The phosphonate anion HPO₃^2- is the active molecule and is derived from phosphonate (HPO(OR)₂). The phosphonate is placed on the market under more than one name: potassium phosphonate, dipotassium phosphonate, monopotassium monopotassium phosphonate, disodium phosphonate, ... The phosphonate ion (and/or salt) is sometimes incorrectly called ‘phosphite’.

All products leading to the release of the phosphonate ion in plants are affected by the following information.

EC Regulation 1107/2009 Plant protection products

Phosphonate has a direct action on the inhibition of Oomycetes fungi. It is also used as an elicitor of the natural defences in the plant. Disodium phosphonate and potassium phosphonates (potassium hydrogen phosphonate and dipotassium phosphonate) were approved for this reason as active substances under Regulation 1107/2009 on the placing of Plant protection products on the market. In order to place on the market products containing these active substances, an application must be submitted to the Plant Protection Products and Fertilisers Service.

Plant protection products containing potassium phosphonates or disodium phosphonate as an active substance are already authorised in Belgium. An application for authorisation of products based on other phosphonates than disodium phosphonate or potassium phosphonates will only be possible after approval of the substance in question as an active substance in the context of the Regulation 1107/2009.

Phosphonates and Fertilisers Regulation

Phosphonate HPO₃^2-, contrary to phosphate PO₄^3-, cannot be used by the plant as a nutrient source of phosphorus. In addition, the plant cannot oxidise phosphonate HPO3^2- to phosphate PO₄^3-.

For this reason, phosphonate cannot be considered as a nutrient and cannot fall under Regulation (EC) No 2003/2003 relating to fertilisers or Royal Decree of 28/01/2013 on fertilisers.

In December 2011, a Spanish company proposed to include potassium phosphonate in Annex 1 of Regulation (EC) No 2003/2003 relating to fertilisers. The Fertilisers Working Group, a working group bringing together the Member States and the Commission, rejected its inclusion as a result of the absence of nutritional effects of the substance.

This position was confirmed by the European Commission in its note to the Member States of 13 January 2015. This note was a follow-up to a Commission communication to the stakeholders of 1 September 2014.

Phosphonates, fosetyl & MRLs

The use of products containing phosphonate on crops can lead to residues on harvested products. In the context of Regulation 396/2005, maximum residue limits (MRLs) were set for fosetyl(-aluminium). Phosphonates (salts of phosphonic acid) are covered by the residue definition for fosetyl(-aluminium). The presence of phosphonates (analytically determined as phosphonic acid) in a foodstuff of plant origin is therefore an indication that fosetyl(-Al) and/or phosphonates were used. Controls on maximum residue levels are carried out regularly by the Federal Agency for the Safety of the Food Chain.

Conclusion

Any product containing phosphonate falls within the scope of Regulation 1107/2009 "plant protection products". A fertiliser containing phosphonate is considered not to comply with Regulation (EC) No 2003/2003 and with the Royal Decree of 28/01/2013 on fertilisers.

The use of fertilisers containing phosphonate may lead to residues exceeding the maximum residue limits.