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Environmental fate & pathways

Biodegradation in soil

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According to REACH Regulation Annex VII, column 2, the study does not need to be conducted if the substance is inorganic.

However, literature data regarding the degradation of phosphite in soil has been included below for information purposes, indicating very slow degradation of phosphite in the absence of phosphate.


The effects of phosphite as a fertilizer was first described in 1953 (Adams, 1953)

They observed that phosphite added to soil samples disappeared with a corresponding increase in phosphate concentration. The oxidation of phosphate proceeded only when microbial activity was not restricted by the presence of a bactericidal agent such as toluene.


The levels of ethyl phosphonate and phosphonate in avocado seedlings and soil were determined using high-performance ion chromatography at 1, 2, 4, 6, and 8 wk following foliar or soil applications of either 3 mg/ ml of fosetyl-Al or 2.1 mg/ ml of potassium phosphonate. After soil treatment with either potassium phosphonate or fosetyl-Al, phosphonate persisted in soil for 2 and 4 wk, respectively. Following both soil and foliar applications of the two fungicides, high phosphonate levels were maintained in avocado tissues for the 8-wk period of the experiments, suggesting that phosphonate is stable in plants. The phosphonate levels found in roots after either soil or foliar applications were sufficiently high to account for a direct antifungal effect in controlling avocado root rot caused by Phytophthora cinnamomi But the experiment was conducted with pot soil and no extrapolation to agricultural soil is granted (Ouimette D., 1989)


Several articles are describing the use of phosphate as source of phosphate:


A variety of bacteria grown on a glucose and salts medium were capable of utilizing orthophosphite as a sole source of phosphorus. Two organisms, Pseudomonas fluorescens 195 and Serratia marcescens 24, were studied in detail. Growth rates and total cell yields of the bacteria grown on phosphite indicated that the bacteria utilized phosphite as efficiently as phosphate. The ability to oxidize the anion was shown to be inducible. A period of adaptation was required prior to growth on phosphite when phosphate-grown cells were transferred to a medium containing a limiting amount of phosphate and excess phosphite. No phosphite-oxidizing activity could be detected in whole cells or cell-free extracts of phosphate-grown cells. Both whole cells and cell-free extracts of phosphite-grown cells possessed phosphite-oxidizing activity (Malacinski G., 1966).


Twenty-three microbial cultures were surveyed for their ability to utilize orthophospite phosphorus for heterotrophic growth and to accumulate the oxidation product, orthophosphate, in the growth medium. Of these 14 utilized orthophosphate for growth, but only Pseudomonasfluorescens strain 195 accumulated orthophosphate. A study of orthophosphate accumulation by this pseudomonad revealed that the accumulation occurred in a linear fashion after an initial 24-hr incubation period and that the rate decreased when most of the carbohydrate had been removed from the medium. All variations tried in the growth medium resulted in reduced accumulation (Casida L. 1959).


Evidence that Phosphite can be directly used by plants as a sole source of nutritional P is lacking. When Phosphite is administered in such as way as to allow it to come into contact with bacteria, either associated with plant root systems or in the soil, then the oxidation of Phosphite to phosphate (HPO2ÿ 4 ; Pi) may take place. By this indirect method Phosphite could thus become available to the plant as a P nutrient. The rates at which this occurs are slow, taking months or as much as a year, depending on the soil type (McDonald, 2001).



In the Adams and Conrad Study (Adams, 1953) DT50 for phosphorous acid in soil was not calculated but it was < 16 weeks. No reliable data are existing for soil dissipation /transformation in different soils. RMS calculation provide a DT50 value of 96d for phosphite application to bare soil at 28°C using 1st order kinetic. At 20°C the corresponding value would be 157d and 49 d respectively.



Adams F, Conrad JP. (1953) Transition of phosphite to phosphate in soils. Soil Science 75: 361- 371.

Ouimette, D. G., and Coffey, M. D. 1989. Phosphonate levels in avocado (Persea americana) seedlings and soil following treatment with fosetyl-Al or potassiumphosphonate. Plant Disease 73:212-215.

MALACINSKI, GEORGE (Indiana University, Bloomington), AND WALTER A. KONETZKA. Bacterial oxidation of orthophosphite. J. Bacteriol. 91:578-582. 1966

L. E. CASIDA, JR. MICROBIAL OXIDATION AND UTILIZATION OF ORTHOPHOSPHITE DURING GROWTH' Department of Bacteriology, Pennsylvania State University, University Park, Pennsylvania December 23, 1959