Fact Sheets And Publications
Measurement and Management of Soil pH for Crop Production in Delaware
What is Soil pH and Why is it Important?
Soil pH is a measure of soil acidity or alkalinity, which are a function of the presence or absence of active hydrogen ion (H+) concentration in the soil solution, and is calculated using the following equation:
pH = -log10 [H+]
The pH scale ranges from 0 to 14; a pH value of 7 is considered neutral, while pH values less than 7 are acidic, and pH values greater than 7 are alkaline. The pH values of soil usually range from pH 4.0 to pH 8.0; higher or lower pH values are very rare and are normally found only in severely disturbed soils or in soils that have been amended with some type of acidic or alkaline material.
Soil pH is one of its most important properties because it influences many aspects of soil productivity, including nutrient availability, microbial activity, and pesticide efficacy. Plant growth problems are common in soils that are too acidic or too alkaline. Therefore, maintaining pH in the proper range for the desired use of the soil is critical for successful production of agronomic and horticultural crops.
Delaware soils tend to be slightly to moderately acidic due to the nature of the parent materials from which the soils formed. In addition, natural weathering processes in humid areas such as Delaware leach base cations (e.g., Ca, Mg) from the topsoil, leaving acidic cations, particularly aluminum (Al), as the dominant exchangeable cation. Soil acidification also occurs due to organic matter decomposition (e.g., crop residues, manures, sludges), the use of fertilizers containing ammonium (e.g., ammonium sulfate, monoammonium phosphate, diammonium phosphate), or atmospheric deposition (e.g., acid rain, mist, solid particles). Management problems associated with acid soils include:
- Increased aluminum (Al) or manganese (Mn) plant toxicity
- Reduced plant availability of macronutrients including phosphorus (P), calcium (Ca), or magnesium (Mg)
- Reduced microbial populations and activity
- Increased bioavailability of hazardous trace elements including cadmium (Cd), nickel (Ni), and lead (Pb)
- Reduced pesticide efficiency
Soils that are too alkaline for plant growth are very rare in Delaware due to the factors described previously. Alkaline soils are mainly found in arid regions where the leaching of basic cations is greatly reduced. If excessively alkaline soil conditions are encountered in Delaware, it is likely the result of over-liming soils. Over-liming is usually caused by improper calibration of lime application equipment but can also occur if lime applications were not based on results of an appropriate lime requirement test. Management problems associated with alkaline soils include:
- Reduced bioavailability of micronutrients including copper (Cu), iron (Fe), manganese (Mn), or zinc (Zn)
- Increased volatilization of surface applied ammonium fertilizers
- Reduced availability of phosphorus (P)
Managing Soil pH
Most plants grow best in Delaware soils when pH values are between 5.5 and 7.0. As such, the first step in managing soil pH is to select the target pH. Target pH is defined as the soil pH value associated with optimum plant growth and varies based on crop type and soil characteristics. Some crops, like blueberries grow best under more acidic conditions, while other crops, like alfalfa grow best when soil pH is near neutral. In addition, plants grown in high organic matter (soil organic matter > 6%) often have a lower target pH than would the same plant grown on a soil with a lower organic matter content. The lower target pH for high organic matter soils is related to the ability of soil organic matter to moderate some of the effects of excessive soil acidity. Six different soil target pH values are identified for crops in the University of Delaware Nutrient Management Recommendations (Shober et al., 2018; Taylor et al., 2018) or the Mid-Atlantic Commercial Vegetable Production Recommendations (Wyenandt and van Vuuren, 2019):
- pH 4.8 for blueberries,
- pH 5.2 for non-scab resistant potatoes,
- pH 5.6 for black, high organic matter soils in commercial production,
- pH 6.5 for crops with a high Ca demand,
- pH 6.8 for alfalfa,
- pH 6.0 for most other crops and soils.
