Amending Soils with Lime or Gypsum (NRCS 333)

What is it: 

The use of lime and gypsum as soil amendments can enhance crop production. The two types of material provide different outcomes in the soil profile which are important to differentiate to know when and where to best utilize these products. Lime, also known as agricultural limestone, neutralizes soil acidity and provided calcium and magnesium available for plant uptake. Gypsum is a calcium sulfate containing product that provides both calcium and sulfate to the soil system. No change in pH can be expected from a gypsum application. The calcium in gypsum can replace sodium in sodic soils or aluminum where aluminum toxicity might be a concern. Neither of these situations are common in Ohio.

Liming

Materials used for neutralizing soil acidity are called agricultural liming materials. Several different materials have the characteristics needed of a calcium and magnesium oxide, hydroxide, carbonate, silicate or combination that are sold from ag purposes. The most common material is ground limestone which is calcium/magnesium carbonates. Ground limestone can have two different names that reflect the percentages of calcium to magnesium in the product. Dolomitic limestone is a combination of calcium (55-85%) to magnesium (45-15%) carbonates. Calcitic limestone is predominately calcium (95%) but still contains a small amount of magnesium (5%) carbonates. A variety of materials have neutralizing capabilities but are generally less effective, more costly than or not as available as limestone.

Gypsum

Gypsum is a soluble source of calcium and sulfur (in sulfate form) that can be used as a plant nutrient source. The chemical composition of gypsum is calcium sulfate dihydrate. Naturally mined sources of gypsum are available but much of the readily available supply is flue gas desulfurization (FGD) gypsum resulting from power plant air scrubbers that clean the air exhausted from coal burning power plants or other industrial smoke stacks. The mined and FGD products compare well on calcium and sulfur content, but FGD is lower in other plant micronutrients or heavy metals except for cadmium and selenium.

Where is it used: 

Both agricultural lime and gypsum can be used on agricultural fields. Lime is surface applied with activity of neutralizing soil pH enhanced by incorporation through tillage and soil mixing. Gypsum is surface applied and left on the surface where changes to surface structure may be seen.

    Why install it: 

    Liming

    • Adjusts acidic soil pH which affects the availability of plant nutrients in the soil
    • Increases biological activity favorable to plant growth including organic matter cycling resulting in nutrient turnover
    • Supplies plant available calcium and magnesium
    • Improves soil structure and tilth
    • Reduces plant toxic concentrations of aluminum, manganese and iron that occur under acidic pH conditions

    Gypsum

    • Provides moderately soluble forms of sulfur and calcium that are plant available
    • Most frequently used to reclaim sodic soils (not a condition seen in Ohio)
    • Physical properties of some soils may be improved by use of gypsum to reduce surface crusting through dispersion of soil particles
    • Increases water infiltration at soil surface
    What do I need to know about it: 

    Effectiveness

    Liming
    Liming is a very effective agronomic tool. It affects soil nutrient availability (see Figure 30) and soil microbial activity when used to adjust pH into the range suggested for different plant species. A water quality benefit has not been attributed to liming. Soil structure can be improved in acidic soils due to the change in soil calcium content resulting from the lime application.

    Gypsum
    Gypsum might benefit water quality by a reduction of DRP in water leaving a field site. The current NRCS Conservation Practice Standard 333, recommends application to soils with a greater than two times the “maximum optimal phosphorus level” for crop production or when the field phosphorus index rates high or very high.

    One field experiment in Ohio measured water quality at the edge-of-field for both surface and subsurface (tile) water with an application of one ton of gypsum per acre applied in two consecutive years on a field with a 480 PPM Mehlich 3 phosphorus soil test (ten times the agronomic optimum). The experiment produced the following results:

    • Following the first application, DRP and Total P (TP) mean concentrations were reduced by 21 percent and 10 percent, respectively, in the tile. DRP was reduced by 14 percent in the surface water. Total loading was not affected (King et al., 2016).
    • After the second application, DRP and TP loads were reduced by 41 percent and 40 percent, respectively, in the surface water. The tile flows DRP and TP loads were reduced by 35 percent and 15 percent, respectively (King et al., 2016).
    • Further field experimentation is being conducted, and results will be added as they become available.

    Considerations

    Liming

    Figure 30. Relative availability of elements essential to plant growth at different soil pH levels for mineral soils. Credit: Ohio Agronomy Guide, 15th Edition, The Ohio State University.

    pH effectsMost standard soil tests provide the information needed to evaluate the need for lime and determine rates of application of selected material. A standard soil test provides two measures that relate to soil pH and lime management.

