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Toxicity to soil microorganisms

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Description of key information

Klimisch 1 study: 96d-EC50 = 8.7 g/kg soil dw. and 96d-NOEC = 4 g/kg soil dw. for calcium dihydroxide for dehydrogenase activity (Schulz, 2007a)
Klimisch 1 study: 96d-EC50 > 12 g/kg soil dw. and 96d-NOEC >= 12 g/kg soil dw. for calcium dihydroxide for nitrogen transformation (Schulz, 2007b)
Rationale for read-across: in the environment, lime substances rapidly dissociate or react with water. These reactions, together with the equivalent amount of hydroxyl ions set free when considering 100mg of the lime compound (hypothetic example), are illustrated below:
Ca(OH)2 <-> Ca2+ + 2OH-
100 mg Ca(OH)2 or 1.35 mmol sets free 2.70 mmol OH-
CaO.MgO + 2H2O <-> Ca2+ + Mg2+ + 4OH-
100 mg CaO.MgO or 1.04 mmol sets free 4.15 mmol OH-
From these reactions it is clear that the effect of calcium magnesium oxide will be caused either by calcium ions, magnesium ions or hydroxyl ions. Since calcium and magnesium are abundantly present in the environment and since the effect concentrations are within the same order of magnitude of their natural concentrations, it can be assumed that the adverse effects are mainly caused by the pH increase caused by the hydroxyl ions. Furthermore, the above mentioned calculations show that the base equivalents are within a factor 2 for calcium magnesium oxide and calcium hydroxide. As such, it can be reasonably expected that the effect on pH of calcium magnesium oxide is comparable to calcium hydroxide for a same application on a weight basis. Consequently, read-across from calcium hydroxide to calcium magnesium oxide is justified.

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:
4 000 mg/kg soil dw

Additional information

The chronic study on the effect of calcium dihydroxide on the dehydrogenase activity in an agricultural loamy sand soil (Schulz, 2007a) was conducted according to German guidelines for testing of plant protection products (BBA VI, 1-1, 1990). The methods and results are well documented. As such a Klimisch 1 score was assigned to the study. Reported 96d-EC50 and 96d-NOEC values for dehydrogenase activity equalled 8.7 g Ca(OH)2 /kg soil dw and 4 g Ca(OH)2 /kg soil dw and ≥12 g Ca(OH)2 /kg soil dw.

The chronic study on the effect of calcium dihydroxide on the nitrogen transformation in an agricultural loamy sand soil (Schulz, 2007b) was carried out according to OECD 216. The study is well documented, all validity criteria are fulfilled. As such a Klimisch 1 score was assigned to the study. Reported 96d-EC50 and 96d-NOEC values for nitrogen transformation equalled >12 g Ca(OH)2 /kg soil dw and ≥12 g Ca(OH)2 /kg soil dw.

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