The effectiveness of phosphite applications in suppressing certain diseases and promoting turf health is well documented, but little is known about its role in plant nutrition or where applied phosphorus ends up in the plant and soil. Photo by John J. Dempsey
Editor’s note: This article is reprinted from the July 21, 2023, issue of the USGA Green Section Record. Copyright USGA. All rights reserved. The original article can be accessed at https://bit.ly/3TG9jrs.
Phosphate (PO43-) is the sole phosphorus-containing compound utilized for plant growth, leading to the widespread use of phosphate-containing fertilizers. An alternative form of phosphorus (P), phosphite (PO33-), has increasingly been used in many crop
and turfgrass systems. In particular, the use of phosphite-containing products has grown significantly on golf courses, primarily as a pesticide but increasingly as a nutrient source and biostimulant. The plant-health properties of phosphite and how
they aid in suppressing disease have been well documented, but its role as a nutrient source and as a biostimulant is less researched.
The results of previous research investigating the effect of phosphite on turf quality have demonstrated that phosphite products consistently provide a significant increase in turf quality. Enhanced turfgrass quality was reported on creeping bentgrass
(Agrostis stolonifera L.), velvet bentgrass (Agrostis canina L.) and annual bluegrass (Poa annua L.) swards treated sequentially with phosphite over two years. Phosphite treatments also produced significantly better turfgrass quality when compared
to plots treated with identical nutritional inputs minus the phosphite component (1, 2).
These results, however, were secondary to the main focus of all the studies from which they were cited, which was assessment of the disease-suppressive properties of phosphite. Therefore, the question arises: Was this enhanced quality due to the disease-suppressive
properties of phosphite or due to phosphite acting as a nutrient source or biostimulant?
To answer this question, research was conducted at the University of the West of England to determine and describe the uptake, vascular translocation and accumulation of phosphite in treated turfgrass tissues. Scientists also assessed the value of phosphite
as a source of P nutrition, specifically to determine the effect phosphite had on turfgrass growth, P deficiency responses, conversion of phosphite to phosphate and soil P accumulations.
To assess how foliar-applied phosphite was taken up, translocated and accumulated in turfgrass tissues, researchers initiated a two-year study to evaluate the fate of phosphite within cool-season turfgrass plants.
Figure 1. Take-up and accumulation of phosphite in creeping bentgrass (top) and annual bluegrass (bottom) leaf and root tissues, from 0 to 6 weeks post application in February 2011. Bars indicate 95% confidence intervals, n=6. Results from 2012 (not shown) were similar for uptake and translocation; however, phosphite tissue levels decreased more rapidly during periods of vigorous turf growth.
Research study: Uptake, translocation and accumulation of foliar-applied phosphite
Materials and methods
Phosphite was derived using lab-grade phosphorous acid and reagent-grade potassium hydroxide. The phosphite solution was then applied to creeping bentgrass and annual bluegrass in February 2011 and July 2012 at a rate of 1.15 ounces per 1,000 square feet
(3.5 kilograms per hectare) of actual PO33-. Analysis of tissue content for phosphite and phosphate was carried out over a six-week period using High Performance Ion Chromatography (HPIC).
Results
The results showed that foliar-applied phosphite was rapidly taken up into the leaf tissues and was detectable in the roots within hours, confirming phloem mobility and suggesting phosphite has the rare ability both to move up a plant’s xylem and
down its phloem, giving it the quality referred to as symplastic ambimobility. Results from February 2011 (Figure 1) show that in creeping bentgrass and annual bluegrass, maximum leaf accumulations were attained within one week after treatment (WAT)
with 3,332 parts per million in creeping bentgrass and 4,395 ppm in annual bluegrass. Leaf accumulations of applied phosphite progressively declined and by six weeks had reduced to 14.9% and 19.6% of the maximum levels in creeping bentgrass and annual
bluegrass respectively.
