Calcium Hydroxide
Crops
___________________________________
April 4, 2002 National Organic Standards Board Technical Advisory Panel Review Page 1 of 1
Compiled by OMRI for the USDA National Organic Program
Executive Summary
Calcium hydroxide was petitioned for use as a soil amendment to supply calcium for crops. Calcium hydroxide (also
known as hydrated lime) is produced from the slow addition of water to crushed or ground quicklime (calcium oxide),
which is produced by burning various forms of limestone. It may also be produced as a by-product of industrial
processes such as from the generation of acetylene from calcium carbide.
The NOSB refined the definition of synthetic in 1995 and considered the combustion of minerals to be synthetic.
Although the NOSB approved use of calcium hydroxide as a component of Bordeaux mix and lime sulfur for
fungicide use, it did not approve use as a soil amendment.
Reviewers all agreed that calcium hydroxide is a synthetic material. Two of the reviewers support continued
prohibition, due to availability of non-synthetic alternatives, concerns about worker safety, effect on soil
microorganisms, and possible contaminants. One reviewer supports allowance with restrictions on formulated
products, but did not provide any support for validity of these measurements, which were found to be without basis
by a research chemist.
Summary of TAP Reviewer’s Analyses1
Synthetic/
Nonsynthetic
Allow without restrictions?
Allow only with restrictions?
(See Reviewers’ comments for
restrictions)
Synthetic (3)
Nonsynthetic (0)
Yes (0)
No (3)
Yes (1)
No (2)
Identification
1 This Technical Advisory Panel (TAP) review is based on the information available as of the date of this review. This review addresses the requirements of the
Organic Foods Production Act to the best of the investigator’s ability, and has been reviewed by experts on the TAP. The substance is evaluated against the
criteria found in section 2119(m) of the OFPA [7 USC 6517(m)]. The information and advice presented to the NOSB is based on the technical evaluation
against that criteria, and does not incorporate commercial availability, socio-economic impact, or other factors that the NOSB and the USDA may want to
consider in making decisions
Chemical Names: calcium hydroxide, Ca(OH)2
Other Names:
Calcium hydrate, slaked lime, hydrated lime; carboxide,
calcium dihydroxide, lime water.
CAS Number: 1305-62-0
Other Codes:
RTECS EW 2800000
ICSC 0408
DOT-UN 1759
EPA PC no. 075601
Characterization
Composition: Calcium 54.09%, hydrogen 2.72%, Oxygen, 43.19%. Contains at least 95% Ca(OH)2. The molecular weight
is 74.10 (Budavari, 1986).
Properties: Crystals or soft, odorless, granules or powder. It has a slightly bitter, alkaline taste. It readily absorbs CO2
from air, forming calcium carbonate. It loses water when heated, forming calcium oxide. It is slightly soluble in water.
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Solution is a medium strong base, and reacts violently with acids. It attacks many metals in presence of water forming
flammable explosive gas (H2) (Budavari, 1986; Hardy, 2001).
How Made:
See description of calcium oxide production in the calcium oxide TAP Review. Hydrated lime is produced from the slow
addition of water to crushed or ground quicklime (calcium oxide). This is done in a premixing chamber or vessel that
mixes and agitates the lime and water at prescribed levels. The addition of water generates considerable heat of the reaction
and produces steam. Although the theoretical amount of moisture needed is 24.5%, in practice about 50-65% water is
added to counteract loss from steam. After hydration, the slightly moist slaked lime is conveyed to a separator where
coarse fractions are removed, and the powder dries. It is then bagged in paper sacks or shipped in bulk by truck or rail. The
reaction can be characterized as:
CaO + H20 Ca(OH)2 + heat
high calcium quicklime high calcium hydrate
Depending on the source parent rock, different grades of hydrated lime may be produced. A high calcium hydrate is 71-
74% calcium oxide, with 0.5-2% magnesium oxide and 24-25% water.
Slaked lime may also be produced as a waste product. Carbide lime is a waste lime hydrate by-product from the generation
of acetylene from calcium carbide, and occurs as a wet powder or dry powder of varying purity (Kirk-Othmer 1991b).
Specific Uses:
More than 90% of the lime (burned or hydrated) produced in the United States is used for basic or industrial chemistry.
The primary use is for steel manufacture (30%), metallurgy, air pollution control and water and sewage wastewater
treatment (24%), cement and mortar, chemical manufacture, manufacture of glass and paper, diluents and carriers of
pesticides such as lime-sulfur and Bordeaux mixture, bleach production, and other chemical manufacture (Kirk-Othmer,
1991a, b; Budavari, 1986). Beet sugar refineries typically operate on site lime kilns to provide both calcium oxide and
carbon dioxide for the refining process, while cane sugar refining uses quicklime at a lesser rate (Kirk-Othmer, 1991b).
Calcium oxide or calcium hydroxide is used as liming materials for agricultural use, particularly when a rapid change in pH
is desired. Calcium hydroxide is the second most effective of commonly used liming materials, with a neutralizing value
(calcium carbonate equivalent-CCE) of 136% for the pure material. Pure calcitic limestone has a rating of 100%, however
most agricultural limestone rate the CCE as 90-95% because of impurities (Tisdale, 1985). More than 90% of the
agricultural lime used in the US is calcium carbonate, next is magnesium-calcium carbonates (dolomitic limestone) and a
much smaller percentage is calcium oxide or hydroxide (Miller, 1990). This is due to higher cost, lack of stability, caustic
nature, and difficulty in handling (Miller, 1990; Parnes, 1990; Troeh, 1993; Brady, 1975; Zimmer, 2000).
Action:
Calcium hydroxide is more stable than calcium oxide, which is very vulnerable to moisture. However it has a strong affinity
for carbon dioxide, which causes recarbonation.
