Soils

John Hart and Don Horneck

Soil Acidity and Liming, Part III
How Much Lime is Needed?

Soil pH is an easily measured soil chemical property used to assess the condition roots experience. Low pH or acidic soil increases the solubility of iron, zinc, manganese, copper, and aluminum. Concentrations of manganese and aluminum can reach levels that are toxic or at least inhibit root growth. Bacteria that assist with nitrification, the conversion of ammonium to nitrate, are also adversely affected by low soil pH. Crop sensitivity to soluble manganese and aluminum varies not only by species but also by variety. This sensitivity can also be soil dependant. Some varieties may show manganese or aluminum toxicity, while others remain productive.

The adaptation and selection process sorted plants for tolerance to soil pH. Blueberries and cranberries tolerate high acidity (low soil pH) while garlic and alfalfa grow well in slightly acidic to neutral soil (moderate soil pH). Historically, we plant crops or ornamentals in soil modified to fit the pH requirements of the plant rather than choosing to plant blueberries in a field because the soil pH is appropriate for that crop.

When the soil pH is below a suitable level for the species we wish to plant, lime is added to raise the soil pH. Table 1 presents suitable soil pH levels for selected plants.

Table 1. Suitable soil pH for some common plants. 

After the need for lime is established, the next question to address is “how much lime should be applied?”. In Oregon, the amount of lime needed to raise soil pH is estimated by a soil test called the SMP buffer, lime requirement (LR), or buffer pH test. SMP are the initials of the Shoemaker, McClean, and Pratt, the people that published the procedure used for rapidly measuring lime requirement.

The first step toward understanding the SMP Buffer soil test is to understand a buffer. A buffer has the ability to resist change. Let’s use a monetary example to explain a buffer. We will use a wallet containing $10 and bank account containing $100,000. Consider the change if $1 is removed from both the wallet and the bank account. The amount of money in the wallet is reduced by 1/10 while the bank account is relatively unchanged. We could also add money to the wallet, $10, from the bank account. The $10 added to the wallet doubles the amount of money in the wallet, but changes the amount in the bank account relatively little. When $10 is moved from the bank account to the wallet, no change is made in the total amount of money, but the amount in the wallet is changed dramatically. Our example shows that the bank account is buffered and the wallet is not buffered.

Let’s apply the concept of a buffer to soil and soil acidity. In the first article of this series, soil pH was defined as the amount of hydrogen ions in soil solution. Using the wallet/bank account analogy, hydrogen in the soil solution would be money in the wallet. Hydrogen ions are also attracted to the soil surface. Think of these hydrogen ions as those in the bank account. The wallet/bank account ratio of money is approximately the same ratio of hydrogen ions in soil solution to hydrogen ions attracted to the soil particles. The hydrogen ions attracted to soil particles are extremely important, as these ions must be reduced to effectively raise soil pH for plant growth.

The number of hydrogen ions attracted to soil particles is primarily a function of clay and organic matter content. A sandy soil low in organic matter would have fewer hydrogen ions attracted to the soil particles (small bank account) than would a clayey soil high in organic matter (large bank account). The sandy soil is not as well buffered as the clayey or high organic matter soil. When a sandy and a clayey soil both have the same pH, the same amount of hydrogen ions are in the soil solution but fewer hydrogen ions are attracted to the soil particles in a sandy soil. Since the hydrogen ions attracted to the soil surface is a much greater quantity than the hydrogen in soil solution, less hydrogen is found in the sandy soil and less lime is required to raise the pH of the sandy soil than for the clayey soil.

To change soil pH, the hydrogen in solution and attracted to the soil particles must be reduced. To determine the amount of hydrogen that needs to be reduced, or conversely, the amount of lime to add, the hydrogen buffer needs to be measured. The hydrogen buffer is measured with the SMP buffer. The SMP buffer has an initial pH of 7.5. When soil is added, the pH of the mixture will be lower than 7.5. The lower the pH of the mixture of soil and the SMP buffer, the greater the amount of lime needed. The SMP Buffer pH usually is approximately 0.6 to 0.8 units higher than the soil pH in Western Oregon.

A commonly asked question is “if the SMP Buffer measures much more hydrogen than does soil pH, why isn’t the SMP Buffer pH lower than the soil pH?”. You can answer this question now that you know the SMP Buffer’s pH initially is 7.5 and it is a stronger buffer than soil.

Let’s look at some data. A soil has a pH of 5.1 and SMP Buffer pH of 5.9. How much lime is needed? First a target pH is needed. Assume a cool season perennial grass such as tall fescue is being grown. Table 1 shows that the minimum soil pH should be 5.5. We don’t want to add just enough lime to raise the pH to 5.5, but have a slightly higher soil pH after adding lime. So let’s use 6.0 as our target soil pH. We estimate the amount of lime needed to raise the soil pH from 5.1 to 6.0 when the SMP Buffer pH is 5.9 from Table 2. The amount of 100 score lime needed is 3.3 tons/acre.

To find the lime rate using Table 2, follow the left-hand column, “SMP Buffer” until you find 5.9. For this example, 5.9 is in bold type. Now move horizontally to the right to the column with “6.0” on the top. Remember that a soil ph of 6 was our target soil pH. In bold, “3.3” is found. The rate of lime to raise the soil pH from 5.1 to 6 with an SMP Buffer of 5.9 is 3.3 tons per acre.

The table heading tells us that the lime rate is for 100 score lime. An explanation of lime score and sources can be found in an extension publication FG 52, Fertilizer and Lime Materials. It is available electronically at: http://eesc.orst.edu/agcomwebfile/edmat/html/fg/fg52/fg52.html

Table 2. Estimation of lime requirement in t/a using the SMP buffer.

Soil pH indicates if lime is needed and the SMP Buffer is used to estimate how much lime should be applied to raise soil pH to a target level. The question to be addressed in the next issue is another commonly asked question "how often should lime be applied?".

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