Results from the July 2012 study (data not shown) were similar to February 2011 with regard to rapid uptake and translocation rates, with maximum leaf accumulations at 1 WAT. However, there was a more rapid decline in leaf amounts during this period of
higher turfgrass growth rates. In creeping bentgrass, phosphite amounts in leaf tissues at 6 WAT in 2012 had decreased to 7.6% of the maximum accumulation, compared to 14.9% during the 2011 study. Similarly, in annual bluegrass, accumulations in leaf
tissues had dropped to 6.3% of the maximum accumulation in 2012, compared to 19.6% in 2011.
Demonstration of symplastic ambimobility — i.e., that the foliar-applied phosphite translocated and was detected in the roots of treated turfgrasses — was an important outcome of this study. Although the maximum root accumulations, 479 ppm
in creeping bentgrass (February 2011) and 457 ppm in annual bluegrass (July 2012), were much less than in the leaf tissues, it remains a significant result, as no other compound used for pathogen suppression in turfgrasses demonstrates symplastic
ambimobility, and this was the first time that this mobility of phosphite had been reported in turfgrass.
Implications for turf managers
These results are of particular significance to many golf course superintendents who utilize phosphite on a two- to three-week interval as part of their maintenance programs. Many apply phosphite prior to and during periods of high disease pressure, as
phosphite treatments have been shown to suppress disease incidence and stimulate defense responses (2, 3, 4). The results show that sequential foliar applications need to be made at regular intervals to maintain phosphite in the leaf at a level necessary
for effective disease suppression. Favorable growing conditions, like those observed in the 2012 study, can cause a significant decline in the amount of phosphite in leaf tissue. Additionally, grass species impacts persistence of phosphite within
the plant. Both these factors should be taken into consideration when planning spray intervals.
Table 1. Root zone phosphorous (P) content for two cool-season grasses in parts per million, prior to the start of sequential phosphite and phosphate treatments in 2012 and at the conclusion of treatments in 2014.
Research study: Conversion of phosphite to phosphate in turfgrass tissues
Turf managers and golf course superintendents have often expressed interest in understanding the fate of P from phosphite applications and whether any conversion to plant-available phosphate takes place. Determining phosphate levels following phosphite
treatment formed part of this study since this question about conversion of phosphite to phosphate in the plant is often raised, with numerous commercial suppliers also claiming phosphite as a P source following conversion of phosphite to plant-available
phosphate.
Materials and methods
The same method outlined in the research study section above was used to assess how much, if any, foliar-applied phosphite was converted to phosphate in two cool-season turf species, creeping bentgrass and annual bluegrass.
Results
The results were conclusive. In creeping bentgrass and annual bluegrass, during both assessment periods, there was no significant difference in the level of leaf phosphate following treatment by phosphite (data not shown). Foliar applications of phosphite
made to cool-season turf as part of a plant-health-and-protection program will not result in any phosphorus fertilization by way of conversion to phosphate.
Implications for turf managers
The study concluded that foliar-applied phosphite does not convert into phosphate within creeping bentgrass or annual bluegrass plants. Therefore, if testing or visual cues indicate a P deficiency, superintendents should rely on fertilizers containing
phosphate, not phosphite, to meet the plant’s nutrition needs. If turfgrass quality improves following foliar applications of phosphite, it is likely due to overall plant-health factors or disease suppression.
Research study: Root-zone nutrient analyses following sequential phosphite treatments
Another question often posed by turfgrass managers regarding the fate of foliar-applied phosphite is if any amount of P, in any form, ends up in the soil. Knowing the impact foliar-applied phosphite has on root-zone P levels would aid superintendents
and other turf managers in making fertilization decisions.
Materials and methods
To evaluate the effect of long-term, sequential phosphite treatments on turfgrass root-zone P levels, phosphite and phosphate were applied at 1.15 ounces per 1,000 square feet of actual PO33- and PO43- at monthly intervals to greenhouse samples of creeping
bentgrass and annual bluegrass from July 2012 to July 2014.
Results
Root-zone nutrient levels prior to the start of the two-year treatment program and at the conclusion of the study are shown in Table 1. Sequential foliar applications of either phosphite or phosphate significantly increased soil P levels in the root zones
of both turfgrass species compared to pre-treatment levels.