Calcium hydroxide ionizes readily in water to form Ca++, and OH-, forming a medium strong base or alkali. This effectively
neutralizes acid soil solutions and the calcium cation replaces aluminum on the cation exchange complex. In humid
climates, such as the eastern US, most soils are acid due to leaching of soils and gradual depletion of bases in the soil cation
exchange complex (Troeh, 1993; Engelstad, 1985). A neutral soil pH is desirable for most crops in order to tie up
aluminum and iron, which can be toxic to plants at low pH. Other nutrients such as phosphorous and trace elements are
more available at neutral pH (Parnes, 1990). Bacterial conversion of ammonium nitrogen to plant available nitrate forms is
also stimulated by increased amounts of exchangeable base cations (Brady, 1975). Biological activity is also improved at
neutral pH (Parnes, 1990; Brady, 1974; Pankhurst, 1997). Calcium is also an important plant nutrient, aside from its role in
pH modification. Calcium is needed in cell membranes and growing shoots and root tips.
Combinations:
Calcium hydroxide used for agricultural applications is rarely in pure form; more commonly it contains various percentages
of oxides found in the parent material. Other minerals that may be present include magnesium (MgO), silica (SiO2), iron
(Fe2 O3), and aluminum (AlO3). “Air-slaked lime” contains a mixture of the oxide, hydroxide, and carbonate of calcium or
calcium and magnesium, derived from the exposure of quicklime (AAPFCO, 2000). Limestone parent materials may also
include clay in the form of alumina-silicates (Kirk-Othmer, 1991b). Kiln dust is a by-product of cement or burnt lime
manufacture and contains calcium oxide and calcium hydroxide as well as some potassium and sulfur (Zimmer, 2000).
Slags are a group of industrial by-product materials and are used for their liming properties. Blast furnace slag is a byproduct
of pig iron, produced by reduction of iron by calcium carbonate, which produces calcium oxide. This calcium
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April 4, 2002 Page 3 of 3
oxide combines the molten silica impurities from the iron to form a calcium silicate (CaSiO3) that is cooled and ground.
Another type of slag is know as basic or Thomas slag, and is the product of open hearth steel making from high
phosphorus iron ores. Lime (calcium oxide) is used to flux the impurities of silica and phosphorous, and is usually applied
for phosphorus content as well as liming qualities. Electric furnace slag is produced when phosphate rock is reduced to
produce elemental phosphorous, and calcium oxide and silica fuse to produce a calcium silicate (Tisdale, 1985). Fly ash
from coal burning power plants is produced when crushed coal and finely ground limestone are suspended and burned, so
that the sulfur in the coal reacts to form gypsum (CaSO4). The ash residue is a granular mixture of calcium oxide, calcium
sulfate and small amounts of metal oxides. Other industrial by-products that contain calcium hydroxide include flue dust
from cement manufacturing, pulp mill lime, carbide lime, and by-products from the tanning industry (Tisdale, 1985;
AAPFCO, 2000).
Status
Historic Use:
Neither calcium oxide nor the related form of calcium hydroxide is generally allowed for fertilizer use in organic farming.
The NOSB refined the definition of synthetic in 1995 to declare that, “Heating and combustion of plants, animals, and
microorganisms shall not be considered synthetic unless expressly prohibited in the National List. The combustion of
minerals shall be considered synthetic and reviewed for compatibility under OFPA Sec 2119(m)(1-7)” (NOSB, 1995c). At
the meeting in April 1995, the NOSB approved use of calcium hydroxide as a component of Bordeaux mix and lime
sulfur, for fungicide use only (NOSB, 1995a). In November of 1995, NOSB approved use of hydrated lime for livestock
with a suggested annotation that: “not permitted for soil application or to cauterize mutilations or deodorize animal
wastes” (NOSB, 1995b).
OFPA, USDA Final Rule:
OFPA states in Sec. 6508(b):
“Soil Amendments. For a farm to be certified under this chapter, producers on such farm shall not
1) use any fertilizers containing synthetic ingredients or any commercially blended fertilizers containing
materials prohibited under this chapter or under the applicable State organic certification program; or
2) use as a source of nitrogen, phosphorus, lime, or potash or any materials that are inconsistent with the
applicable organic certification program.”
Calcium hydroxide is listed as an approved synthetic nonagricultural substance allowed as an ingredient in organic
processed food at 205.605(b)(6). The NOP rule allows hydrated lime at 205.601(i)(3) and lime sulfur at 205.601(i)(5) for
plant disease control. Lime sulfur is also listed as an insecticide at 205.601(e)(4). The NOSB voted in October 2001 that
synthetic sources of calcium chloride (non-brine sources) are prohibited for crop use, and that nonsynthetic sources from
the brine process may be used only as a foliar spray to treat physiological disorders related to calcium uptake.
Regulatory: EPA/NIEHS/Other Sources
EPA: Office of Pesticide Programs Output reporting shows that of the two formerly active registrations, both have been
cancelled (EPA, 2002).
OSHA: legal airborne permissible exposure limit (PEL) is 15 mg/m3 over an 8 hour workshift.
NIOSH: airborne exposure limit is 5mg/m3 over a 10 hour workshift.
ACGIH: American Conf. of Governmental Industrial Hygienists: threshold limit value 5mg/m3
NFPA (National Fire Protection Association) rates as hazardous chemical: health – 3, extremely hazardous to health but
areas may be entered with extreme care, requires full protective clothing, no skin surface should be exposed. Nonflammable,
reactivity – 0, normally stable, not reactive with water.
NTP(NIEHS) - not listed as a known carcinogen on National Toxicology program website.
(Sources: MSDS- Aldrich, 1992; Hardy, 200; Richardson, 1993; NTP, 2002)
Status Among U.S. Certifiers
US certifiers prohibit the use of calcium oxide and calcium hydroxide as fertilizer. This includes OCIA, Farm Verified
Organic, Oregon Tilth, MOSA, CCOF, NOFA chapters, MOFGA, QAI, WSDA, TDA (Texas Dept of Ag.), and New
Mexico Organic Commodity Commission.
International
IFOAM: Basic Standards 2000; prohibited since not explicitly listed as approved.
EU 2092/91—Prohibited, as Annex IIA does not list calcium oxide or calcium hydroxide, though it does list basic slag,
and “Industrial Lime from sugar production.”
Canadian General Standards 1999 CGSB-32.310-99 – prohibited since not listed
Japanese Agricultural Standards – lists “Calcined lime” to be limited for use in Bordeaux mixture as fungicide.
CODEX- not listed. Does permit Basic slag.
