Inorganic phosphites (containing ) have been applied to crops to combat fungus-like pathogens of the order oomycetes (water molds).  The situation is confusing because of the similarity in name between phosphite and phosphate (a major plant nutrient and fertilizer ingredient), and controversial because phosphites have sometimes been advertised as fertilizers, even though they are converted to phosphate too slowly to serve as a plant's main phosphorus source. In fact, phosphites may cause phytotoxicity when a plant is starved of phosphates. Lemoynie and others have described this complicated situation and noted that calling phosphites fertilizers avoided the regulatory complication and negative public perceptions that might have been incurred by registering them as fungicides.
A major form of inorganic phosphite used in agriculture is monopotassium phosphite. This compound does serve as a potassium fertilizer.