This is a Sunset Preliminary Review by a Subcommittee of the National Organic Standards Board (NOSB). Sunset preliminary reviews are posted for 

public comment and the NOSB will refer to them to complete the sunset review process. They are not final Board recommendations or NOP policy. 

For more information, see the Sunset Review and Renewal Process fact sheet, and Federal Register notice of Sept. 16, 2013.

Sunset 2015 Review 

Meeting 2 - October, 2014

Crops Subcommittee

Aqueous Potassium Silicate

August 19, 2014

As part of the National List Sunset Review process, the NOSB Crops Subcommittee has evaluated the need for the 

continued allowance for or prohibition of the following substances for use in organic crop production.

Aqueous Potassium Silicate

205.601(e)), 205.601(i)

Synthetic

Use: As insecticides (including acaricides or mite control). As plant disease control.

Listing: Aqueous potassium silicate (CAS #-1312-76-1)—the silica, used in the manufacture of potassium silicate, must be 

sourced from naturally occurring sand.

Technical Reports: 2003 (PDF); 2014 (PDF)

Petition(s): Aqueous potassium silicate; Aqueous potassium silicate supplemental

Past NOSB Actions: NOSB review and recommendation for addition to the National List 11/30/07. 

Regulatory Background: Proposed rule (including justification) 6/3/2009 (74 FR 26591). Added to National List 

12/13/2010 (75 FR 77521). 

Sunset Date: 12/14/2015

Reference: 7 CFR 205.601(e)(2)

Subcommittee Review

The Crops Subcommittee believes that the full Board should have the opportunity to complete the review of each 

sunset material by voting. The NOP has stated that to do this a motion to remove should be brought from the 

Subcommittee for each substance. If the Subcommittee motion to remove fails to receive a majority, the motion will 

still be put forward to the full board for review. The motion to remove is voted by the full Board and needs to receive 

a 2/3 majority to recommend removal.

Summary:

In 2007, the Crops Subcommittee recommended against listing Aqueous Potassium Silicate1 (APS) because “multiple 

substitutes are available” and it is a “synthetic soil applied fertilizer not compatible with organic farming regulations.” 

The rationale given for NOSB approval was, “Public comment at Nov. 2007 NOSB meeting well supported listing the 

substance as plant disease control by providing historical 2003 NOSB consideration of the material as well as more 

information from petitioner and other interested stakeholders.”

New information has been provided in a new Technical Review. That information supports the conclusions below. 

(Citations are line numbers in January 6, 2014 TR.)

• Dermal exposure can lead to low to medium systemic toxicity and skin irritation (577-579);

1 Aqueous Potassium Silicate is the active ingredient in products such as the brand Sil-Matrix Fungicide/Miticide.

1

 
• Silicon reduces the availability of elements such as manganese, iron, and aluminum to roots (471-473);

• Treatment with potassium silicate may not be appropriate when crops are used for feeding or as forage for 

livestock because it makes some forages less digestible (477-481);

• The addition of potassium silicate as a foliar nutrient may result in the production of less tender fruits and 

vegetables or forage for grazing animals (479-481);

• Silica supplementation can result in elongation and thickening of stems, delayed antithesis and flower 

deformation in some species (487-490);

• In addition to morphological changes, changes in micronutrient in plants may occur as a result of silica 

supplementation (490-491);

• New alternative materials suggested include other forms of silica that are available as approved supplements for 

the soil that can provide the same protection over a longer term against plant disease and compost made with 

silica-rich plants (592-594);

• The TR suggests the following alternative practices: soilscaping, choice of variety and planting time, balancing 

silica accumulators and nonaccumulators, moisture management, choice of mulch and ground cover, and 

scouting (661-706); and

• Internationally (Japan, Canada, EEC, CODEX, or IFOAM), natural sources of silica, not APS, are allowed (258-296).

The Crops Subcommittee invited comment on these conclusions, as well as well as on two major issues of concern at the 

Spring 2014 meeting:

1. Potassium silicate makes plants more resistant to disease and herbivory, at least in part by concentrating silica. 

Humans and livestock are herbivores who might be consuming the treated plants. Does the foliar application of 

potassium silicate might have impacts on the nutritive value of treated foods that would exceed the impacts of silica 

obtained by the plant from natural soils? The TR addressed this to some extent (See #3, 4, and 6 above.) How should 

the NOSB weigh this impact on the nutritive value of treated plants?

2. The central issue in the essentiality question is whether organic management systems that conserve and build 

available silicon in the soil are alternatives to potassium silicate. Thus, the subcommittee received some information 

on this issue (see #7 and 8 above) and is interested in comments concerning nonsynthetic materials and practices 

(involving soil management as well as foliar treatments) that would build comparable resistance to insects and fungi, 

while precluding the need for synthetic potassium silicate.