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Section 2119 OFPA U.S.C. 6518(m)(1-7) Criteria
1. The potential of the substance for detrimental chemical interactions with other materials used in organic farming systems.
Liming materials are generally considered benign or beneficial when used to improve the soil pH for agronomic use.
The basic effect of limestone to raise pH is due to the reactions:
CaCO3 + H20 Ca++ + HCO3 + OHOH-
+ H+ (in soil solution) H20
The rate of this reaction depends on the amount of hydrogen ions (acidity) of the soil.
When limestone of medium fineness is added to soil, it results in calcium and magnesium being available in three
forms: 1) as solid calcium or magnesium carbonates, 2) as exchangeable bases (Ca++) that are adsorbed onto the soil
colloids, or 3) as dissociated cations in the soil solution, mostly in association with bicarbonate ions (Brady, 1974). Use
of an oxidized material such as calcium hydroxide produces a more rapid pH change, as it breaks down directly into
calcium ion and the hydroxyl anion. This will favor a more rapid effect on the calcium ions in solution and adsorbed
on the soil colloids. Both of these forms are subject to leaching. The reverse reaction is also possible: calcium
hydroxide reacts with carbon dioxide in the soil to form calcium carbonate and bicarbonate. So, although calcium
oxide and hydroxide will affect a rapid pH change, they are not stable and may eventually revert to insoluble forms
(Brady, 1974; Parnes, 1990). Early studies comparing rates of reaction for different liming materials concluded that
while hydrated lime reacted more quickly than high calcium limestone, differences largely disappeared after two
months (Metzger, 1933).
Detrimental effects may occur due to difficulties in mixing, resulting in uneven concentrations, and the possibility of
overliming. Excess lime can tie up available nutrients such as manganese, copper, zinc, and phosphorous. It also ties
up boron. Rapid pH changes can also affect plants and microorganisms negatively when they cannot adjust quickly.
For this reason, a well-buffered soil is considered more desirable (Brady, 1974). Overliming is possible where diverse
soil conditions exist within a field, and in sandy conditions that may be deficient in micronutrients that will be further
reduced in availability at high pH (Troeh, 1993).
Calcium hydroxide has a strong interaction with humic acids (Richardson, 1993). Lime application also increases the
rate of organic matter decomposition (Troen and Thompson, 1993).
2. The toxicity and mode of action of the substance and of its breakdown products or any contaminants, and their persistence and areas of
concentration in the environment.
The LD 50 oral for redwing blackbird is 111 mg/kg. The Ld 50 oral for rat is 7340 mg/kg. (Richardson, 1993). It is
not classed as a carcinogen by NIEHS/NTP.
It is reactive in soil, solubilizes into ionic form, or re-carbonates into calcium carbonate (Tisdale 1985; Brady, 1974).
Some potential exists for leaching; this is similar to natural leaching of calcium from the soil complex.
Both calcium carbonate and calcium hydroxide applications to freshwater lakes suffering from eutrophication resulted
in reduction of total phosphorus and macrophyte biomass resulting in improved water quality (Prepas, 2001).
Treatments of ponds and canals with relatively high levels of slaked lime (210 mg/l for 65 hours) eliminated aquatic
plants a month later. This result could not be reproduced in laboratory studies, and researchers theorized that the
decline in biomass was due to short-term rise in pH in warm water, which resulted in low concentrations of free CO2
and bicarbonate for biosynthesis (Chambers, 2001).
3. The probability of environmental contamination during manufacture, use, misuse, or disposal of the substance.
Calcium hydroxide is produced by hydrating calcium oxide, derived from heating mined calcium carbonate. The
calcinations process requires a large amount of energy. Dust is the major problem that must be closely monitored at
various stages of mining and processing (Kirk-Othmer, 1991). Combinations found in by-product sources such as flue
ash may be contaminated with iron, copper, lead, or other heavy metals (Stout, 1988; Pichtel and Hayes cited in
Pankhurst, 1997).
4. The effects of the substance on human health.
Calcium hydroxide can severely irritate and burn the eyes and skin and mucous membranes. It produces third degree
alkali burns, and can irritate nose throat and lungs (Richardson, 1993; NJDH, 1998). If ingested, causes burning
sensation, abdominal pin, vomiting, diarrhea, collapse and delayed esophagal damage (NIOSH, 1993; NIH, undated).
Penetrates the skin more slowly than calcium oxide, extent of damage depends on length of contact. One of the
commonest causes of severe chemical burns of the eye, known as “lime burns.” Dusts are considered an important
industrial hazard (NIH, undated).
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5. The effects of the substance on biological and chemical interactions in the agroecosystem, including the physiological effects of the substance on
soil organisms (including the salt index and solubility of the soil), crops and livestock.
Soil pH has a significant effect on the range and composition of soil organisms. Early researchers noted that low pH
was injurious to some bacteria, especially the nitrifying bacteria, but favorable to development of fungi (Waksman,
1952). Addition of lime in the form of calcium carbonate was found to stimulate the increase in soil bacteria and
increased decomposition of organic matter. Waksman noted that excess calcium carbonate could become injurious to
soil bacteria, although no empirical studies were cited.
Earthworm populations are affected by pH, and are mostly absent from soils with extreme pH - over 8 or under 4
(Pankhurst, 1997). The addition of fertilizers such as ammonium salts was found to result in absence of earthworms,
but specific comparison of liming materials was not reviewed. Different species of earthworms were found to favor
different pH ranges between 5-8. The use of fertilizers is one of many agricultural practices that may have a large
impact on population fluctuations of earthworms (Fraser 1994, cited in Pankhurst, 1997). Use of fly ash as a liming
material was found to have a negative impact on dehydrogenase enzyme activity, indicating suppression of overall
microbial population. This was attributed to heavy metal contaminants in this material (Pichtel and Hayes, cited in
Pankhurst, 1997). Changes in soil pH cause a rapid effect on protozoan populations, which respond positively to lime
applications. Nematode populations also increase with liming (Pankhurst, 1997).
