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Soil test pH and liming: Common questions and answers

lime ready to be added to soil

By: Dan Kaiser, Extension nutrient management specialist

When it comes to soil tests, I commonly receive questions about pH and liming. Below are some common questions I get and my answers.

What exactly is the buffer pH? 

Soils are routinely analyzed for water pH where equal weights of water and soil are mixed, and a pH electrode is used to determine the pH of the solution. The water pH represents what we call the “active” pH of the soil. When the water pH drops below a certain point, most labs will then run a buffer pH on the soil, which is used to determine the amount of limestone needed to raise the water pH to a desired value. Buffer pH methods use various extraction solutions and are meant to measure both the active and reserve acidity of the soil, which provides a better indication of how the active acidity will change when limestone is applied. The two tests will not return the same value and it is common for the buffer pH value to be higher than the water pH in nearly all soils. 

If I have a water pH value, but no buffer pH value, can I still determine how much lime I need?

While having a buffer pH value is best for determining lime needs, the University of Minnesota does have suggested rates of lime based solely on water pH. In fact, for organic or peat soils, there is no suggestion that uses a buffer pH to determine lime requirement. Organic or peat soils differ from mineral soils in that they tend to be more acidic and the current Minnesota guidelines for liming only suggest raising these soils to a pH of 5.5. For mineral soils, it is suggested to raise soil pH to 6.0 or 6.5, depending on the crop. It is important to know what crops you are growing and what their optimal pH is to determine how much lime your soil requires.

Would raising my soil to a pH of 7.0 be better?

Some states may suggest liming soils to a pH of 7.0, which is considered to be neutral pH. However, crops like corn and soybean tend to show positive yield benefits to lime only when the soil pH is below 6.0. For alfalfa, yield may be impacted when pH is less than 6.5. Raising your soil pH to 7.0 may help to increase the time between lime applications but there is generally no economic justification to do so for most crops. It should be noted that there are separate lime guidelines in the state of Minnesota for western versus eastern Minnesota. This is because there are higher levels of soil carbonates near the soil surface in western Minnesota, where leaching has resulted in the carbonate layers being deeper in the soil profile than in eastern Minnesota. 

I have multiple options for lime. Which one should I choose?

Effective neutralizing power, or ENP, can be used to assess differences in liming sources. The ENP rating should provide a level playing field to determine the overall economics of different sources of limestone. An ENP rating is made on a per ton of material basis. So, if you want to look at costs, you can take your suggested application rate of ENP per acre and divide it by the ENP rating per ton of material. This will give you the total cost when comparing materials. However, the ENP ratings do not factor in how easy it is to spread certain materials. Lime needs to be spread evenly to have the greatest impact and materials high in moisture can form large clumps, which can make it difficult to spread evenly across a field. This should be taken into account when deciding on the best material to use.

Is it possible to over-lime a field?

Excessive lime application can result in soil pH values that may cause some issues for crops such as soybean. We do know that nutrient availability in soils is impacted by pH and that high pH soils can present challenges when it comes to phosphorus acquisition. Also, for soybean, high pH soils can impact the acquisition of iron, causing iron deficiency chlorosis. The maximum pH that typically can be achieved with liming sources is around 8.4 to 8.5. While it is possible to find soils with even higher pH values, those soils typically have other issues, such as high salts or high sodium content, which are separate issues. 

Can I acidify high pH soils?

While it would be possible to acidify a soil where too much lime was applied, there is no economically feasible way to acidify soils natively high in lime, such as those in western Minnesota. The layers of carbonates in soils in western Minnesota can go from the soil surface to several feet deep in the soil profile. Acidification in the upper soil surface would only be temporary and have a very minimal impact on crops. In these types of soils, it is better to deal with the high lime in other ways and not fall for any of the misinformation on using elemental sulfur or AMS to acidify natively high pH soils.

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Comments

  1. I have fields next to gravel roads that test above 8. P levels are good, but still have I assume iron chlorosis by your description. Can I add iron?

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  2. What is the pH in the rest of your field. It is not uncommon to find high pH as you go up to 100' or more from the field boundary that borders a gravel road. You may not need to apply iron though unless you see any visible yellowing of the soybean plants especially if the subsoil is more acidic. I don't think I would be too concerned unless there is some visible yellowing. If that is the case and you wish to apply iron it needs to be applied directly on the seed to work. However, do not apply any other liquid fertilizer directly on the soybean seed.

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