Phosphonic acid, potassium salt (1:1)

Toxicity to soil microorganisms

Description of key information

No effect (Nitrogen and Carbon) at 25 and 125 mg a.i./kg soil (dry weight). 

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:

125 mg/kg soil dw

Additional information

All studies reported are well conducted on the similar substance Reaction mass of dipotassium phosphonate and Phosphonic acid, potassium salt (1:1). Rationale for Read Across is attached at point 13.

The effects of Potassium phosphonate (KH2PO3/K2HPO3) on soil microbial nitrogen and carbon transformation were assessed according to OECD Guideline No. 216 (2000) and No. 217 (2000).

Untreated (control) and treated soil (at 25 mg active ingredient/kg soil dry weight and 125 active ingredient mg/kg soil dry weight) were incubated for 28 days.

The test item was mixed into a sandy loam soil at the application rates of 25 mg active ingredient /kg soil (dry weight) and 125 mg active ingredient/kg soil (dry weight). Test concentrations were determined based on the typical application rate of potassium phosphonate multiplied by the number of applications/year. Soil microbial nitrogen and carbon transformation rates of the treated soil were compared to a control without the test item.

The variation between replicate control samples should be less than ± 15 % as indicated by the validity criterion for OECD Guidelines 216 and 217. Since the maximum variation (RSD) between control samples was 5.3% (nitrogen transformation) and 7.5% (carbon transformation) for the respective tests, the results in the here presented study are considered valid.

Concerning nitrogen transformation, since on the 28th day differences (mean value) between untreated soils and treated soils (at the highest concentration) were greater than 25 %, the incubation was continued for further 14-day intervals until differences were lower than 25 % (56th day), as required by OECD No. 216 (2000).

Concerning carbon transformation, since on the 28th day differences (mean value) between treated and untreated soils were lower than 25 %, the incubation was stopped as required by OECD No. 217 (2000).

The influence of the test item on nitrogen transformation was investigated by determining nitrate concentration in treated soil and comparison with untreated soil.

After 28 days, at the highest dose applied (125 mg a.i./kg soil dry weight) the deviation from the control, as absolute value, was higher than 25 %. Therefore the measurements were continued in 14-days intervals until the deviation from control soil was lower than 25 %, which occurred between day 42-56 and measurements were stopped as indicated in OECD Guideline 216.

The influence of the test item on carbon transformation was investigated by measuring the oxygen consumed by soil microorganisms during a glucose-induced respiration test and comparing it to untreated samples.

After 28 days, at both the lowest applied dose (25 mg a.i./kg soil dry weight) and the highest applied dose (125 mg a.i./kg soil dry weight) the deviation from control in treated samples was lower than 25%, as absolute value. Therefore the measurements were stopped according to the OECD Guideline 217.

Potassium phosphonate (KH2PO3/K2HPO3) has no long-term effects both on nitrogen transformation and on carbon transformation at 25 mg a.i./kg soil (dry weight) and 125 mg a.i./kg soil (dry weight).