The TAP Review document prepared for NOSB in 1995 for hydrated lime states, “In direct soil application, however
it would create a strong imbalance of soluble calcium which would negative affect soil microbes and cause rapid
oxidation of other soil nutrients” (NOSB, 1995d). While extensive reviews in Pankhurst did not directly consider
effect of calcium oxide on microbes, extreme pH levels were found to generally reduce population levels. Other texts
note the generally beneficial impact of lime on earthworms and microbes when pH is adjusted to optimal levels
(Brady, 1974; Tisdale, 1985). An early study on the impact of limestone applied at different mesh sizes compared to
hydrated lime in regards to nitrification rate found that both forms stimulated nitrification, with hydrated forms giving
greater results (Walker, 1935).
The petitioner submitted data from experimental work with a proprietary complex containing calcium oxide and
calcium hydroxide in combination with other materials to show that this complex did not negatively impact microbial
degradation of cotton test strips as compared to a control, limestone application, or burnt lime treatment. Results were
highly variable depending on site. Only one of three test sites showed a reduction in degradation rate due to
application of burnt lime.
6. The alternatives to using the substance in terms of practices or other available materials.
The petitioner requested use of this material not as liming material but as calcium source for crop nutrition (Preston,
2001). Usually calcium deficiencies are associated with low pH soils and soils with a low cation exchanges capacity,
such as a leached sandy soil. A standard remedy is to add limestone and build up soil organic matter (Magdoff, 2000).
Mined sources can and do provide adequate available calcium to soils and plants in organic farming systems. The
principle sources used by organic farmers are limestone, gypsum, and rock (tricalcium) phosphate. Rotations and
compost can serve to increase the availability of these amendments. Wood ash and poultry manure also have readily
available calcium (Parnes, 1990).
Gypsum (calcium sulfate) is widely used to supply calcium at neutral or higher pH (Tisdale, 1985; Troeh, 1995; Parnes,
1990; Brady, 1974). Gypsum (CaSO4-2H20) has been used as a fertilizer since early Greek and Roman times. Deposits
are found at several locations in the US and Canada, and elsewhere in the world (Tisdale, 1985). Gypsum is widely
used on saline-sodic soils to replace sodium on the soil colloids with calcium. The sulfur in the gypsum then
precipitate with sodium, and can be leached out of the soil. Gypsum is not a liming agent, and does not substantially
affect soil pH but also provides sulfur, a necessary plant nutrient (Tisdale, 1985).
In non-organic systems, calcium fertilizers such as superphosphate and triple superphosphate contain 12-18%
calcium. Calcium chelated materials may also be applied as foliar sprays.
Normally the large amount of exchangeable calcium on the colloidal complex satisfies the calcium needs of most
crops (Mortvedt and Cox in Engelstad, 1985; Troeh, 1995). Some crops have shown calcium deficiency when soil
acidity is not a problem; these are usually localized in fruits, storage organs, or shoot tips. Examples include poor
kernel formation in peanuts, bitter pit in apples, blossom end rot in tomatoes, tipburn in lettuce, and black heart of
celery (Mortvedt, in Engelstadt, 1985). Gypsum applications are effective at supplementing calcium in peanuts
(Tisdale, 1985). Foliar applications of calcium chloride are effective when translocation of calcium is a problem to
actively growing shoots and fruits (see 2001 TAP review of calcium chloride).
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Calcium ions on the soil exchange surfaces are not very mobile, and the plants take up calcium ions more readily from
the soil solution (Troeh, 1993). Calcium in solution is in dynamic equilibrium with the exchangeable form, so that
when calcium is removed by plants or by leaching, there is replacement by the adsorbed phase (Tisdale, 1985). If there
is a sudden addition to calcium in solution, then more adsorption on to the exchange complex can also be expected.
Factors determining calcium supply to plants include a number of factors, including total calcium supply, soil ph,
CEC, percent calcium saturation of soil colloids, type of soil colloids, and the ratio of calcium to other cations in
solution (Tisdale, 1985). A study comparing the effectiveness of different liming materials found that the order of
effectiveness in neutralizing acidity was hydrated lime, basic slag, cement kiln dust, and ground limestone. All of these
different materials showed increasing response in plant uptake of calcium when applied at increasing rates, based on
calcium equivalency, but there was no difference between lime sources (Oguntoyinbo, 1996). Another study
comparing liming materials found that efficiency increased with decreased Mg content and that silicates were less
effective than carbonates or oxides. Efficiency of carbonate forms increases with fineness of grinding (Gutser, 1987).
Limestone fineness directly affects the rate of solubility of calcium cations (Brady, 1974; Miller, 1990). Where a rapid
pH change is desired, use of a more finely ground limestone will effect a quicker change. Limestone can be evaluated
for effective calcium carbonate equivalent (ECCE) or effective neutralizing value (ENV) based on a formula derived
from the fineness and chemical nature of the liming material (Troeh, 1993). If at least 50% of a limestone will pass
through a 100 mesh screen it is considered fine limestone and gives good results. Theoretically, if 100% of a ground
stone passed through a 100 mesh screen, a pH change would be complete within 3 months (Miller, 1990). Limestone
that passes a 60% screen should be completely reacted in soil within 3 years. Fluid limestone formulations are also
available, consisting of finely ground limestone suspended in water with clay (Miller, 1990).
Fertilizer sources of calcium (Parnes, 1990)
Ca content lbs./ton lime equivalent, lbs./ton
poultry manures 80
legume hay 28
clam and oyster shells 680 1700
wood ashes 700 1750
rock phosphate 420-660 800-1200
calcitic limestone 760 1900
dolomitic limestone 500 1900
gypsum 460
7. Its compatibility with a system of sustainable agriculture.
The Principals of Organic Production and Handling adopted by the NOSB include as goals the “use [of] cultural,
biological, and mechanical methods, as opposed to using synthetic materials to fulfill specific functions within the
system” and describes an organic systems production system as one designed to
“Optimize soil biological activity; Maintain long-term fertility;….Recycle materials of plant and animal origin in order
to return nutrients to the land, thus minimizing the use of non-renewable resources, (and) Minimize pollution of soil,
water, and air” (NOSB, 2001),
The use of nonsynthetic liming materials such as limestone (a nonrenewable resource) is unavoidable in humid
climates where soil will naturally become acid and leached of base cations over time. Increased attention to organic
matter additions and use of renewable resources such as poultry manures and wood ashes may be practical in some
systems but not all. Use of synthetic calcium oxide contributes indirectly to increased energy use and fossil fuel
consumption due to the burning process of manufacture. Natural sources of mined limestone or gypsum are more
slowly soluble and contribute to soil fertility and neutralization over a longer term. Some sources of mixed calcium
oxides and silicates are usable recycled materials from industrial pollution control systems, iron production, or cement
kilns, but these may also contain metal and other contaminants. A neutral pH is beneficial to the soil organisms, but
rapid pH changes may cause short-term fluctuations and disruptions in population levels.