Over the two-year period, P levels in phosphite-treated root zones increased significantly more than phosphate-treated root zones. In creeping bentgrass root zones, P levels following phosphite treatments increased from 37 ppm to 51 ppm but only increased
to 40 ppm following phosphate treatments. Similarly, in the annual bluegrass root zones, P levels increased from 37 ppm to 57 ppm following phosphite treatments but only increased to 44 ppm after phosphate treatments.
Implications for turf managers
Soil type, organic matter, microbial activity and other factors likely play a role in the amount of P accumulation in the root zone. The significantly higher level of root-zone P accumulation following phosphite treatment could be due to it being locked
into the rhizosphere by soil microorganisms, since oxidation of phosphite to phosphate in soil relies on microbial activity, which is a slow process. More research is needed on the mechanism by which this occurs. The movement of P out of a turfgrass
system is not desirable due to its impact on water bodies. Phosphorus in the root zone following phosphate treatment would be less persistent and more easily leached, keeping in mind the cation exchange capacity status of the studied root zones was
extremely low. Whatever the reason, the steady increase of soil P levels following sequential phosphite treatments may pose a problem for turfgrass management. Higher levels of soil P are often correlated with increased populations of annual bluegrass
— a species that is dominant in many golf greens but is still widely regarded as an undesirable weed. Turf managers should monitor P levels in their regular soil tests if they are making foliar phosphite applications to be sure that an undesirable
accumulation does not develop.
Figure 2. Treatment effect on the growth of leaf, crown and root tissues of annual bluegrass growing in a P-sufficient root zone (top) and P-deficient root zone (bottom), following sequential treatments over a six-month period, of phosphate, phosphite and potassium chloride (control). Bars are 95% confidence intervals, n=6. Letters indicate significant differences within tissue type as determined by Tukey HSD post hoc analyses at p = 0.05.
Research study: Effects of phosphite on turfgrass growth
The major aim of this final aspect of the research study was to assess the properties of phosphite as a source of P nutrition for turfgrass and its effects on growth and root-zone P-deficiency responses. As stated earlier, the disease-suppression and
plant-health properties of foliar-applied phosphite are well researched, but little is known regarding its effect on turfgrass growth and development.
Materials and methods
Foliar treatments of phosphite and phosphate at 1.15 ounces per 1,000 square feet as PO33- and PO43- and potassium chloride (KCl) as a control were applied to perennial ryegrass (Lolium perenne L.) and annual bluegrass biweekly over a six-month period.
Two soil P levels were used in the study: P-sufficient (P greater than 35 ppm); and P-deficient (P less than 6 ppm). Treatment effect on shoot growth was determined by the cumulative dry leaf weights. Crowns and roots were collected at the end of
the trial and weighed for dry mass determination and calculation of root-to-shoot ratios.
Results
The results shown in Figure 2 illustrate that for turf plants growing in a phosphorus-sufficient root zone (P greater than 35 ppm), the phosphite treatment significantly increased dry weights in leaf, crown and root tissues, compared with phosphate- and
KCl-treated plants (perennial ryegrass data not shown). Phosphite treatments can inhibit pathogenic soil microorganisms that have a deleterious effect on turfgrass development. Phosphite has proven efficacy in reducing these organisms, allowing for
the healthier development of turfgrass, and may account for the increase in dry weights. The enhanced growth may also be caused by a growth-regulatory or phytohormonal factor, affecting sugar metabolism, stimulation of the shikimic acid pathway, or
internal hormonal and chemical changes.
Conversely, the results determined that phosphite treatment to both perennial ryegrass and annual bluegrass growing in phosphorus-deficient rootzones (P less than 6 ppm) significantly reduced dry weights in leaf, crown and root tissues compared with phosphate-
and KCl-treated plants (Figure 2, perennial ryegrass data not shown). This is likely due to phosphite suppressing many of the plant’s responses to limited P, such as enhanced root growth and increased root-to-shoot ratios. Phosphite also competes
with phosphate for uptake via the same plant transport system, which leads to an even further reduction in usable P in the soil.
