Hexacalcium hexaoxotris[sulphato(2-)]dialuminate(12-)

Toxicity to terrestrial plants

Description of key information

Ettringite is not expected to have detrimental effects on terrestrial plants.  See discussion in this section.

Additional information

There are no studies available on terrestrial plants for Ettringite.

Due to its chemical nature Ettringite is not stable under natural environmental conditions. The main degradation products are calcium sulfate (dihydrate) with limited solubility resulting in free calcium and sulfate ions and insoluble aluminium hydroxides and insoluble aluminium oxides (at neutral pH range).

The relevant compound to consider with regard to terrestrial toxicity of Ettringite is aluminium.

With regard to Calcium sulfate, calcium and sulfate ions:

Calcium sulfate, calcium and sulfate ions are ubiquitous in the environment. Calcium is an important constituent of most soils and the minerals found in soil are mostly compounds of calcium with other substances. Furthermore, calcium sulfate, as Gypsum, is used as an inorganic fertiliser to improve soil quality. Important applications include:

•for the reclamation of sodic soils through ion exchange (calcium replacing sodium)

•to reduce run-off water and its resulting erosion in dry agricultural areas as an ameliorant for acidic subsoils and soils in forestry

•to improve Ca- and S-nutritional elements in agriculture (rape and cereals)

•Gypsum is also useful as an additive for soils with high levels of sodium

The European Committee Joint Research centre have summarized a number of papers as part of an assessment of the effect of gypsum on plants. Van Alphen and de los Rios Romero (1971) conclude that up to 2 percent gypsum in the soil favours plant growth, between 2 and 25 percent has little or no adverse effect if in powdery form, but more than 25 percent can cause substantial reduction in yields. They suggest that reductions are due in part to imbalanced ion ratios, particularly K:Ca and Mg:Ca ratios. Hernando et al. (1963, 1965) studied the effect of gypsum on the growth of corn and wheat by varying the gypsum level in the soil up to 75 percent. They show that high levels of gypsum caused poor growth of corn, especially as the soil moisture was maintained at 80 percent of field capacity. However, wheat showed minimum growth where the soil contained 25 percent gypsum at all soil moisture levels ranging from 15 to 100 percent of field capacity. Akhvlediani (1962) concludes in general, that agricultural production on gypsiferous soils is not affected when the gypsum content is between 15 and 30 percent.

The various reports quoted by JRC suggest that the effect of gypsum on plants is extremely variable depending on the plant, soil type and location. However, the overall results indicate that apart from special crops (e.g. certain fruit trees) the gypsum concentration in soils should be limited to 15 %.

Investigations by Sanderson (2004), on behalf of Agriculture & Agri-Food Canada, on the use of gypsum as an organic amendment in lowbush blueberry production, indicated that gypsum application, with or without fertiliser application, was an effective method to increase nutrient uptake in the lowbush blueberry. Gypsum significantly influenced nutrient uptake and general plant health more than any other fertiliser application evaluated in this region.

Sulfur (as sulfate) is a major plant nutrient, and is essential for crop growth.

Calcium is an important constituent of most soils and the minerals found in soil are mostly compounds of calcium with other substances. Soil calcium is necessary for proper plant functions and helps in producing healthy fruits and flowers.

Some of the functions that require soil calcium include enzyme activity for the absorption of other nutrients, proper cell formation and division, increased metabolic activities, starch breakdown and nitrate uptake. Without soil calcium plants tend to lose their colour, have a short life and produce little or no fruit.

Many fertilisers available today make use of calcium and calcium-rich salts to neutralise soils and make them less acidic. Calcium has strong relationships with other substances found in the soil like magnesium, potassium and sodium. Together these nutrients make the soil so rich that almost all kinds of plants can be grown with it.

Soil calcium is mainly important for lowering the pH level and the associated acidity. It is often recommended to include up to 40 – 50% of calcium in any fertilizer to account for its deficiency in the soil. The resulting plants will have stronger roots and better growth rate than a calcium deficient soil. In addition to this, calcium also helps in regulating the flow of water and air in the soil for proper absorption by the plant cells.

Calcium is known as an essential nutrient for higher plants and one of the basic inorganic elements of algae. Calcium plays crucial roles in strengthening cell walls and plant tissues, reducing the toxicity of soluble organic acids, elongating roots, and so on.

The calcium content of plants varies between 0.1 and > 0.5% of the dry weight depending on the growing conditions, plant species, and plant organ. In well-balanced growing nutrient solutions with controlled pH, maximal growth rates were obtained at calcium supply levels of 2.5-100 uM. Also, calcium can be supplied at higher concentrations and might reach more than 10% of the dry weight without symptoms of serious inhibition of plant growth, at least in calcicole plant species.

Typical symptoms of calcium deficiency are the disintegration of cell walls and the collapse of the affected tissues, such as the petioles and upper parts of the stems. Lower calcium contents in fleshy fruits also increase the losses caused by enhanced senescence of the tissue and by fungal infections.

Given the extensive and continued use of calcium sulfate as a fertiliser and for chemical treatments of soils and its natural occurrence in the environment, it is considered that calcium sulfate would not have a detrimental effect on plants at the concentrations released to soil. Therefore, the performance of a short-term toxicity test to plants is not scientifically justified.

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With regard to aluminium:

Aluminium is the most abundant metallic element in the Earth's crust. Based on its ubiquitous occurrence the present natural background concentration far outweighs anthropogenic contributions of aluminium to the terrestrial environment. As detailed in the endpoint summary on terrestrial toxicity in general further toxicity testing on terrestrial organisms is considered unjustified and waiving based on exposure and scientific considerations is applied.