TAP Reviewer Discussion
Reviewer 1 [Ph.D. plant pathology, M.S. soil science. Research, consulting, and administrative activities related to waste treatment and
reuse of waste as soil amendments and fertilizers. Southeast US]
Comments on Database
The following information needs to be corrected or added to the database:
Under the Combinations section the review mentions that kiln dust is a by-product of cement or burnt lime
manufacture. In addition to the calcium oxide and calcium hydroxide mentioned in cement kiln dust, there are a
variety of other chemicals. Some cement kilns are permitted to burn hazardous waste as a fuel source. The MSDS for
cement kiln dust lists a variety of heavy metals that it may contain including lead antimony, arsenic, mercury and
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others. The route of exposure for many of these metal is through inhalation or ingestion (Ash Grove Cement Co.,
2000). The USEPA review of CDK lists a variety of other compounds that may be present including dioxins (US
EPA, 1998).
(1) The potential of such substances for detrimental chemical interactions with other materials used in organic farming systems;
I agree with the criteria evaluation.
(2) The toxicity and mode of action of the substance and of its breakdown products or any contaminants, and their persistence and areas of
concentration in the environment;
I agree with the criteria evaluation.
(3) the probability of environmental contamination during manufacture, use, misuse or disposal of such substance;
Calcium hydroxide is also present in other combination materials such as cement kiln dust, which contains calcium
oxide, free silica, and various heavy metals (MSDS, Ash Grove, 2000). There is increasing interest in the agricultural
use of industrial combustion by-products such as fly ash that have a high pH (Pankhurst, 1997). Cement kilns often
own limerock mines because limerock is the primary feedstock of cement kilns. Cement kiln dust may come from
kilns that are permitted to burn hazardous waste, which can be used as a fuel source (Peters Chemical Co., website;
Hansen, 1990).
In 1999, the US EPA proposed management of cement kiln dust under the Resource Conservation and Recovery
ACT (RCRA). The US EPA included proposed limitations on various pollutants in cement kiln dust that would be
used for agricultural purposes (US EPA, 1999; US EPA, 1998). Cement kilns are also significant point sources for air
pollution. Pollutants include carbon monoxide, sulfur dioxide, nitrogen oxides, and particulates.
(4) the effect of the substance on human health;
I agree with the criteria evaluation.
(5) the effects of the substance on biological and chemical interactions in the agroecosystem, including the physiological effects of the substance on
soil organisms (including the salt index and solubility of the soil), crops and livestock;
The use of nitrogen fertilizers results in an increase in soil acidity, which creates the need for more frequent lime
applications (Kamprath and Foy, 1985; Troeh and Thompson, 1993). Waksman (1952) found that calcium carbonate
stimulated soil bacteria, which led to an increased decomposition of soil organic matter. The humus fraction of soil
organic matter provides long term benefits including pH buffering, increased cation exchange capacity which is pH
dependent, and water holding capacity (Bohn et al., 1979; Haynes and Naidu, 1998).
Although this petition is for calcium hydroxide, the petitioner also submitted information pertaining to a proprietary
product called Bio-Cal. Included in this information was research that examined the effects of Bio-Cal, calcium oxide
and calcium carbonate on soil biological activity. Bio-Cal contains a proprietary mix of calcium hydroxide and calcium
oxide in combination with other ingredients. Buried cotton strips were used as a bioassay to measure microbial
activity. This is an indirect method to assay microbial activity. It was not accompanied by soil assays for target
decomposing organisms but rather for broad classes of microorganisms. The results of this field research were
statistically quite variable and did not correlate with the Formazan bioassay results for microbial activity. The authors
also mention a previous study that showed increased microbial activity with Bio-Cal rather than no difference. At this
point in time their results are too variable to draw conclusions about the impacts of different lime materials on
microbial activity.
(6) the alternatives to using the substance in terms of practices or other available materials; and
The calcium content of a soil can be enhanced through the use of manures like poultry manure and also the use of
appropriate leguminous green manures. The addition of organic matter to the soil will also enhance the buffering
capacity of the soil and increase the cation exchange capacity (Brady, 1974).
(7) its compatibility with a system of sustainable agriculture.
I agree with the criteria evaluation.
Reviewer 1 Conclusion – Summarize why it should be allowed or prohibited for use in organic systems.
Calcium hydroxide is synthetically produced by the combustion of calcium carbonate to produce calcium oxide. This
is then converted to calcium hydroxide by the addition of water in an exothermic reaction.
Lime to correct calcium deficiency is rarely needed. If calcium is needed, an option is gypsum, which will provide
calcium without significantly affecting the soil pH (Tisdale et al., 1985). Over liming is a risk and can result in the
unavailability of iron, manganese, zinc, and other micronutrients (Brady, 1974). Fields are often limed to a pH above
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the optimum nutrient availability range of pH 6-7. Poultry manure and legume green manures are also sources of
calcium.
Lime is predominately used to correct soil pH. The use of inorganic nitrogen fertilizers has increased the need for lime
through their acidifying affect (Kamprath and Foy, 1985; Troeh and Thompson, 1993). Current organic standards
allow the use of calcium carbonate and dolomite to correct soil pH thus providing an alternative method to the use of
calcium hydroxide.
Calcium hydroxide is also a caustic material to handle and causes damage to the respiratory tract.
Finally calcium oxide is also a component of cement kiln dust, which is a by-product of the cement kiln industry
(MSDS Ash Grove Cement Co.). Many cement kilns burn hazardous waste and tires for fuel (Hansen, 1990 and 1992).
