Liming an acid soil is the first step in creating favorable soil conditions for productive plant growth. Crops vary in their ability to tolerate an acidic (low pH) soil. In addition, evidence has shown that soil acidity may influence other crop management problems such as herbicide activity. Soil pH is a good indicator of the need for liming. A soil pH of 5.5 or lower will often result in significant negative impact on most crops. The general goal of liming agricultural soils continues to be a soil pH of 6.0 to 7.0.
Raising soil pH requires a quantity of agricultural liming material that is determined by the amount of acidity in the soil and the quality of the liming material. Soil acidity is measured by soil testing; the quality of agricultural liming material is determined by its purity and particle size distribution. In Pennsylvania, agricultural liming materials must, by law, meet minimum quality standards.
Limestone is applied to neutralize the acidity in the soil and thus raise the soil pH to the optimum range for crop growth. The limestone recommendation is based on the amount of exchangeable acidity measured in the soil and the optimum soil pH level for the crop. The recommended limestone application is a one-time application to cover the three years on the test. For most agronomic crops, the optimum pH is 6.5. For alfalfa and barley, the pH goal is 7.0. The pH requirements vary from crop to crop. However, because only one limestone recommendation is made for three years, the recommendation on the report will adjust the pH for the most sensitive crop to be grown during the three years. The actual pH goal used to make the limestone recommendation is indicated next to the recommendation on the report.
Lime requirement, as pounds of calcium carbonate equivalent (CCE) per acre, can be estimated using the following formulas. The exchangeable acidity is given at the bottom of the Penn State Soil Test Report (see sample soil test report). Some soil testing labs report exchangeable acidity as “Exchangeable H+.”
- For a desired pH of 7.0, the lime requirement in lb CCE/A can be estimated as follows:
- Lime requirement = exchangeable acidity × 1,000
- For a desired pH of 6.5, the lime requirement in lb CCE/A is estimated as follows:
- If the exchangeable acidity is greater than 4.0, then:
- Lime requirement = exchangeable acidity × 840 or
- If the exchangeable acidity is less than 4.0 and the soil pH still less than 6.5, then:
- Lime requirement = 2,000 lb/A
- Otherwise, no lime is recommended.
The recommendation is based on liming an acre furrow slice approximately 7 inches deep. If the limestone will be mixed with a larger volume of soil (i.e., if the plow depth is more than 9 inches), the recommendation is adjusted as follows:
|Plow depth||Adjusted limestone requirement|
|Less than 9 inches and reduced or no tillage
|9 to 11 inches||Basic requirement x 1.5|
|More than 12 inches||Basic requirement x 1.8|
Additional Considerations for Prioritizing Liming Management
Although the best practice is to apply the recommended amount of a liming material, a partial application should be considered rather than no application at all. Below are some important considerations for prioritizing liming management:
- Always maintain soil pH above 5.5. This is a critical pH for root growth for most agronomic crops. With limited money to spend on lime, at least try to apply some limestone to any field with a pH near or below 5.5. To estimate a limestone recommendation for a target pH of 5.5, multiply the “Exchangeable Acidity” on your soil test by 400. This will give you the approximate pounds of calcium carbonate equivalent to apply to at least achieve a pH of 5.5.
- Prioritize fields that will be planted to alfalfa for limestone application. The rhizobia that fix nitrogen in the nodules on alfalfa roots are less productive at low soil pH. This can result in nitrogen deficiency in alfalfa. Since an alfalfa seeding is a long-term investment, this is an especially critical time in the crop rotation to correct the soil pH to at least 6.5–7.0.
- Maintain pH in long-term no-till fields. Because there is no mixing by tillage, regular liming is essential to maintain soil pH in long-term no-till fields. If the pH in a no-till field is allowed to get too low, it can take a long time to correct this condition without going back to tillage.
- For better nutrient use efficiency and herbicide activity, try to maintain the pH at least above 6.0 if possible. Even if you cannot afford the full recommended rate of lime, the next priority would be to get as many fields as possible above 6.0. To estimate a limestone recommendation for a target pH of 6.0, multiply the “Exchangeable Acidity” on your soil test by 600. This will give you the approximate pounds of calcium carbonate equivalent to apply to achieve a pH of 6.0.
- Ideally, apply the amount of limestone recommended on the soil test for the crops to be grown.
In a tillage system, when less than the full limestone requirement is applied, make the application after primary tillage and incorporate it by secondary tillage, thus mixing the limestone 4 inches into the surface. This ensures a better environment for germinating seeds and young seedlings, as well as more effective herbicide action in the surface soil layer.
When high rates of limestone are recommended (4 tons or more per acre), the applications should be split. See liming material conversion for guidance on splitting high limestone recommendations. Separate the applications by 6 months or at least by tillage operations. In a tillage-based management system, plowing down part of the limestone and then applying the balance and incorporating with secondary tillage is an excellent way to correct the pH in the whole plow layer.
As soil acidity increases (soil pH becomes lower), the need to apply a liming material to any crop and the expected returns of applying it become greater. If soil acidity is limiting crop production, other production inputs such as fertilizer, seed, pesticides, machinery costs, and labor will not realize the kind of return that would be possible on a well-limed soil.
See Soil Acidity and Liming for details on limestone recommendations, liming material quality, and liming practices.