How do I use phosphite products in my garden?

Phosphite products really started to gain popularity within the turf industry in the 1990s as an effective and relatively less toxic alternative to standard chemical fungicides. Phosphites are effective at stopping the development of and controlling the spread of the Pythium fungus, commonly referred to as root rot. However, phosphite products do not contain any phosphorus (P) in the phosphate form, which is the form that is readily available for uptake by plants’ roots and is involved in, but not limited to, the formation of DNA as well as the facilitation and storage of chemical energy produced through photosynthesis.

There are a number of phosphite fertilizers available that claim to increase flower and fruit production. These products are usually labeled with an N-P-K ratio of either 0-30*-20 or 0-30*-30 with the asterisk denoting non-plant-available phosphite. Phosphite products are made by mixing phosphorus acid and potassium hydroxide. The resulting solution, called potassium phosphite, renders the phosphorus unavailable in the phosphite form with the potassium existing as the soluble potash (K20) ion, a plant-available form. Any increase in flower production from the application of a fertilizer containing phosphite will undoubtedly have occurred due to the elevated level of plant-available potassium. Although current research has shown that in certain conditions, phosphites can indeed be converted, over time, to the phosphate form by specialized soil bacteria, it is not considered a reliable way of providing phosphorus to developing plants and is best used as a foliar or soil application to suppress the growth of fungal pathogens.