Phosphite in Soil and Ryegrass.

Phosphate (H₃PO₄) has long been used as a P fertilizer source, while phosphite (phosphorous acid) (H₃PO₃) has long been used as a known and proven fungicide.  The two P sources are sometimes found (combined or separately) in commercial fertilizer products.  However, the exact role of the phosphite is unclear - is it functioning as a fertilizer, fungicide, or both?  Previous work in crop production has shown that phosphite can be detrimental to plant growth, especially if the soil is low in phosphate.  Other work shows that there is a conversion of phosphite to phosphite in the soil, with estimates of around 60 days for that conversion.  However, there is little work that examines turfgrass responses (not related to a specific disease) to phosphite application, and how that response may vary with extractable soil P.  Thus, the objective of this work was to examine growth of perennial ryegrass (Lolium perenne L.) as affected by:  1) phosphite source, 2) initial soil P, and 3) soil type.  The study consisted of a series of greenhouse experiments in which soil (typically a Marvyn loamy sand) was fertilized with rates of P (as triple superphosphate or the various phosphite sources) from 0 to 125 mg P₂O₅ kg^-1.  This P was mixed thoroughly with the soil, and each pot seeded with ryegrass.  Phosphite sources were two commercial liquid fertilizers with potassium phosphite as the P source, or reagent grade potassium phosphite.  Over a 12 week period collected data included weight of harvested tissue (at weeks 4, 8 and 12), P content of that tissue, and extractable soil P (both CaCl₂ and Mehlich).  In general the presence of phosphite did not affect plant growth, and dry matter yield was unaffected at the P rates used in this study.  Additional work with separate phosphite and phosphate soil analyses (via ion chromatography) confirm a roughly 60 day conversion period for phosphite to phosphate.