The US EPA has proposed restrictions on various pollutants in cement kiln dust that will be used for agricultural
purposes (USEPA, 1998).
Reviewer 1 Recommendation Advised to the NOSB:
The substance is Synthetic
For Crops, the substance should be Not Added to the National List.
Reviewer 2 [Ph.D. Research chemist, serves on an organic certification committee, Eastern US]
Comments on Database
Calcium Hydroxide is a crystalline solid. Its crystal structure is Trigonal with space group P3_m1. Its heat of solution
with water is 11.7 kJ/mole (Kirk-Othmer, 1991b). Commercial Ca(OH)2 is synthetic and is produced by hydration of
CaO. The CaO is formed by heating limestone in a kiln.
Natural Ca(OH)2 is called the mineral Portlandite. It is extremely rare, however, and has only been described from two
locations: Kongsberg in Sweden and the Kalahri manganese fields in South Africa (www. mindat.org).
The three basic Ca compounds derived from limestone--CaCO3, Ca(OH)2 , and CaO--are all used as agricultural field
amendments. Limestone can be simply crushed to a desired particle size, while the other two compounds are only
used for ag purposes if their powder size or purity are not acceptable for higher value applications.
These basic calcium compounds serve two main purposes as field amendments. The anion raises the pH of acidic
soils. The ability of these compounds to raise pH is determined on the amount of acid each molecule can neutralize
(CaO has the highest neutralizing value, Tisdale et al., 1985). The particle size of the liming agent is also important in
pH control. Coarse particles have lower surface area than fine ones and take longer to react with soil water and acids.
Milling CaCO3 very fine can speed the kinetics of the neutralization and make the limestone behave more like CaO. A
mole of CaCO3, however, neutralizes less acid than CaO (Norton & Zhang, 1998), and more limestone would be
needed to achieve the final pH of a given amount of CaO. Many ag crops grow best in neutral pH soils. Neutral to
slightly basic soil pH binds toxic ions like Al and Fe while releasing other ions needed for plant growth and health
(Zimmer, 2000).
Calcium released by solution of the carbonate or hydroxide also competes with Na + and Al +3 ions for absorption
sites on clay particles. Increasing exchangeable Ca +2 loosens soil structure and facilitates plant uptake of other
minerals (Norton & Zhang, 1998; Tisdale et al., 1985). Gypsum (CaSO4) behaves like calcium compounds in its effect
on soil texture and plant mineral uptake. The sulfate anion, however, does not change soil pH and does not affect
mineral availability in the soil like lime does.
Effective use of gypsum and basic calcium liming materials can have very positive effects on the yield and nutrition of
crops and grass forages (Nation, 95; Zimmer, 2000). Any of the three basic calcium compounds can be used to
achieve these effects. The amount of Ca applied and the neutralizing capacity is different for the three compounds,
but can be tuned by varying the amount of each compound applied to the soil. The kinetics can also be partially
controlled by the choice of particle size used.
OFPA Criteria Evaluation
I agree with the criteria evaluation in the database, and offer additional supporting information.
(1) the potential of such substances for detrimental chemical interactions with other materials used in organic farming systems;
Calcium hydroxide is very caustic. The pH of a saturated solution is about 12.5 (Weast, 1981). It is incompatible with
acids and boric oxide, and the reaction with strong acids liberates large quantities of heat (MSDS). It is easier to
NOSB TAP Review Compiled by OMRI Calcium Hydroxide Crops
April 4, 2002 Page 9 of 9
overlime fields with Ca(OH)2 than with limestone (pH 9.4). Calcium is more available and the high pH can harm soil
biota (see below).
(2) the toxicity and mode of action of the substance and of its breakdown products or any contaminants, and their persistence and areas of
concentration in the environment;
Solid Ca(OH)2 is an irritant causing chemical burns and inflammation of plant or animal tissue (Gosselin et al., 1976).
Aqueous Ca(OH)2 harms organisms primarily through high pH. A study of the use of Ca(OH)2 as a disinfectant
against Cryptosporidium parvum oocysts in water supplies found that calcium hydroxide only killed oocyst through high
pH (Robertson et al., 1992). The hydroxide has no effect on C. parvum when the pH is adjusted down to 6. In the
Eastern US, Ca(OH)2 will not persist in the soil. Reaction with atmospheric CO2 and acidic rain water will destroy any
hydroxide. This conversion takes some time, however, and local high pH conditions can persist around the lime
particles.
(3) the probability of environmental contamination during manufacture, use, misuse or disposal of such substance;
Dust is the main environmental concern with calcium hydroxide. The dust is caustic and a sever irritant of the eyes,
and mucus membranes in the nose and throat. As dust Ca(OH)2 is considered a major industrial hazard (Toxnet,
2002). When heated to 580°C., Ca(OH)2 can decompose to CaO, which can spread as dust during a fire (MSDS).
(4) the effect of the substance on human health;
Irritation of skin and eyes and inflammation of mucus membranes and lungs are the main immediate health effects of
Ca(OH)2 exposure. Calcium hydroxide is one of the most common causes of chemical burns of the eyes (Toxnet,
2002).
(5) the effects of the substance on biological and chemical interactions in the agroecosystem, including the physiological effects of the substance on
soil organisms (including the salt index and solubility of the soil), crops and livestock;
Little literature exists on the effect of Ca(OH)2 on soil organisms. The aqueous hydroxide of calcium is used as an
antimicrobial and there exists an extensive literature on its effect on microbe pathogens. (OH) - ions are antimicrobial
at pH > 9 and will inhibit most bacteria and many viruses (Aiello, 1998). Aqueous Ca(OH)2 is also used to disinfect
premises (Aiello, 1998).
In human dentistry, Ca(OH)2 is used as an antibacterial agent in root canals (intracanal dressing) and in periodontal
work (Estrela et al., 2001; Molander et al., 1999). Stuart et al. (1991) found Ca(OH)2 to be effective in vitro against
Streptococcus mutans, Actinomyces viscosus and Bacteroides gingivalis.
Scarification of seeds to enhance germination can favor colonization by pathogenic bacteria. Holliday et al. (2001)
found reduction in Salmonella and Escherichia coli O157:H7 when alfalfa seeds (for sprouts) were treated in 1 %
Ca(OH)2. The presence of organic material in the seed did not affect the antimicrobial activity.
