Oxalate "occurs in most plant tissues and the amount depends on growing conditions, season, and plant part." [TNS] In plants it is synthesized by the incomplete oxidation of carbohydrates.[Wiki-1]

(Left) Scanning Electron Micrograph of the surface of a kidney stone showing tetragonal crystals of Weddellite (calcium oxalate dihydrate) emerging from the amorphous central part of the stone. Horizontal length of the picture represents 0.5 mm of the figured original.[Kempf] [Wiki-1]

The following species are known to contain one or more Oxalates (species in bold are local);

  • The goosefoot family (Chenopodiaceae) [CPPlantMush]
  • The buckwheat family (Polygonaceae) [CPPlantMush]
    • Rumex acetosa, R. crispus' - Dock [TNS]
  • Beta vulgaris - Beets [CPPlantMush]
  • Camellia sinensis - Tea [Wiki-1]
  • Chenopodium album - Lamb's Quarters [Wiki-1]
  • Halogeton glomeratus [TNS]
  • Oxalis Sp. (Oxalis caerulea, O. corniculata) [TNS]
  • Rheum Sp. (including; R. officinale, palmatum, rhabarbarum, rhaponticum) [TNS]
  • Portulaca oleracea [TNS]
  • Salsola kali - Russian Thistle [CPPlantMush]
  • Tetragonia tetragonioides - New Zealand spinach [TNS]

Further details regarding the qualitative and quantitative properties of oxalates can be found under the hazard and/or phytochemical headings on the plant in question's profile. [Personal Note]

"Under normal circumstances, oxalates in moderate amounts can be broken down by bacteria in an animal's digestive tract. Usually oxalic acid salts must comprise 10% or more of a plant's dry weight to be seriously toxic." [CPPlantMush]

In the body, oxalic acid combines with divalent metallic cations such as calcium and iron to form crystals of the corresponding oxalates which are then excreted in urine as minute crystals. Calcium oxalate is the primary cause of kidney stones. [Wiki-1]

Some sources recommend milk and calcium supplementation [Wiki-1], but other studies have claimed that supplemental calcium increases risk, while dietary intake decreases risk. [3][20] Another says that "high intakes of dietary oxalates and low fluid intake" are the greater factor in kidney stone development.[22][Wiki-5]

Ingesting small amounts of oxalate-containing plants generally causes only minor GI disturbance. Serious leaf toxocity is not well-documented. "Fatalities associated with the ingestion of rhubarb probably involved other agents, toxins, or etiologies." 20 mg/Kg (IV) sodium oxalate produced profound hypocalcemia and cardiac arrest.[21][TNS] "Recovery from severe oxalate poisoning is possible, but permanent liver and kidney damage may have occurred." [Wiki-3]

Soluble Oxalates
Water-soluble oxalates include potassium oxalate and sodium oxalate. "Unfortunately, the soluble oxalates can be absorbed from the digestive tract and then react with calcium in the blood to form calcium oxalate. This situation may tie up the calcium to the point where an ionic mineral imbalance could occur in the blood, resulting in internal hemorrhaging due to an anticoagulatory effect." This may lead to coma and death.[WildFoodsForum]

Insoluble Oxalates
See also Insoluble calcium oxalates. Other symptoms can include slurred or unintelligible speech and "superficial necrosis developing days after initial contact." Antihistamines (Diphenhydramine (Benadryl))can be used as "treatment for significant oral and/or laryngeal edema." [Kearney et al.]

Shamrock - Oxalis hedysaroides
Contains soluble and insoluble oxalates;
"soluble oxalates may cause damage to the kidney, brain, liver, or heart."
"Signs and symptoms of overdose include dermatitis, hypocalcemia, hypocalcemic arrhythmia, metabolic acidosis, renal damage, tetany." "The onset of symptoms is 2-12 hrs." Drink milk/water to dilute.
"For treatment of soluble oxalates, which may cause systemic oxalate toxicity, keep well hydrated, monitor renal function, fluid/electrolytes; hypocalcemia may occur which can be treated with calcium gluconate, monitor urine for crystals; treat other symptoms supportively."
Toxicity is unlikely unless without large amounts.[PTH]

Calcium binds with available oxalate in the gastrointestinal tract, thereby preventing its absorption into the bloodstream, and reducing oxalate absorption decreases kidney stone risk in susceptible people.[69] There is a significant correlation between lack of fluid intake, and excessive sodium, refined sugar and animal protein. People with underlying kidney disease and Chrohn's Disease are at increased risk of kidney stones.[Wiki-5]

Oxalic Acid

Melting point 102 to 103 °C (216 to 217 °F; 375 to 376 K); 101.5 °C (214.7 °F; 374.6 K) dihydrate Solubility in water: 143 g/L (25 °C)[2] Solubility:237 g/L (15 °C) [2] in ethanol; 14 g/L (15 °C) in diethyl ether [1][Wiki-6]

Used as a mordant for dyeing and also used in baking powder.[7][Wiki-6]

