Soil pH and Lime Requirement

The optimum (target) pH for a soil depends on the crops that will be grown. Table 4.2 lists the optimum pH levels for crops grown in Wisconsin. The amount of lime recommended is the amount needed to reach the target pH for the most acid-sensitive crop (the one with the highest target pH) that is to be grown during the next 4 years. If alfalfa will be grown on a field in the future but is not indicated in the present rotation, the lime needs for the field may be underestimated. 

Once a soil reaches the desired pH level, it will tend to remain at that level for a relatively long time without additional application of lime. This is because soils are naturally highly buffered against changes in pH. Coarse-textured soils (sands and loamy sands) are not as highly buffered against pH change as medium- and fine-textured soils, so they will generally not maintain their pH level as long. Sandy soils may need to be limed more frequently, but at much lower rates. 

Table 5.1. Formulas used to calculate lime requirement at various target pH levels.

Lime requirement calculations

Lime should be applied if the soil pH is more than 0.2 units below the target pH. Minor fluctuations inherent in both sampling and pH measurement preclude calculating lime needs when the pH is within 0.2 units of the target. The lime requirement equations listed in Table 5.1 use soil pH and buffer pH values in calculating lime requirement for a sample. 

The recommendations obtained using equations in Table 5.1 are for liming materials with a neutralizing index (NI) of 60–69. Because 80–89 NI lime is commonly used in much of the state, the necessary rate of 80–89 lime is normally listed on a soil test report along with the 60–69 rate. If using lime with an NI other than 60–69, adjust the lime requirement using the following formula: 

Lime requirement for 60–69 lime should be rounded to the nearest ton, while lime requirement for liming materials with a greater NI should be rounded to the nearest 0.5 ton/a. The lime requirement for potato should be rounded to the nearest 0.1 ton/a because potatoes are typically grown on poorly buffered soils and it is not desirable to over-lime potato fields.

Plow depth adjustment

Adjusting the lime requirement for the depth of tillage is critical for reaching the desired soil pH. In the past, most tillage operations were limited to the top 7 inches of the soil, so lime needs are based on that assumption. If tillage extends below 7 inches, the lime requirement is greater, as more soil is being mixed with the applied lime. To adjust the lime recommendation for deeper tillage, multiply the lime requirement by the factor listed in Table 5.2. 

An application rate of 1 ton/a of topdressed 60–69 lime or 80–89 lime is recommended for fields that have been under no-till management for more than 5 years and have a surface (0–2 inches) pH that is more than 0.2 units below the target pH. These fields should be retested in 3 to 4 years to determine if additional lime applications are needed. 

Averaging the lime requirement

On fields where multiple samples have been taken, a field average is normally used to determine the best overall rate. For samples where the lime requirement exceeds the field average by more than 2 tons/a, apply a higher rate of lime to the more acid part of the field. If a sample from the field indicates that the lime requirement is more than 2 tons/a below the mean, that sample should be excluded and an adjusted mean calculated using the remaining values. If only three or four samples were submitted from a field, no more than one sample will be eliminated from consideration. If five or more samples are taken to represent the field, no more than two samples will be excluded. This adjusted average is the value that is used to determine the lime needs for fields that are to be amended by applying a single uniform rate. If fewer than one-half of the samples in a field have a lime requirement, then the field lime requirement should be considered to be zero. However, growers should be aware that some parts of this field may benefit from liming and should consult the laboratory results section of the soil test report. If at least one-half of the samples in a field have a lime requirement, the field lime requirement should be based on the average of the samples with a lime requirement. Again, the laboratory results section of the soil test report should be consulted to determine which parts of the field may not benefit from liming. 

Table 5.2. Plow depth adjustment: multiply lime requirement by adjustment factor based on plow depth.

Other factors affecting lime recommendations

Coarse-textured soils are not as well buffered against changes in soil pH as are medium-and fine-textured soils. To help prevent over-liming on sandy soils with an average organic matter content of less than 1%, only 1 ton/a of lime should be applied when the calculated lime requirement is less than 1.5 tons/a. For sandy soils with more than 1% organic matter content as well as silt loam and clay soils, the minimum application should be 2 tons/a of 60–69 NI lime or 1.5 tons/a of 80–89 grade lime. The rate of lime applied should never exceed 8 tons/a for potato or 12 tons/a for other crops even though more lime may be required to completely neutralize soil acidity. Where the lime need is greater than these levels, the field may not reach the desired target pH, but the smaller application is recommended for economic reasons. 

