Phosphite in Soil and Turfgrass
Type of DegreeMaster's Thesis
Agronomy and Soils
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Phosphate, derived from phosphoric acid (H3PO4), has long been used as a P fertilizer source, while phosphite, derived from phosphorous acid (H3PO3), has long been used as a known and proven fungicide. However, the exact role of 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. Two greenhouse studies (repeated in time) and one incubation study were used to evaluate the behavior of phosphite in the soil and its effects on ryegrass and bentgrass growth. Phosphite-containing materials (both labeled fungicides and phosphite-containing fertilizers) were applied based on P rate or at labeled rates. Collected data included root and shoot growth, and P uptake. In early sampling periods (1 month) application of P as phosphite did negatively affect root and shoot growth. However, by two months after fertilization phosphite had likely converted to phosphate, and plant growth was either unaffected or improved. Fungicide-only treatments indicated that this was a function of P and not fungicidal activity of phosphite. Results from the incubation study support this conclusion as phosphite was converted to phosphate largely within the second sampling month. Further work should investigate if similar results are observed in the field, and at what point in time effects of phosphite are mitigated by conversion to phosphate.