Application of liming or acidifying materials will ensure that soil pH is near the agronomic target pH. Yet, it is also important to consider the critical pH before making soil pH management decisions. The critical pH is defined as “the maximum soil pH value at which liming increases crop yield” (Adams, 1984). The critical pH reflects the practical and economic considerations of changing soil pH to the value most suited for plant growth. University of Delaware recommended target and critical pH values are available for agronomic, fruit, and vegetable crops in Table 1 and for forage, hay, and woody crops in Table 2.
Table 1. Target and critical pH values used by the University of Delaware Soil Testing Laboratory to calculate lime requirements, mineral soils – Agronomic, fruit, and vegetable crops.
|Crop||Target pH||Critical pH|
|Onions, Bulbs, and Scallions||6.5||6.0|
|Potatoes, White, Non-scab Resistant||6.2||5.5|
Table 2. Target and critical pH values used by the University of Delaware Soil Testing Laboratory to calculate lime requirements, mineral soils – Forage, hay, and woody crops.
|Crop||Target pH||Critical pH|
|Forages and Hay Crops|
|Annual or Italian Ryegrass||6.5||5.8|
|Christmas Trees (except Douglas Fir)||6.0||5.5|
|Christmas Trees (Douglas Fir)||5.6||5.0|
Measuring Soil pH
Once a target pH is selected for the appropriate crop-soil pair, the next step is to determine the soil pH. For conventional tillage systems, soil pH measurements should be taken on a soil composite sample that was collected from the top 8 inches (“plow depth”). For fields under reduced tillage, we also recommend measuring soil pH on a 2-inch composite sample. (Note, an 8-inch composite sample may also be needed to determine management for reduced tillage systems.) As always, be sure to collect enough core samples to ensure that the sample is representative of the management area.
Soil testing laboratories typically offer soil pH as part of a routine soil test. When soil samples are submitted for analysis, the laboratory will measure soil pH using an electronic pH meter that is inserted into a soil-water or soil-weak salt slurry. The University of Delaware Soil Testing Program (and other regional laboratories) use the soil-water slurry (water pH) method with a 1:1 soil to solution ratio (Table 3). It is important to note that other soil testing laboratories may use different soil to solution ratios (1:2, 1:5) or different solutions (e.g., weak salt solutions such as 0.01M CaCl2) to measure soil pH. Make note of the soil pH method used by the soil testing lab that you choose, as the soil pH method will impact management decisions for liming or acidification.
Table 3. Methods for water and buffer pH tests used by regional soil testing laboratories.
|Laboratory||Soil pH Method||Lime Requirement Method|
|University of Delaware||Water (1:1)||Adams-Evans|
|Brookside Laboratories, Inc.||Water (1:1)||SMP/Sikora|
|Penn State University||Water (1:1)||Mehlich|
|Rutgers University||Water (1:1)||Adams-Evans|
|Spectrum Analytical Laboratories||Water (1:1)||Sikora|
|Virginia Tech||Water (1:1)||Mehlich|
|Waters Agricultural Laboratories, Inc.||Water (1:1)||Adams-Evans (KY location); Mehlich (NC location)|
|Waypoint Analytical||Water (1:1)||Mehlich; SMP (by request only)|
Portable pH meters and inexpensive hand-held “pH pens” are also available and can provide reasonably accurate estimates of soil pH in the field. Field measurements of pH are especially useful in reduced tillage systems to spot check soil pH levels. If soil pH in a 2-inch (surface) composite sample is at or below the critical pH for the specific crop or soil type (Tables 1 and 2), the University of Delaware recommends also collecting an 8-inch composite sample and submitting that samples to a commercial laboratory for soil pH analysis.
The use of indicator dye kits to measure soil pH is not recommended for making soil pH management decisions because of the difficulty in obtaining accurate, reproducible pH values with these color-based procedures.
Interpreting Soil pH Results
If the measured soil pH is below both the target pH and critical pH: The soil testing laboratory will run the lime requirement (buffer pH) test and provide a lime recommendation.