Implications for turf managers
If foliar-applied phosphite is a regular part of superintendents’ agronomic programs, it’s important to maintain plant-available P amounts in the root zone at sufficient levels. Foliar phosphite applications do not increase the amount of usable
P in the soil and can enhance the negative effects of a P-deficient root zone on turf plants, so applications of phosphate fertilizer are still required when soil tests or visual cues indicate a P deficiency.
Conclusion
Foliar phosphite applications are a valuable tool for golf course superintendents to manage disease throughout the year and promote plant health during periods of stress. This research showed phosphite is rapidly taken up and translocated by turfgrass,
and that sequential applications at label rates can maintain beneficial levels within the plant. However, foliar-applied phosphite does not supply any P fertilization, since there is no conversion to phosphate in cool-season turfgrass plants. Long-term,
sequential phosphite treatments can also lead to cumulative increases in soil P levels. The mechanism by which this occurs and how much of this soil P becomes plant-available requires further research, but turf managers regularly applying phosphite
should monitor soil P, since increased levels favor annual bluegrass and can have other undesirable consequences. Finally, use caution when applying phosphite if P is deficient in the root zone, as foliar-applied phosphite decreases the growth and
quality of cool-season turfgrass in this situation. Turf managers should keep the results of this research in mind to maximize the benefits of foliar-applied phosphite on turf health and limit any negative impact on turf quality.
The research says
Researchers determined that phosphite was rapidly taken up, translocated and accumulated in leaf and root tissues. However, foliar-applied phosphite does not supply any phosphorus (P) fertilization since there is no conversion to plant-available phosphate
in cool-season turfgrass plants. • Results showed that sequential foliar applications need to be made at regular intervals to maintain phosphite in the leaf at a level necessary for effective disease suppression. • Sequential applications
of phosphite over two years led to significant increases in root-zone P levels. This could have undesirable implications depending on the specific situation. • In P-sufficient root zones, foliar-applied phosphite increased the growth and quality
of treated turfgrass. In P-deficient root zones, foliar-applied phosphite had a detrimental effect on growth and quality.
More information
This article is based on research published in the Journal of Plant Nutrition. You can find additional information, including all tables and figures, at https://bit.ly/47gYmQc.
Literature cited
- Cook, J., P.J. Landschoot and M.J. Schlossberg. 2006. Phosphonate products for disease control and putting green quality. Golf Course Management 74(4):93-96 (https://www.gcsaa.org/docs/default-source/research-and-information/disease/phosphonate-products-for-disease-control-and-putting-green-quality.pdf?sfvrsn=2).
- Dempsey, J.J., I. Wilson, P.T.N. Spencer-Phillips and D.L. Arnold. 2012. Suppression of Microdochium nivale by potassium phosphite in cool-season turfgrasses. Acta Agriculturae Scandinavica, Section B – Soil & Plant Science 62 (Suppl. 1):70-78
(https://doi.org/10.1080/09064710.2012.681392).
- Dempsey, J.J., I. Wilson, P.T.N. Spencer-Phillips and D.L. Arnold. 2022. Phosphite-mediated enhancement of defence responses in Agrostis stolonifera and Poa annua infected by Microdochium nivale. Plant Pathology 71(7):1486-1495 (https://doi.org/10.1111/ppa.13584).
- Mattox, C.M., A.R. Kowalewski, B.W. McDonald, J.G. Lambrinos and J.W. Pscheidt. 2020. Combinations of rolling, mineral oil, sulfur, and phosphorous acid affect Microdochium patch severity. Agronomy Journal 112(5):3383-3395 (https://doi.org/10.1002/agj2.20191).
John J. Dempsey is an independent turfgrass researcher in Kildare, Ireland; Ian Wilson is a senior lecturer in the Department of Biomedical, Biological and Analytical Sciences at the University of the West of England; Peter T.N. Spencer-Phillips is an
associate professor of applied microbiology at the University of the West of England; and Dawn L. Arnold is a professor and associate pro-vice chancellor (research) at Harper Adams University, UK.