If the field has been limed in the last 2 years, additional lime may not be needed, even though the target pH has not been reached. 

No additional lime should be applied until the most recent application has had 2 to 3 years to equilibrate with the soil and the pH has been retested. 

Choosing a liming material

When choosing a liming material, several factors should be considered: the amount of pH change required, how long it will be before the most acid-sensitive crop is planted, the availability of local liming materials, and land ownership or tenure. 

If the grower either owns the land or is assured of long-term use of a field, applying the full recommended rate of lime is justified in most cases. On the other hand, if the access to a field is uncertain from year to year, smaller applications may be worth considering. If a local source of ground limestone is available, it will most likely be the most economical source of lime because shipping costs are a large part of the cost of purchasing ground limestone. Local lime deposits are found in many parts of the state, with the exception of the north central and northern areas. 

It is advisable to base the selection of a liming material on the cost per acre after adjusting for the neutralizing index. The cheapest lime is not always the best choice. Multiply the rate per acre required by the cost per ton to determine the cost per acre. For example, if 60–69 grade lime costs $25/ton and you need to apply 4 tons/a, the total cost would be $100/a. By contrast, the equivalent amount of 80–89 required would be 3.1 tons/a. If this material costs $30/ton, the total cost would be $93/a. In this case, the more expensive material is actually the better buy. 

Several other factors, which are difficult to quantify economically, can also influence your choice of a liming material. For example, if you are liming a no-till field or will be seeding an acid-sensitive crop like alfalfa immediately after liming, you may want to select a finer ground lime to react more quickly with soil acidity even if you have to pay a premium. If large changes in pH are required, it may be wise to delay the planting of acid-sensitive crops for a year and select the most economical liming material for your situation following the guidelines above. 

Lowering soil pH 

Most horticulture and agronomic crops grow best when soil pH is between 6.0 and 6.8. Many crops can adapt to higher or lower pH levels with no drop in crop quality or yield. However, some crops, like blueberries, require acid soil conditions (soil pH of 5.5 or less) to grow and perform as expected. 

Many soils, especially those in southeastern Wisconsin, are alkaline (high pH) and may contain free carbonate, which is a source for alkalinity. Such soils require high levels of management to successfully grow crops that require acid soil conditions. If the soil pH is 7.5 or greater, growing crops that require low soil pH conditions is not recommended. 

In the rest of Wisconsin, most soils with a pH of less than 7.5 can be amended to lower the pH to the desired level (Table 5.3). The most common materials used are elemental sulfur (S) and aluminum sulfate. To lower the soil pH, elemental sulfur must be converted (oxidized) to sulfate by soil bacteria. As a result, the change in pH takes several months or longer. Sometimes the soil contains very small numbers of this special kind of bacteria. Under these conditions, the process may take 6 or more months. The oxidizing reaction brought about by the organisms is as follows: 

S + 1.5O₂ + H₂O ⟶ 2H⁺ + SO₄^2- 

Applying more than 20 lb S/1,000 sq ft per year is not recommended. If more is required, use split applications of 20 lb S/1,000 sq ft and apply in succeeding years. Check the soil pH before making a second application to see how much change has taken place. Aluminum sulfate [Al₂(SO₄)₃] can also be used to lower soil pH. Its effect is nearly immediate, but the cost is higher than using elemental sulfur. The amount of aluminum sulfate needed to achieve the same decrease in pH is six times the amount of elemental sulfur required.

Because too much aluminum can be toxic to plants, aluminum sulfate should not be applied at rates exceeding 50 lb Al₂(SO₄)₃/1,000 sq ft at any one application.

Keep in mind that fertilizer products containing sulfate-sulfur are not effective in lowering soil pH. This includes products such as potassium sulfate (K₂SO₄) and gypsum (CaSO₄).

Table 5.3. Amount of finely ground elemental sulfur (S) needed to lower soil pH (increase acidity).