If the measured soil pH is between the critical pH and the target pH: The soil testing laboratory will run the lime requirement (buffer pH) test and may provide a lime recommendation. If you receive a lime recommendation, you may choose to wait to apply lime and continue to monitor until the pH approaches the critical level. Waiting to apply lime until the soil pH is closer to the critical pH can be a good strategy for crops that are sensitive to micronutrient deficiency (e.g., small grains or soybean) or if current field conditions prohibit application of lime.
If the measured soil pH is at or above the target pH: The soil test lab will not run the lime requirement (buffer pH) test. No lime will be recommended. However, if the soil pH is above the target pH, it may be prudent to reduce the soil pH for certain types of plants such as blueberries or non-scab resistant potatoes. Acidification is difficult on a large-scale basis and is only recommended if growing acid-loving plants.
Determining the Lime Requirement
When soil pH (1:1 soil to water ratio) is below the target pH, regional soil testing laboratories will also measure soil pH in a buffer solution. The soil pH in the buffer solution provides an indication of how easily the pH can be changed (also known as the “the buffering capacity of the soil”).
The University of Delaware Soil Testing Laboratory uses the Adams-Evans buffer pH test to determine the lime requirement. The Adams-Evans buffer was specifically developed for the coarse-textured, poorly buffered soils of the Atlantic Coastal Plain including those in Delaware. Adams-Evans buffer can detect small changes in lime requirement on soils which can be easily over-limed, thus resulting in problems with plant growth such as micronutrient deficiency (Sims and Eckert, 2011). Calibration of the Adams-Evans buffer test was conducted by Sims and Dennis (1989) using 20 Delaware soils. Other regional soil testing laboratories use the Mehlich buffer test or the SMP buffer (Table 3). The SMP buffer is also listed for use in the Northeastern U.S. However, the SMP buffer test was designed for heavier soils of the Northeastern and North Central US that have higher CEC and lime requirements and does not predict actual lime requirement for Delaware soils as well as the Adams-Evans or Mehlich buffers (Sims and Dennis, 1989).
The water pH value is typically listed as “soil pH” on the soil test report. The buffer pH may also be reported on the soil test report as the “buffer pH”. In lieu of the listing the buffer pH value, laboratories that use the Mehlich buffer test may report exchangeable acidity (meq/100 cm3); exchangeable acidity is calculated based on the pH of the soil in the Mehlich buffer (Hardy, 2014).
Determining the base lime requirement
Lime requirement is determined based on the mathematical relationships between the soil pH (1:1 soil to water) and pH in the buffer solution (or exchangeable acidity when using the Mehlich buffer test). Given the complexity of the lime requirement equations, we prepared tables that identify the base lime rate (tons/ac) of agricultural grade limestone based on water pH and buffer pH. Lime requirement tables are available for all target pH values for the Adams-Evans (Soil and Plant Analysis Council, 2000), Mehlich (Hardy, 2014), and SMP (Sikora, 2014) buffer tests based on the appropriate mathematical equations. These tables should be used to determine the appropriate base lime application rate for an 8-inch composite soil sample as follows:
- Conventional tillage: Run the lime requirement test and determine the appropriate base liming rate from the lime recommendation tables.
- Reduced tillage: If the soil pH of the 2-inch composite sample is below the critical pH, run the lime requirement test on an 8-inch composite sample (Beegle and Ligenfelter, 2014).
- If lime is recommended for the 8-inch composite sample, determine the appropriate base liming rate for conventional tillage systems.
- If lime is not recommended for the 8-inch composite sample, surface apply 1.5 tons/ac of agricultural grade limestone.
More detailed information on the different lime requirement tests and the equations used to determine the lime requirement for regional buffer pH tests is available in the University of Delaware fact sheet Methods to Determine Lime Requirement.
Adams-Evans buffer lime requirement tables
Mehlich buffer lime requirement tables
Note: The Mehlich buffer cannot be used to determine lime rates needed to raise soil pH above pH 6.6 due to the chemical nature of the test.