The NOSB received two comments supporting renewed listing and eight comments opposing renewal. Some specific 

comments that were received are:

• When APS enters the soil from plant treatment it is indistinguishable from silicates already present in the 

ground. 

• APS is used as a foliar application not for roots. 

• “Management systems can be used to build the Si in the soil to improve the plant’s resistance to disease and 

reducing the likelihood of needing a pesticide treatment. However, when an infestation occurs and a treatment 

is required, APS should be an available option for organic farmers.”

• Information is needed on accumulation of silica in plants.

• International standards do not allow aqueous potassium silicate in crop production.

• Organic methods of soil conservation make its use unnecessary.

Conclusion

In reviewing the 2014 Technical Review and materials submitted by commenters, the Crops Subcommittee finds new 

evidence relating to OFPA criteria. There are concerns about impacts on human health and the environment, essentiality 

given alternative materials and practices, and the fact that APS is not allowed in organic production in other countries. 

There is also information that APS fills a need when a problem occurs. The subcommittee supports research to gather 

additional information needed to address the issues identified in the attached checklist. 
Motion to Remove:

This proposal to remove will be considered by the NOSB at its public meeting. 

Based on the Subcommittee’s review, the Subcommittee proposes removal of this substance from the National List 

based on the following criteria in the Organic Foods Production Act (OFPA): [OFPA criteria at 7 U.S.C. 6158(m), (7) its 

compatibility with a system of sustainable agriculture.

Motion to remove Aqueous Potassium Silicate from the National List

Motion by: Harold Austin

Seconded by: Carmela Beck

Yes: 4 No: 3 Abstain: 0 Recuse: 0 Absent: 0

Minority Statement on Motion to Remove

While the minority of the CS agrees with the majority that the full NOSB should vote on sunset materials, in voting 

against this motion it is following what we believe are required procedure of AMS/USDA as established by the 

September 16, 2013 Federal Register notice (78 FR 56811), which states that motions to remove be justified by criteria 

established by the Organic Foods Production Act. Because of concern that a change in NOSB procedures should be 

disclosed to the public before taking effect, the minority does not accept the compatibility criteria from 7 U.S.C. 6158(m)

(7) that was provided in this case. Furthermore, AMS/NOP has said that no action by the NOSB maintains a sunset 

material on the National List. 
Evaluation Criteria (see attached checklist for criteria in each category)

Criteria Satisfied? 

1. Impact on Humans and Environment ☐ Yes ☒ No ☐ N/A 

2. Essential & Availability Criteria ☐ Yes ☒ No ☐ N/A

3. Compatibility & Consistency ☐ Yes ☒ No ☐ N/A 

Substance Fails Criteria Category: 1, 2, 3

NOSB Evaluation Criteria for Substances Added To the National List (Optional Checklist)

Category 1. Adverse impacts on humans or the environment? Substance: Aqueous Potassium Silicate 

Question Yes No N/A Comments/Documentation (TAP; petition; 

regulatory agency; other)

1. Is there a probability of environmental 

contamination during use or misuse? 

[§6518(m)(3)]

X Since potassium silicate is sprayed onto 

plants, there is a probability of 

contamination. 

2. Is there a probability of environmental 

contamination during, manufacture or 

disposal? [§6518(m)(3)]

X CO2 produced by manufacture. High energy 

cost, use of fossil fuels. Strong alkaline 

solution produced in manufacture. TAP, pp. 

2, 4, 9-12.

3. Are there any adverse impacts on 

biodiversity? (§205.200)

X Potassium silicate has not been tested for 

ecotoxicity. It is not persistent in aquatic 

systems, but is highly alkaline in solution 

form and can be harmful to aquatic life if not 

diluted and disposed of properly. TAP, p. 4.

4. Does the substance contain inerts classified 

by EPA as ‘inerts of toxicological concern’? 

[§6517 (c)(1)(B)(ii)]

X It is/was a List 3 “inert.” TAP, p. 3.

5. Is there potential for detrimental chemical 

interaction with other materials used in 

organic farming systems?

[§6518(m)(1)]

X The substance may react in storage with 

ammonium salts to form hydrogen gas, and 

care should be taken to avoid contact with 

raw manure in closed storage. Potassium 

silicate solutions have a high pH, and 

applications may have adverse effects if used 

on alkali sensitive crops. TAP, p. 3. During 

storage of the compound, care must be 

taken to avoid wetting the material. Spills are 

slippery. Reacts with acids, ammonium salts, 

reactive metals and some organics. TAP, p. 9 

Potassium silicate gels and generates heat 

when mixed with acid and may react with 

ammonium salts resulting in the evolution of 

ammonia gas. Flammable hydrogen gas may 

be produced on contact with aluminum, tin, 

lead, and zinc. TR lines 451-453.2

6. Is there a toxic or other adverse action of the X No carcinogenicity, mutagenicity, or 

2 Line numbers for TR refer to January 6, 2014 final.

 
material or its breakdown products?