Field studies of the effective of liming on soil fauna are difficult to control. Raising soil pH higher than 6 favor fungi
and worm populations over bacteria (Waksman, 52, **** 97). pH shifts also alter the availability of nutrients. Nitrogen
mineralization can be effected by both pH and changes in the bacteria/fungi populations. Weyman-Kaczmarkowa and
Pedziwilk (2000) studied the effect of liming on soil microbes in sandy loam and loose sand soil. They used Ca(OH)2
to increase the soil pH and found that the total microbial biomass (bacteria + fungi) decreased by ~ 50 % when pH
was raised from the natural 4.5 to 9.0 in the sandy soil. In the loam (natural pH of 7.7) using Ca(OH)2 to raise the pH
to 9.0 decreased the biomass by 30 %, and an increase to pH = 11 decreased biomass by 40%.
A study of different liming materials on soil microbial activity is attached to the petition. An analysis of the study is
given in the CaO review.
(6) the alternatives to using the substance in terms of practices or other available materials
As stated above, all the basic Ca lime materials affect soil pH and Ca availability the same way. The higher the acid
neutralizing power and the calcium solubility are, the faster the effects will be seen in the field. Limestone and gypsum
are slower acting than the synthetic calcium hydroxides but do the same job. Quality pastures with high nutrient
grasses are possible using mined limestone and gypsum. Three to five years may be needed to optimize pastures using
the mined minerals, however (Nation, 1995). The synthetics work faster, but possible can do harm to soil biota.
(7) its compatibility with a system of sustainable agriculture
Mined liming and Ca field amendments are compatible with sustainable agriculture. The main US limestone deposits
are found in the Mid-West, but mineable deposits are found through out the Appalachian and Rocky Mountains.
(Brobst & Pratt, 1973). Ca(OH)2 is usually produced at limestone quarries. It is synthesized at high temperatures
NOSB TAP Review Compiled by OMRI Calcium Hydroxide Crops
April 4, 2002 Page 10 of 10
(1000° C.) in open kilns that require large inputs of energy. Since the un-fired limestone is an adequate liming material,
the additional processing and energy use seems unnecessary.
Ca(OH)2 appears on the National List of synthetic substances in Bordeaux mix and as a sanitizer. In both of these
uses, the compound is used to kill microbes, not to enhance soil microbial populations. Both these uses rely on the
high pH of a calcium hydroxide aqueous solution to kill microbes.
A discussion of the petitioned annotation for Ca(OH)2
The petition requests that Ca hydroxide be placed on the National List only if it is in a form that yields less than 1
degree F [increase in temperature] when mixed with 1:1 vol. water. This request is based on crude caliometry data
provided by an analytical testing lab. The lab measured a heat of solution for the product Bio-Cal of 0.6 cal./gm.
Assuming that all the material was Ca(OH)2, the measured heat of solution would be 0.19 kJ/mole, far lower that the
traditional Ca(OH)2 value of 11.7 kJ/mole. No controls of known chemistry are reported to validate the test method.
[The reported number] 0.6 cal/gm is a much lower heat of solution than any of the Ca liming materials used to make the
product. There are several possible reasons for the low heat of solution of the product Bio-Cal: (1) The number in the
petition is wrong. The lab did not use a calorimeter, and their set up could have easily under-measured the heat
generated. (2) Calcium hydroxide is not present in the final product, or (3) its effect is masked by other phases present in
the product.
Bio-Cal is a processed liming material that uses feed stocks of kiln dust (CaO + Ca(OH)2 ), gypsum, and power plant fly
ash. Fly ash varies in chemical composition, but is basically a Mg, Fe alkaline earth alumino silicate (Bone & Himus,
1936). These feed stocks are mixed/milled and heated by a proprietary process to give a final product which has
apparent low heat of solution and high Ca availability (see petition). Neither the chemical composition of Bio-Cal nor
the processing temperature is given. The final product probably is probably more than just a mixture of Ca(OH)2,
CaCO3, and CaSO4 . These phases may exist with reaction rims and precipitated particles of complex hydroxy
carbonates and hydroxy sulphates like Ca2(H2O)[SO3]2 (Bassanite) or Ca(H2O)6[CO3] (hexahydrocalcite) (Povarennykh,
1972).
Without knowledge of the phases present in Bio-Cal it is not possible to review calcium hydroxide with an annotation
that it must be part of a proprietary product which reduces the heat of solution from that of pure Ca(OH)2.
Reviewer 2 Conclusion
Calcium hydroxide (CAS #1305-62-0) is a synthetic compound. It should be prohibited without annotation for use as a
field amendment. Calcium hydroxide is used as an antimicrobial in the medical and food industries. Limited field data
suggest that it is harmful to soil biota. All the benefits of Ca(OH)2.additions to soil can be obtained by the addition of
mined limestone and gypsum.
Calcium hydroxide, which has an unusually low heat of solution, cannot be reviewed due to a lack of consistent
properties. Bio-Cal cannot be reviewed. The chemical composition and processing have not been revealed by the
manufacturer and there is insufficient information to conduct a review.
Reviewer 2: Recommendation Advised to the NOSB
The substance is Synthetic
For Crops and Livestock, the substance should be Not Added to the National List.
Reviewer #3 [Organic farmer, organic inspector, advises an organic certifier. Western US.]
Comments on Database
Materials provided with TAP review appeared to be consistent and helpful in understanding what the detrimental and
positive effects of calcium hydroxide and calcium oxide can be.
This material should be considered synthetic. It is currently allowed for use for disease control as a foliar but is prohibited
as a fertilizer. The concern over its use as a fertilizer is that it creates a strong imbalance of soluble calcium, which
negatively affects soil microbes and causes rapid oxidation of other soil nutrients.
Producers certified by some chapters of OCIA and OGBA have used these materials in formulations [that meet the
proposed restrictions] for many years. In this sense, the materials have previously been accepted for use in organic
agriculture….