However, for reasons of completeness existing data on the terrestrial toxicity of aluminium are provided in addition and summerised here.

Kinraide & Parker (1987) in a test with Triticum aestivum determined an EC50 of 1.2 uM referring to the inhibition of root elongation of shoots after 2 days exposure. In a study investigating the same effect but using Alium cepa as test organism and a four days exposure period by Berggren & Fiskesjo (1987) an EC50 of 25 uM was determined based on monomeric labile aluminum. Increassing labile monomeric aluminum from 0 to 85.2 uM and increasing monomeric aluminum from 0 to 69.7 uM resulted in a decreasing root length from 53.7 to 5.8 mm and 47.6 to 5.3, respectively. Lazof et al. (1994) demonstrated an inhbition of root growth (root elongation) of 60% after 6 hours of exposure to 38 uM AL3+. They showed that a substantial aluminum acculmulation in the root tip region after 30 minutes of exposure and report 20 to 25 layers of undiferrentiated cells around 0.3 to 0.8 mm from the cell apex. In this region they found an aluminum signal up to 60 um inwards from the surface. Root growth in aluminum sensitive and tolerant cultivars was compared by Calba et al. (1999). These others found that second order root growth was affected by Al for both the sensitive and the tolerant cultivar. The relative growth of second order roots of the tolerant cultivar remained relatively constant in the range of pHs studied, while the sensitive ones declined when the pH was below 4.2. The relative root length of the sensitive species was approx 20% at pH 3.8 (final pH in the rhizosphere) and approx. 60% at pH 4.5 (final pH in the rhizosphere after 5 days). With the tolerant cultivar, growth ranged from 75% (in comparison to control) at final rhizosphere pH 3.6 and approx.60% at final rhizosphere pH 4.5.

Although results are diverse as a result of various test designs it might be concluded on a strongly generalised basis, that both decreasing pH and/or increasing concentrations of aluminum pose negative effects to roots of terrestrial plants. However, several factors need to be considered in detail , e.g. knowledge of aluminum species responsible for effect, pH regime, soil characteristics, organic matter present, plant species and tolerance mechanisms, in order to assess aluminum toxicity appropriately.

The toxicity of aluminum to vascular plants against the background of such factors was reviewed by Andersson (1988). According to this author soil acidification has the potential to induce aluminum toxicity in plants, as the solubility of aluminum increases exponentially as the pH decreases below 4.5. From the different species of aluminum found it is mainly the labile, monomeric, inorganic species that constitutes the toxic fractions. In terms of measuring toxic concentrations the sum activity of monomeric aluminum species in the soil solution is a better measure than the total concentration of soluble or exchangeable aluminum. A toxicity of aluminum can primarily be expected in mineral soils which have a low content of organic matter and organic acids since these are capable of complexing aluminum, thus reducing the bioavailability of aluminum to plants. The availability of aluminium is also depending on the mineral and soil characteristics. For instance, soils rich in clay have large aluminum fraction, which can be mobilised during an acidification event.

Symptoms of toxicity are first observed in the roots, the development of which is some way hampered, e.g. the elongation of the main root axis diminishes and laterals roots often fail to develop. Roots might also show deformations, they might become stubby, short, swollen, gnarled, or brittle with bent, brown tips. Vascular bundles may not develop properly and the root system can be restricted to soil horizons low in soluble aluminum. As a consequence, the absorption of water and nutrients is often strongly reduced and adds to the adverse influence of aluminum. Seed germination is not as strong affected by aluminium than the survival of seedlings. Higher concentrations and longer exposure times are necessary in order to cause effects to shoots than to roots. Effects observed for shoots include weight decrease and delayed leaf development, more severe effects are wilting, shedding of leaves and death. Such symptoms might be caused by nutrient deficiencies, e.g. inhibition of phosphorous transport by aluminium or uptake and distribution of calcium and other nutrients. On the other hand high concentrations of calcium in the soil can reduce aluminium activity. In general in nutrient-rich soils, plants can cope better with high concentrations of soluble aluminum. On a subcellular level aluminum may disturbe cell devision and DNA replication, membrane flexibility and permeability is affected, coagulation of proteins occurs, enzymes are influenced negatively. All these effects result in hampered transport mechanisms, decreases sugar phosphorylation and root respiration.

However, not all plant species are affected to the same extent and a variety of tolerance strategies is found. Species adapted to acid conditions are more Al tolerant than others. Evolved strategies include active exclusion mechanism, immobilization of aluminum in roots, tolerance to high tissue levels of aluminium due to inactivation and storing at specific sites, the ability to absorb and use phosphorous and calcium in the presence of aluminum, or low requirement for these nutrients.

References:

Van Alphen and De Los Rios Romero F (1971) Gypsiferous soils, notes on characteristics and management. Int Inst of Land Recl and Improv. Bulletin 12. Wageningen. The Netherlands

Hernando V., Sanchez Conde. MP and Contreras JG (1965) Study of the mineral nutrition of maize on soils rich in gypsum Zolfo in Agricoltura Palermo 1964; 398-411

Sanderson K (2004), Gypsum as an Organic Amendment in Lowbush Blueberry Production, Agriculture & Agri-food Canada, Report 2004F-05E

Nachtergaele F, FAO, Rome – Italy, Criterion 6.3 “Soil gypsum content” cited in Common bio-physical criteria to define natural constraints for agriculture in Europe, JRC Scientific and Technical Reports, Draft report EUR XXXXX EN.