SMP buffer lime requirement tables
Note: The SMP buffer test was designed for soils with high buffering capacity and lime requirements. Delaware soils tend to have low buffering capacity. Use caution when applying lime at the rates based on the SMP buffer, as over-liming is possible on sandy loam and loamy sand soils (Sims and Dennis, 1989).
Adjusting the base lime requirement to account for past lime applications
The base lime requirement obtained from these tables can be used if no additional lime was applied within the last 18 months. If lime was applied within the last 18 months of the current soil test, then the base lime requirement must be adjusted to account for previous lime recommendations because limestone reacts relatively slowly in the soil to neutralize soil acidity. In fact, it can often take up to two years for lime to be fully effective. As a result, soils tested for lime requirement within two years of a lime application may still show a need for pH adjustment even though enough limestone was applied initially. Applying the full rate of lime recommended by the current soil test could easily result in over-liming of Delaware’s poorly buffered, low organic matter soils and lead to problems such as Mn or Zn deficiency.
To avoid the risk of over-liming soils, the University of Delaware calculates a lime credit for soils limed in the previous 18 months. This credit (reported in tons/ac) is based on the previous liming rate and time since application and calculated using the equation:
Lime credit (ton/ac) = Previous lime rate (ton/ac) × Lime availability factor
The previous lime rate (tons/ac) refers to the amount of any liming material applied within 18 months of the soil sampling event and the lime availability factor is based on the length of time since last application (Table 4). The net lime requirement is determined by subtracting the lime credit from the base lime recommendation, as determined from the current soil test by the following equation:
Net lime requirement (tons/ac) = Base lime requirement (ton/ac) × Lime credit (ton/ac)
Table 4. Availability factor used to adjust lime rates when liming materials were applied within 18 months of the current soil test.
|Time Since Last Lime Application (months)||Lime Availability Factor|
Recommendations for Applying Lime
Lime is most effective when it is mixed with the soil. Therefore, lime should be applied and incorporated into soils by plowing prior to planting pastures or converting an operation to conservation tillage or no-till. Repeated, smaller applications of lime to the surface of soils managed under conservation tillage, no-till, or pasture will be effective at maintaining soil surface pH near the target pH in subsequent years. The University of Delaware recommends applying no more than 2 ton/ac of lime per surface application under conservation tillage, no-till, or pasture.
Larger lime applications are feasible during tillage operations. However, there is little benefit to applying lime at a rate > 4 ton/ac in a single application. If the net lime requirement is >4 ton/ac, the University of Delaware recommends splitting the total amount of lime into 2 to 3 smaller applications for best results. Applications should be made more than 6 months apart or (at a minimum) by tillage operation. When a single lime application is 3-4 ton/ac, the University of Delaware recommends applying half the recommended rate prior to plowing and then disc the remainder of the lime in after plowing.
Selecting a Liming Material
The selection of a liming material is dependent upon several factors including the need to increase soil Ca or soil Mg concentrations, economics, material availability, desired speed of reaction, and ease of use. In this section, lime type selection will be discussed solely in terms of whether to use calcitic (“Hi-Cal”) or dolomitic (“Hi-Mag”) agricultural grade limestone. Agricultural limestone is assumed to be 67% as effective as pure calcium carbonate. For a more detailed discussion of liming material selection and application, see the University of Delaware fact sheet Liming Materials and Management.
In most cases, the lime requirement of the soil can be adequately met by applying either calcitic or dolomitic agricultural grade lime. Growers can make their choice of agricultural grade limestone based on availability and cost. However, in certain situations, it may be desirable to choose one or the other to have the added benefit of increasing either the soil Ca or soil Mg concentrations without applying additional Ca or Mg fertilizers.
