Phosphonates as fertilizers

Phosphonates were first investigated as fertilizers in Germany and the U.S. during the 1930s and 40s. At that time, agricultural officials were concerned that war activities would disrupt vital shipments of rock phosphate for fertilizer production, so alternative sources of fertilizer phosphorus were explored (6). Results of studies conducted in both countries demonstrated that phosponates were not effective substitutes for phosphate fertilizer. Scientists found that yields of legumes and grasses treated with calcium phosphite were lower than phosphate-treated plants, and in most cases, lower than controls plants receiving no phosphorus. However, a second crop seeded into the same soils that were treated with calcium phosphite showed improved yields. The authors attributed the delayed phosphorus response to the conversion of phosphite to phosphate in the soil (9). Subsequent research revealed that phosphite could be converted to phosphate primarily by soil-borne bacteria, but that these bacteria would not use phosphite until most phosphate was depleted (1). Based on the results of these studies, phosphonate fertilizer was viewed as an inefficient and costly means of supplying phosphorus to plants and scientists eventually lost interest in this compound as a phosphorus fertilizer.
Despite previous research findings, phosphonate compounds are marketed by some companies as a source of phosphorus and potassium fertilizer. Preliminary results with turfgrasses growing in sand culture and treated with equal amounts of potassium phosphite and potassium phosphate have supported claims that potassium phosphite does not supply usable phosphorus to turfgrasses ( see fig below . Although potassium phosphite can be converted to phosphate in soil, turf managers should realize that this is an inefficient means of supplying phosphorus to plants when compared with phosphate fertilizer.

Figure --. Annual bluegrass treated with a nutrient
solution containing potassium phosphate as the source
of phosphorus (left); and the same nutrient solution
with potassium phosphite as the source of phosphorus
(right). Annual bluegrass treated with potassium
phosphite shows phosphorus deficiency symptoms
(stunted growth and a red tint to foliage) indicating
that this compound is not supplying usable
phosphorus to the plants.

Claims that phosphonates consistently enhance rooting are debatable and more evidence is needed to support these claims. A two-year study performed at North Carolina State University showed that bentgrass root mass was unaffected by phosphonate products (4). Certainly, more research using precise root measurement techniques is needed to determine if enhanced rooting due to phosphonates occurs under different environmental and management conditions. If enhanced rooting does occur, it could be due to product formulation, or from the suppression of minor root pathogens (Pythium spp.) due to fungitoxic action of the phosphonate product, leading to healthier and more extensive roots.

Literature cited

1. Adams, F. and J.P. Conrad. 1953. Transition of phosphite to phosphate in soils. Soil Science 75:361-371.
2. Anonymous. 2005. Greenbook turf and ornamental reference for plant protection products. Vance Communication Corp., New York, NY.
3. Brown, S., S.T. Koike, O.E. Ochoa, F. Laemmlen, R.W. Michelmore. 2004. Insensitivity to the fungicide fosetyl-aluminum in California isolates of the lettuce downy mildew pathogen,Bremia lactucae. Plant Disease 88:502-508.
4. Dorer, S.P. 1996. Nutritional effects of a fungicide combination on summer bentgrass decline. Master of Science Thesis, North Carolina State University, Raleigh, NC.
5. Griffith, J.M., A.J. Davis, and B.R. Grant. 1992. Target sites of fungicides to control oomycetes. pp. 69-100. In: Target sites of fungicide action. W. Koller (ed.), CRC Press, Inc., Boca Raton, FL.