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An anion, (HPO3)2-, which is more typically called phosphanate. It can be used as a mild fungicide as well as acting as a biostimulant source, although it is not involved in the growth of a plant, unlike the application of the fertiliser nutrient phosphate. Phosphite is typically sourced from potassium phosphite (either mono-potassium (KH2PO3) or di-potassium (K2HPO3) salts of phosphoric acid).

Phosphite’s main use is by acting as a plant protectant to reduce the likelihood of disease attack and also slows the speed of infection, especially Anthracnose, Dollar Spot and Fusarium Patch, as well as Pythium diseases, and also reduce the extent of algae in fine turf. Phosphite needs to be applied to the grass sward on a regular basis to help protect the leaf, with the plant vascular tissues translocating it throughout the plant to provide for a more resilient plant.

Where phosphate levels are very low within a soil the application of phosphite may actually reduce growth, so it is important to ensure adequate soil nutrient levels are maintained as part of overall management practices. Where phosphate is adequate then stimulated root growth can occur from the application of low rates of phosphite, and this can also increase the efficiency of nutrient uptake from the soil.

References:
• John C. Inguagiato, Ph.D.; John E. Kaminski, Ph.D.; and Timothy T. Lulis
(March 2018) ‘Effect of phosphite rate, source on cyanobacteria greens colonization’, GCM, https://gcmonline.com/course/environment/news/cyanobacteria-putting-greens, accessed 10th March 2024.
• Dr Ranjan Swarup, Dr Umar Mohammed, Dr Jayne Davis, Dr Steve Rossall, (April 2020) ‘Role of phosphite in plant growth and development’, School of Biosciences, University of Nottingham, https://www.nottingham.ac.uk/biosciences/documents/research/2020-research-documents/swarup-uon-phosphite-white-paper-april-2020.pdf, accessed 11th March 2024.
• Peter Landschoot, ‘Understanding the Phosphanate Products’, PennState Extension, Jan 1, 2023, https://extension.psu.edu/understanding-the-phosphonate-products, accessed 10th March 2024.
• Dr John Dempsey, ‘Phosphite in turfgrass’, Grounds management, Winter 2023.