Health Risks
There "is a possible risk of congenital malformation in the fetus", it is "extremely destructive to tissue of mucous membranes and upper respiratory tract", it can cause burns to the skin and eyes, and "burning sensation, cough, wheezing, laryngitis, shortness of breath, spasm, inflammation and edema of the larynx, inflammation and edema of the bronchi, pneumonitis, pulmonary edema.[25]" Kidney failure [28] and joint pain are also possible side effects.[Wiki-6]

Oxalic acid has a low oral lethal dose of 600 mg/kg.[26] and a lethal oral dose of 15-30 g/kg.[Wiki-6]

Calcium Oxalate

Properties: Practically insol in water or acetic acid; sol in dil HCl or HNO3 [Merck,2001] Melting point 200 °C (392 °F; 473 K) decomposes (monohydrate); Solubility in water 6.7 mg/L (20 °C) [Wiki-3]

Occur in jack-in-the-pulpit, arrow arum, skunk cabbage (Lysichiton americanus), wild calla, and other members of the Arum family.[WildFoodsForum] "...reported in more than 1000 different genera of plants.[1]"[Wiki-3]

  • Arisaema Sp. - Jack-in-the-Pulpit [Wiki-3]
  • Beerstone - A scale forming in breweries. [Wiki-3]
  • Colocasia esculenta - Elephant Ear [Wiki-3]

Insoluble calcium oxalates occur in some plants as 'raphides', or needle-like crystals. When they penetrate the mouth the pain can last for days.[WildFoodsForum] Effects can last up to 2 weeks.[9][Wiki-2] The quick onset generally induces one to spit them out before swallowing, but they can cause severe, possibly life-threatening inflammation of the airways. Because they usually pass through the digestive system unabsorbed by the bloodstream, they do not cause the kidney damage/kidney stones that the soluble oxalates do.[WildFoodsForum] Raphides may help facilitate the transfer of other toxins through the skin. [Wiki-2]

Species Examples;

Taro (Colocasia esculenta): Globally 11.3 millions tons of Taro produced[4] for their edible corms, leaves and leaf-stems. The plant is inedible when raw and considered toxic due to the presence of calcium oxalate[6][7] crystals, typically as raphides. The toxin is minimized by cooking,[8] especially with a pinch of baking soda. It can also be reduced by steeping taro roots in cold water overnight. [Wiki-4]


  • [Kearney et al.]http://emedicine.medscape.com/article/817016-overview, Oxalate Poisoning, Jason F Kearney and William K Chiang, Medscape, Dec 06, 2013
  • [Wiki]
    • [1]https://en.wikipedia.org/wiki/Oxalate, Accessed July 5, 2015
      • [Kempf]Dr. rer. nat. Eugen Karl Kempf, Emeritus Professor, University at Cologne, Faculty of Mathematics and Natural Sciences, Institute of Geology and Mineralogy, Zuelpicher Str. 49a, D-50674 Koeln, GERMANY. January 2012 , Accessed July 7, 2015
    • [2]https://en.wikipedia.org/wiki/Raphide, Accessed July 5, 2015
      • [9]Watson, John T.; Jones, Roderick C.; Siston, Alicia M.; Diaz, Pamela S.; Gerber, Susan I.; Crowe, John B.; Satzger, R. Duane (2005). "Outbreak of Food-borne Illness Associated with Plant Material Containing Raphides". Clinical Toxicology 43 (1): 17–21. doi:10.1081/CLT-44721. PMID 15732442.
    • [3]https://en.wikipedia.org/wiki/Calcium_oxalate, Accessed July 5, 2015
      • solubility values from W. C. de Baat, Recl. Trav. Chim. Pays-Bas, Vol 45, pp 237, 1926.
      • [Merck,2001]
    • [4]Taro http://en.wikipedia.org/wiki/Taro#Toxicity Accessed July 6, 2014
      • [2] Faostat UN Food & Agriculture Organisation
    • [5]https://en.wikipedia.org/wiki/Kidney_stone, Accessed July 5, 2015
      • [3]Johri, N; Cooper, B; Robertson, W; Choong, S et al. (2010). "An update and practical guide to renal stone management". Nephron Clinical Practice 116 (3): c159–71. doi:10.1159/000317196. PMID 20606476.
      • [20]"Tolerable upper intake levels: Calcium and vitamin D". In Committee to Review Dietary Reference Intakes for Vitamin D and Calcium 2011, pp. 403–56.
      • [22]Liebman, M; Al-Wahsh, IA (2011). "Probiotics and Other Key Determinants of Dietary Oxalate Absorption" (PDF). Advances in Nutrition 2 (May): 254–60. doi:10.3945/an.111.000414. Retrieved 2011-07-27.
    • [6]https://en.wikipedia.org/wiki/Oxalic_acid, Accessed July 5, 2015
      • [1]Radiant Agro Chem. "Oxalic Acid MSDS".
      • [2]solubility values from W. C. de Baat, Recl. Trav. Chim. Pays-Bas, Vol 45, pp 237, 1926.
  • [WildFoodsForum]Vol. 9, No.1, January I February 1998