Choose calcitic (“High-Cal”) lime when:
- Soil test Mg > 100 UD-FIV (based on Mehlich 3)
- Soil test Mg is between 50 and 100 UD-FIV and is > soil test Ca levels
Choose dolomitic (“High-Mag”) lime when:
- Soil test Mg < 100 UD-FIV (based on Mehlich 3)
- Soil test Mg is between 50 and 100 UD-FIV and is < soil test Ca levels
Note: Soil test Mg is based on extraction with Mehlich-3, where University of Delaware fertility index value (UD-FIV) of 50 is equivalent to 131 lb/ac (65.5 ppm) and UD-FIV of 100 is equivalent to 262 lb/ac (131 ppm).
Soil acidification is difficult to achieve on a large scale. As such, it may be better to choose a different crop if the soil pH is more than 1.0 unit higher than the target pH. However, if acidification is desired, the University of Delaware recommends ground elemental sulfur (S) as the acidifying material. While other materials are capable of acidifying soils (e.g., aluminum sulfate, peat), they are often more difficult to manage or less effective than elemental S. For example, aluminum sulfate can be used as an acidifying agent but can cause phytotoxicity if over-applied. Peat moss is sometimes used as a short-term solution to acidify soil in the root zone of newly installed plants, but results are less predictable than with elemental S.
The quantity of acidifying agent required to reduce soil pH in a given soil is dependent upon two factors: the desired change in soil pH and the buffering capacity of that soil. Soils that are more strongly buffered against changes in pH (e.g., fine-textured, high CEC, high OM soils) require more S to achieve the same pH change than those that are less strongly buffered (e.g., sandy, low CEC, low OM soils). The rate of elemental S required to cause the desired change in soil pH is available in Table 5. Rates are expressed as a function of the pH unit decrease and soil type. The pH unit decrease is the difference between the current soil pH and the desired soil pH; hence, if the present soil pH is 6.5 and the desired soil pH is 5.5, the pH unit decrease would be 1.0. Because of differences in soil buffering capacity, a silt loam soil would require an application of 1,000 lb/ac of elemental S to achieve a 1.0 pH unit decrease, while application of 320 lb/ac of elemental S would be required to achieve the same 1.0 pH unit decrease for a sandy loam soil. If aluminum sulfate is the desired acidifying agent used rate in Table 6 (elemental S rate multiplied by 6).
Table 5. Elemental S required to reduce soil pH as a function of pH unit decrease and soil type. Black soils have a soil organic matter content >6.0.
|Desired pH Unit Decrease||Loamy sands, Sandy loams||Black loamy sands, Black sandy loams||Silt loams, Loams||Black silt loams, Black loams|
Caution: To avoid accidentally reducing soil pH to a value that may create problems with plant growth, it is recommended that no more than 60 lb/ac of elemental S be applied in a single application. The soil pH should then be tested again in 4 to 6 months to determine whether additional S is required.
Amy L. Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality, University of Delaware
Karen L. Gartley, Director, University of Delaware Soil Testing Program
J. Thomas Sims, Professor (retired), University of Delaware
Original Publication Date: 1 July 2019
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Shober, A.L., R.W. Taylor, K.L. Gartley, and J.T. Sims. 2017. Nutrient management recommendations – Agronomic Crops. University of Delaware, Newark, DE. Available at: http://extension.udel.edu/factsheets/agronomic-crops/
Soil and Plant Analysis Council. 2000. Soil analysis handbook of reference methods. CRC Press, Boca Raton, FL.
Taylor, R.W., K.L. Gartley, A.L. Shober, and J.T. Sims. 2017. Nutrient management recommendations – Hay and Forage Crops. University of Delaware, Newark, DE. Available at: http://extension.udel.edu/factsheets/forage-and-hay-crops/
Wyenandt, C.A. and M.M.I. van Vuuren, editors. 2019. 2019 Mid-Atlantic commercial vegetable production recommendations. Rutgers University. New Brunswick, NJ. Available at: http://extension.udel.edu/ag/vegetable-fruit-resources/commercial-vegetable-production-recommendations/
Sydney Riggi, Nutrient Management Extension Agent, University of Delaware Extension
Jarrod Miller, Assistant Professor and Agronomy Extension Specialist, University of Delaware
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