    Value number one is pH (sometimes noted as water pH). It is a measure of the soil pH when the soil is mixed with just water. This is the measure monitored to know if the soil is in the correct range for the target crop to be grown. Soil pH affects nutrient form which influences availability of the nutrient in soil solution (see figure 3-1, Source: Ohio Agronomy Guide). Most agronomic crops do well in a pH range of 5.8-6.8 and this range tends to maximize nutrient availability. pH values that are more alkaline or acidic than this range start to affect nutrient availability and deficiencies may be seen. Soil pH also affects microbial activity. For example, nitrogen fixing rhizobia bacteria that fix nitrogen for legumes like soybeans and alfalfa, are hindered at a pH less than 5.8. If a pH is too acidic (<5.5) it is recommended that lime addition occur before using the nutrient values on a soil test, since by correcting pH, nutrients will be more available resulting in increased soil test values.

    Value number two on a soil test report is the buffer pH value, which is used to determine the amount of lime needed to correct a soil water pH into a desirable range. A buffer solution used in the buffer pH determination, gives a measure of both the soil solution pH and reserve acidity (H+ ions) in the soil profile that will need to be neutralized to achieve the desired pH.

    The use of lime to nuetralize soil acidity will also add calcium and magnesium to the soil solution. This addition can correct low nutrient availability of these two nutrients and also change soil structure. It also recommended that pH be corrected prior to using gypsum.

    A factsheet titled Soil Acidity and Liming provides an understanding of soil pH and liming consideration for Ohio soils.

    Gypsum

    Gypsum should be surface-applied and not incorporated. If soil pH is less than 5, the application might be harmful to plants present at the time of the application. In this situation, it would be recommended to use liming to correct soil pH, and then reevaluate for gypsum application as needed.

    Gypsum applications can have positive effects on the physical properties of soil. The physical condition of the soil can be improved, as measured by reduced penetrometer readings, reduced surface crusting, and improved aggregate stability. Infiltration can also improve. Results have been variable and might not be seen with a single application in a single year.

    Cost

    Generally both lime and gypsum are sourced from set locations where either limestone is mined or FDG gypsum accumulates at an industrial plant scrubber. The cost of the material is one part of the charge but trucking can be a significant portion of the total cost per ton of material. Thus a standard cost is difficult to document but some information is provided below.

    Lime is a regulated product in the state of Ohio under the Pesticide and Fertilizer Section of the Ohio Department of Agriculture (ODA). Material sold as a liming material is regularly tested and annual lime testing results are published by ODA. Different liming material should be compared using their effective neutralizing power (ENP), which is reported in lbs/ton. The ENP considers the total neutralizing power (TNP), fineness of grind, and percent moisture of a liming material so comparison can be made between different grades and sources of material. The proceedure for this comparison is provided in Soil Acidity and Liming. Generally, lime costs are $15 to $30 per delivered ton but since quality of material can vary, the ENP should be used determine the value of different lime sources.

    Gypsum is not a regulated item, so it would be prudent to request an analysis of material being considered for purchased. Generally cost will range from $20-$35 per ton of material delivered but several factors may affect the final cost.

    How does it work: 

    Liming

    The most common material is ground limestone which is calcium/magnesium carbonates.  Even though liming materials are not the same, they all follow the same process to neutralize soil acidity. Lime supplies a surplus of the basic cations Ca2+ and/or Mg2+ in a carbonated, hydroxide, or oxide form (CaCO3, MgCO3, CaOH, MgOH, CaO). As the compounds dissolve in soil solution, the carbonate (CO32-), hydroxyl (OH-), or oxide (O2-) react with active acidity (H+) to form carbonic acid (H2CO3) or water (H2O). Also, because H+ is being removed from soil solution, free Al3+ reacts with OH- to form an insoluble compound. Hydrogen held by soil-clay (potential acidity) is released into soil solution to maintain chemical equilibrium as active acidity is neutralized, and Al3+ is released from the soil to form insoluble compounds. The H+ released into the soil solution is then neutralized until the CO32-, OH-, and O2- are exhausted. Ultimately, most of the carbonic acid will dissociate to form water and carbon dioxide. Thus, excess H+ is converted into water, and free Ca2+ and/or Mg2+ replace the released H+ and Al3+ on the soil exchange sites. (Source: Soil Acidity and Liming )

    Gypsum

    Gypsum when soil applied, will react in soil to provide soluble forms of calcium and sulfur that are plant available. Research is needed to determine the best soil types and management practices that lead to potential benefits of using gypsum as a soil amendment for enhancing crop yield and farm profitability. Recommendations for gypsum use are not generally tied to a single soil test value but need to be considered within the context of the chemical makeup of the product and identified crop needs. The cost of gypsum should be compared to other alternative products that can be used to address the specific identified condition of concern that needs to be addressed in the crop production system. 

    Situational uses of gypsum:

    1. Identified need for sulfur.
    2. Identified need for calcium in vegetable crop production especially potato, tomato or peppers.
    3. Aluminum toxicity in acidic subsoils.
    4. Surface crusing in higher clay soil types.
    Design: 

    Extensive design details are provided in Amending Soil Properties with Gypsum Products (NRCS 333). Also see the fact sheet titled Soil Acidity and Liming for Agronomic Production for information on soil pH and liming.