[§6518(m)(2)]

developmental toxicity data are available for 

potassium silicate. TAP, p. 5. Potassium 

silicate has no chronic hazards, does not bioconcentrate in the food chain, nor make 

volatile or toxic organic compounds when 

used as recommended. TAP, p. 11.

7. Is there persistence or concentration of the 

material or breakdown products in the 

environment? [§6518(m)(2)]

X “little to no potential to contaminate or 

persist in the environment.” TAP, p. 9. Use 

will not result in hazardous of 

environmentally persistent byproducts. TAP, 

p. 11.

When dissolved in water, the active 

ingredient potassium silicate dissociates into 

potassium cations, hydroxide anions, and 

mono- and polysilicic acids. When used as a 

pesticide, potassium silicate residues are low 

relative to naturally present concentrations 

and other uses in the environment. TR lines 

522-523; 398-399. 

8. Would the use of the substance be harmful 

to human health or the environment? 

[§6517 (c)(1)(A)(i); §6517 (c)(2)(A)(i); 

§6518(m)(4)]

X Acute overexposure may cause skin and 

respiratory tract irritation. The substance has 

not been tested for primary eye irritation, 

but is regarded as an eye irritant on the basis 

of its high alkalinity and its similarity to 

sodium silicate (Blumberg 2001). Respiratory 

problems in the agricultural sector due to 

inhaled dust are a proven concern (Schenker 

2000). TAP, p. 4

Potassium silicate has not been tested for 

ecotoxicity. It is not persistent in aquatic 

systems, but is highly alkaline in solution 

form and can be harmful to aquatic life if not 

diluted and disposed of properly. TAP, p. 4. 

Not buffered, and a buffered natural system 

is not likely to be affected. TR 387-391. 

Agricultural use of potassium silicate is 

subject to the Worker Protection Standards 

(WPS), requiring Personal Protective 

Equipment (PPE) a long-sleeved shirt, long 

pants, socks, shoes and gloves, plus a 4 hour 

Restricted Entry Interval (REI). TR lines 568-

569. “Results of the acute dermal toxicity 

study indicated moderate to low toxicity at 

the maximum dose tested, although dermal 

irritation was observed.” TR lines 577-579.

9. Are there adverse biological and chemical 

interactions in the agro-ecosystem? 

[§6518(m)(5)]

? Potassium silicate effects on metabolic 

interactions are not well characterized if at 

all. TAP, p. 9. Protective effects would apply 

to weeds as well as crop plants. If used on 

pastures, could affect the pattern of grazing, 

thus affecting species composition. TR lines 

494-497.
10. Are there detrimental physiological effects 

on soil organisms, crops, or livestock? 

[§6518(m)(5)]

X Successive silicate fertilizer applications have 

been shown to increase soil pH to levels that

adversely affect plant growth (Miayke and 

Takahashi 1983), but soils with high organic 

matter content tend to buffer this effect, and 

additions of organic material were effective 

in correcting soil pH. TAP, p. 5. Most impacts 

on soil are positive. TAP, pp. 2, 5, others. . 

Silicon reduces the availability of elements 

such as manganese (Mn), iron (Fe), and 

aluminum (Al) to roots of plants such as rice 

and sugarcane. TR lines 471-473. Treatment 

with potassium silicate may not be 

appropriate when crops are used for feeding 

or as forage for livestock since its addition 

hardens some plants, making them both 

more difficult to chew and digest. 

Furthermore, monosilicic acid naturally 

strengthens the phyto-skeleton, thus the 

addition of potassium silicate as a foliar 

nutrient may result in the production of less 

tender fruits and vegetables or forage for 

grazing animals. TR lines 477-481. Silica 

supplementation can result in elongation and 

thickening of stems, delayed antithesis and 

flower deformation in some species 

depending on the level of accumulation of 

silica by the plant species, the type of silica 

supplement used and the method by which it 

was applied. In addition to morphological 

changes, changes in micronutrient in plants 

may occur as a result of silica 

supplementation. TR lines 487-491 Makes 

certain forage grasses less digestible. TR lines 

494-497.

Category 2. Is the Substance Essential for Organic Production? Substance: Aqueous Potassium Silicate 

Question Yes No N/A Comments/Documentation (TAP; petition; 

regulatory agency; other)

1. Is the substance agricultural? [§6502(1)] X

2. Is the substance formulated or 

manufactured by a chemical process? 

