sugar beet lime and most aglime must be incorporated with tillage (e.g., 4-5 inches with beavertail spike and harrow, or tandem disc harrow) to be effective. It does not move into the soil with rain in dryland conditions.
  • 

    Figure 2. Soil organic carbon before 4-5" tillage to incorporate sugar beet lime, and a year later, by soil depth. Fort Benton and Geraldine, MT, sites (Engel unpub. data).

    The single tillage used to incorporate sugar beet lime did NOT reduce soil organic carbon (SOC) in the top 8-inches of soil a year after tillage (Figure 2).
  • A review (Blanco-Canqui & Wortmann 2020) of occasional tillage (aka, limited, conservation, strategic, minimum, targeted) used to address no-till challenges found:
    • Effect of tillage depends on tillage method, depth, timing, soil temp and water content at time of tillage.
    • Soil organic carbon was not decreased, but there was less vertical stratification of SOC and nutrients.
    • Soil water content and aggregation were not generally reduced.
    • Where occasional tillage did affect soils and crops, the impacts generally lasted less than 2 years.

Sugar beet lime increased soil pH (Figure 3) and crop yields within 2 years after application (Figure 4). Based on the rate of acidification that occurs with suggested ammonium-based N fertilization rates (Figure 6A), yield benefits should last about 15 years (Rick Engel, unpub. data).

Figure 3. 2020 aerial photo of a field that received SB lime in 2017 with lime rate and 2019 soil pH levels at each strip. Photo courtesy Bill Summers.

Figure 4. Pea and durum yields by soil pH 2- and 3-years after SB lime application (Rick Engel, unpub. data).

Other Management Options

• Seed-place phosphorus fertilizer can increase small grain yields by reducing aluminum toxicity in a given growing season.

• Consider planting small grain species or varieties that are aluminum or low-pH tolerant  (Table 1).
• Consider low-pH tolerant perennials (Table 2).
• 

  Figure 5. Soil pH after 14 years of N fertilization at recommended rates in different cropping rotations in Gallatin Valley, MT. On average, 100 lb N/ac caused pH to drop by 0.044 units (Engel et al. unpub. data).

  Ammonium-based fertilizer causes soil acidification, even when used at rates within research-based guidelines (Figures 5, 6A, 7). Including legumes in rotation reduces N-fertilizer input and soil acidification (Figures 5, 6B), while perennials can slowly reverse soil acidification.
• Take steps to increase N fertilizer use efficiency, reduce N rates when possible, keep plant stubble on the field, and include legumes, crops with low N requirements, and perennials in rotation. Prevention is preferred over liming.

Figure 6. Soil pH after 8 years in relation to, A) total N fertilizer added over 8 years on diverse rotations with winter wheat (fallow, spring wheat, pulse, legume green manure), and B) averaged across 0, 0.5, 1, and 1.5 x the recommended N rate. Near Big Sandy, MT, on sandy loam. On average,100 lb N/ac caused pH to drop by 0.14 units (Jones & Miller unpub. data). Bars w/out any of the same letters indicate treatments had different pH levels with at least 90% confidence.

Figure 7. Soil pH on grazed native range w/out N fertilization and crested wheatgrass pasture with 40 lb N/ac each of 43 years (Liebig et al. 2014, ND).

References cited:

Blanco-Canqui, H., and C. Wortmann. 2020. Does occasional tillage undo the ecosystem services gained with no-till? A review. Soil & Tillage. 198:104534 doi:10.1016/j.still.2019.104534

Foy, C.D. 1996. Tolerance of durum wheat lines to an acid, aluminum-toxic subsoil. J. Plant Nutr. 19:10-11. doi:10.1080/01904169609365206

Liebig, M., S. Kronberg, J. Hendrickson, and J. Gross. 2014. Grazing management, season and drought contributions to near-surface soil property dynamics in semiarid rangeland. Rangeland Ecol. Manage.  67:266-274. DOI: 10.2111/REM-D-13-00145.1

Ohki, K. 1984. Manganese deficiency and toxicity effects on growth, development, and nutrient composition in wheat. Agron. J. 76:213–218