Planning to Lime for the 2022 Crop

Larry Oldham, Extension Soils Specialist
By Larry Oldham, Extension Soils Specialist March 20, 2021 09:35 Updated

Planning to Lime for the 2022 Crop

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Corn in soil with pH 4.3, photo by E. Larson.

Wait a minute, aren’t we just now planting the 2021 crop? True, however, some fields being planted now will suffer from soil acidity issues this year. There may be short term band-aids available, but long-term solutions will be necessary. One of my mentors in this business said, “pH easy to spell, more difficult to understand”.

Soils become more acid over time from:

  • Rainfall and subsequent leaching of basic cations such as calcium,
  • Acidic parent material may be a factor in some regions,
  • Organic matter produces acidity as it decays,
  • Harvest of high yielding crops removes base cations such as calcium, and
  • Acidification as ammonium fertilizers change to nitrate after application.

Severe soil acidity issues can present obvious visual symptoms in the field (see accompanying photo); more often the effects are gradual. Poorly performing spots in fields get larger over the years, plants grow slower, there may be some yellowing. Digging up root systems may find them short, stubby, and maybe dark in color.

You see these symptoms because soil pH is the master variable of soil chemistry and controls many factors in the rooting environment. There are more hydrogen ions (the H part of pH) where the roots are trying to grow in low pH soils. Because pH is a negative logarithmic measurement (the p part of pH), the soil is 10 times more acidic when the pH is 1.0 units lower. The soil is 100 times more acidic when the pH is 2.0 units lower. (Discussions have arose in the chemistry community about defining ‘p’. We will not go down that path.)

Aluminum at pH’s over 5.5 is combined with other elements, and benign toward growing plants. As pH decreases below 5.5, the aluminum becomes disassociated and creates serious issues in the soil solution. Many crop roots cannot thrive in such acidic soil environs; therefore, they end up short, stubby, and possibly darkened.

Phosphorus and molybdenum are less bioavailable as pH decreases in the complex soil chemistry system. Conversely, manganese, while essential for plant growth, becomes available at toxic concentrations – potentially the cause of the yellowing noted above, and can be lethal at very low soil pH levels (reference: personal experience). Nitrogen fixation in legumes proceeds at faster rates in less acidic soils; and applied phosphorus and potassium fertilizer efficiency is better.

Soil testing the good versus bad spots should confirm soil acidity issues.

When soil tests confirm acidity as the issue during the current growing season, the long-term solution is to start planning and implementing a liming program before the next one (hence the title), remembering that lime requires moisture and time to react in the soil.

‘Hard rock’ calcitic and dolomitic lime materials are significant investments in Mississippi because they must be imported from other states via truck or barge. As imported materials, even if from Alabama, logistical details need to be addressed including sources, trucking, application method, and storage. The ‘soft rock’ limes available from in-state vendors must be kept dry to prevent caking. Various industrial by-products have been offered as liming materials – check with the Mississippi Bureau of Plant Industry as whether products have been approved for land application under the appropriate regulations.

When evaluating potential sources, remember that the effectiveness of lime depends on the material neutralizing value and fineness. The method for comparing the relative values of different lime materials is available at Agricultural Limestone’s Neutralizing Value.

The title should be evident now because liming soils in Mississippi is very important when needed, but the long-term remedy requires investing management, capital, and time.

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Larry Oldham, Extension Soils Specialist
By Larry Oldham, Extension Soils Specialist March 20, 2021 09:35 Updated
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