[§6502(21)]

X Potassium silicates are manufactured using a 

calcination process that combines silica sand 

(SiO2) and potassium carbonate (K2CO3) at 

1100-2300°F for up to 15 minutes (NOP 

Petition; Rawlyk and McDonald 2001). The 

two substances fuse into glass, which can be 

dissolved with high-pressure steam to form a 

clear, slightly viscous fluid, or cooled and 
ground into a powder. Carbon dioxide is 

evolved from this reaction. The solution can 

be dried to form hydrous powder crystals of 

potassium silicate. TAP, p. 2

3. Is the substance formulated or 

manufactured by a process that chemically 

changes a substance extracted from 

naturally occurring plant, animal, or mineral 

sources? 

[§6502(21)]

X Potassium silicates are manufactured using a 

calcination process that combines silica sand 

(SiO2) and potassium carbonate (K2CO3) at 

1100-2300°F for up to 15 minutes (NOP 

Petition; Rawlyk and McDonald 2001). The 

two substances fuse into glass, which can be 

dissolved with high-pressure steam to form a

clear, slightly viscous fluid, or cooled and 

ground into a powder. Carbon dioxide is 

evolved from this reaction. The solution can 

be dried to form hydrous powder crystals of 

potassium silicate. TAP, p. 2

4. Is the substance created by naturally 

occurring biological processes? 

[§6502(21)]

X Petitioned material is synthetic, but some 

natural aqueous potassium silicate is present 

in volcanic soils. TR, line 72.

5. Is there a natural source of the substance? 

[§ 205.600(b)(1)]

X

6. Is there an organic substitute? 

[§205.600(b)(1)]

X

7. Is there a wholly natural substitute product?

[§6517(c)(1)(A)(ii)]

X Fertilizer: glauconite, TAP p. 6. Azomite, p. 8. 

Disease: A number of foliar treatments to 

control fungal disease are currently used in 

organic agriculture, with research ongoing; 

some of these are agricultural products. In 

one study, an aqueous solution of burnt rice 

husks (400 q/ha) was shown to be as 

effective and economically viable as a 1% 

commercial sodium silicate solution for 

treatment of rice blast (Pyricularia oryzae) 

(Hsieh and Hsieh 1989). Sulfur is by far the 

most widespread treatment for powdery 

mildew and botrytis bunch rot on grapes.

Others...TAP p. 6. More studies are required 

to definitively state that silica is useful to

prevent fungal infections in other crops. The 

necessity of potassium silicate for organic 

production has not been demonstrated. P. 

10. There is no known natural substance 

producing the same short term effect on 

plant health as aqueous potassium silicate in 

a foliar spray. However, other forms of silica 

and application methods for these 

substances are available as approved 

supplements for the soil that can provide the 

same protection over a longer term against 

plant disease. TR lines 591-594. Compost 

made with silica-rich plants. TR lines 596-605. 

Biopesticides. TR lines 609-633. 
8. Are there any alternative substances? 

[§6518(m)(6)]

X See #7.

9. Are there other practices that would make 

the substance unnecessary? [§6518(m)(6)]

X Organic practices including rotation, green 

manures, compost. TAP p. 8, 10. In addition, 

soilscaping, choice of variety and planting 

time, balancing silica accumulators and 

nonaccumulators, moisture management, 

choice of mulch and ground cover, and 

scouting. TR lines 661-706.

Category 3. Is the substance compatible with organic production practices? Substance: Aqueous Potassium Silicate

Question Yes No N/A Comments/Documentation (TAP; petition; 

regulatory agency; other)

1. Is the substance consistent with organic 

farming and handling? 

[§6517(c)(1)(A)(iii); 6517(c)(2)(A)(ii)]

X Unnecessary synthetic input. Synthetic 

fertilizer. Internationally (Japan, Canada, EEC, 

CODEX, or IFOAM), natural sources of silica, 

not APS, are allowed. TR lines 258-296. 

2. Is the substance compatible with a system of 

sustainable agriculture? [§6518(m)(7)]

X Unnecessary synthetic input. Synthetic 

fertilizer. High energy requirement for 

manufacture.

3. If used in livestock feed or pet food, Is the 

nutritional quality of the food maintained 

with the substance? [§205.600(b)(3)]

X

4. If used in livestock feed or pet food, Is the 

primary use as a preservative? 

[§205.600(b)(4)]

X

5. If used in livestock feed or pet food, Is the 

primary use to recreate or improve flavors, 

colors, textures, or nutritive value lost in 

processing (except when required by law)? 

[§205.600(b)(4)]

X

6. Is the substance used in production, and 

does it contain an active synthetic ingredient 

in the following categories: 

[§6517(c)(1)(B)(i);

copper and sulfur compounds

X

toxins derived from bacteria X

pheromones, soaps, horticultural oils, 

fish emulsions, treated seed, vitamins 

and minerals

X Minerals.

livestock parasiticides and medicines X

production aids including netting, tree 

wraps and seals, insect traps, sticky 

barriers, row covers, and equipment 

cleansers

X