OFPA Criteria Evaluation
Alternatives:
NOSB TAP Review Compiled by OMRI Calcium Hydroxide Crops
April 4, 2002 Page 11 of 11
There are many existing alternatives for adjusting pH that are currently used in organic agriculture. This particular form
and formulation has several significant advantages and mitigates some of the disadvantages.
The approved organic alternatives work well but do not have some of the beneficial properties of this formulation. A
distinction needs to be made between the formulation, Bio-Cal, and the generic ingredients, calcium oxide and calcium
hydroxide. The Bio-Cal contains several ingredients of which calcium oxide and calcium hydroxide are a part.
The effects of the formulation are different from the effects of each separate ingredient. The request is to allow the generic
material to be used in a manner that has effects similar to a proprietary formulation. According to some data [provided by the
petitioner] this formulation is several times more efficient at providing calcium to plants and soil than either the approved
organic alternatives or the petitioned [generic] materials.
Is the material compatible with organic production and handling?
With the restrictions requested in the application--specifically the requirements that it be applied in a form that yields less
that 1degree Fahrenheit temperature increase when equal volumes of product and water are mixed and that it be part of a
managed program to remineralize soils--the negative effects on the soil are mitigated. The material with restrictions is
compatible with organic agriculture.
Recent developments and insights into the “soilfoodweb” have developed organic agriculture practices and enhanced our
understanding of what developing soil means in terms of microorganisms. Formulations or amendments that increase the
activity and number of species of soil microorganisms should be considered compatible with organic agriculture. The
restriction that this material be applied to the fields in amounts necessary to raise soil minerals to optimum levels based on
soil tests is also likely to increase diversity of microorganisms.
The same material without restrictions has been found to have detrimental effects to the soil. In common terms, it “burns
the soil and kills the worms.” Some data suggests the effects may not be as harmful for the first applications, instead the
harmful effects build up over time and in conjunction with other practices.
The material is synthetic and alternative materials are available to adjust pH and protect plants from disease.
Other considerations
This petition brings up a larger issue with some important implications for considering what materials should be allowed
for use in organic agriculture. The larger issue is; if a material that can cause harm when used conventionally can be
restricted or reformulated in a manner that does not cause harm; should it be allowed? If it is allowed, what quantitative
measurement of harm should it be judged by? ….
The measurement of harm should be based on our increasing understanding of soil and plant microbiology. What it means
to “build the soil” can now be defined by measurement of the diversity of microorganisms. Soil building is fundamental to
the principals of organic agriculture and specified in the Final Rule in 205.203(a)” “The producer must select and
implement tillage and cultivation practices that maintain and improve.... biological condition of soil….” Soil building is a
method of increasing fertility and pest and disease control for crop production.
If a material is either neutral or increases the diversity of microorganisms, it should be considered beneficial to the soil. A
material that enhances the diversity of microorganisms can be considered safe for the environment, producer, field
workers, and consumer and therefore allowed for use in organic agriculture…. The significance of using soil microbial
components and soil enzyme activities as indicators for monitoring the impact of materials on soil is gaining recognition. It
is also very significant to organic agriculture….
In the case of calcium hydroxide one of the main harmful effects of the material is the heating (oxidation) of the soil. An
evaluation method that measures the amount of heat a given formulation produces is a valid evaluation method. The
restriction that limits the amount of heat produced is appropriate for organic agriculture. Unfortunately, there does not
appear to be protocols established as to how to measure this effect. The manufacturer’s test has the advantage of being
simple to perform at the farm level.
In the petition there were several documented questionnaires from organic growers testifying to their support for Bio-Cal.
[In this reviewer’s opinion] long-term support for a material from organic growers is significant factor in considering a
materials organic status.
The material has long been recommended for use as disease control in “Bordeaux mixture.” This mixture is effective for
its toxicity and is not usually considered part of a soil building regime or beneficial to microorganisms. It does however
show the material has been accepted for other aspects of organic crop production
NOSB TAP Review Compiled by OMRI Calcium Hydroxide Crops
April 4, 2002 Page 12 of 12
Reviewer 3 Conclusion
I recommend that it be approved as synthetic-allowed with requested restrictions on its use [The proposed] restrictions
protect the soil from excessive oxidation (“burning”) that can be caused by using too high a concentration. It also prohibits
direct application of the material in raw form. To meet this restriction, the material must be formulated with other
materials before being applied to the soil. It can be used as an ingredient, but not directly.
The restriction [also] limits the amount of material that can be applied and requires soil testing…[and] requires further
documentation on its use and limits the amount of material used.
Reviewer 3: Recommendation Advised to the NOSB
The substance is Synthetic
For Crops and Livestock, the substance should be Added to the National List with restrictions.
Suggested Annotation: It is applied in a form that yields less than 1 degree Fahrenheit temperature increase when
equal volumes of product and water are mixed. It is applied to fields in amounts necessary to raise soil minerals to
optimum levels based on soil tests. It is applied as part of a managed program to remineralize soils
[End of TAP Reviewer Comments]
TAP Conclusion:
All three reviewers find calcium hydroxide to be a synthetic material. Two of the reviewers support prohibition, and find
that there are non-synthetic alternatives, as well as concerns about worker safety, possible effects on microorganisms, and
possibility of contaminants. One reviewer supports allowance with restrictions on formulated products proposed by the
petitioner, however did not offer any validation or support for this method of determining heat of solution. A research
chemist reviewer noted that, “No controls of known chemistry are reported to validate the test method.” Calcium
carbonate (limestone), gypsum, the use of legume cover crops, and poultry manure are nonsynthetic alternatives widely
used in organic soil building programs.
References
*= included in packet
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*AAPFCO 2000. Official Publication of the Association of American Plant Food Control Officials. Raleigh, NC.
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*Ash Grove Cement Co. 2000. MSDS for Cement Kiln Dust. Overland Park, KS. 7 pp.
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*Engelstad, O.P Ed. 1985. Fertilizer Technology and Use, 3rd Edition. Chapter 4, Lime-Fertilizer-Plant Interactions in Acid
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*Zimmer, G. 2000. The Biological Farmer. pp. 118-130. Acres, USA, Austin TX (submitted with petition)
This TAP review was completed pursuant to United States Department of Agriculture Purchase Order # 43-6395-2900A.