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Are Phosphorous and Phosphoric Acids Equal Phosphorous Sources for Plant Growth?

Asha M. Brunings, Guodong Liu, Eric H. Simonne, Shouan Zhang, Yuncong Li, and Lawrence E. Datnoff

Phosphorus (P) is one of the 17 nutrient elements essential for plant growth and development (nickel is the 17th) (Association of American Plant Food Control Officials 2005; Bai, Reilly, and Wood 2006). Phosphorus is also a key component in some agrochemicals, such as phosphorous acid (H3PO3). Thus, there are two types of P closely associated with crop production. While growers are familiar with P-containing fertilizers, the abundance of similar terms (such as phosphoric acid, H3PO4, and phosphorous acid, H3PO3) may create some confusion about the actual content and efficacy of these products. Table 1 lists some P-containing compounds used for crop production. Some claims found in commercial literature and product descriptions refer to phosphorous acid as a "supplemental fertilizer," while others present it as a fungicide (Table 2). This article explains what phosphorous acid is and examines both its fungicidal activity and nutrient value.

The percentage of phosphorus in a fertilizer, as phosphorus pentoxide (P2O5), is represented by the middle number on the bag expressed (e.g., 5-10-15). The first number represents the nitrogen percentage, and the third number represents potassium percentage as K2O. The P2Ounit used to represent P content in fertilizer is a conventional unit (in reality, there is little or no P in the form of P2Oin fertilizer).

As a nutrient element essential for normal plant growth and development, phosphorus is utilized in the fully oxidized and hydrated form, orthophosphate (H3PO4). Plants absorb and utilize either hydrogen phosphate (HPO42-) and/or dihydrogen phosphate (H2PO4-), depending on the pH of the growing medium. At pH7.2, both H2PO4and HPO42- are approximately equal in amount. In fertilizers, P is normally not found in the form of phosphoric acid (H3PO4) unless the growth medium is very acidic. At pH levels below 2.1, H3POcan become the dominant form, but at pH levels from 5.5 to 6.5 more favorable for plant growth (near neutral pH), the amount of H3POis negligible. Compared with either H2PO4or HPO42-, it is only one out of 100,000 because it always dissociates to H2PO4and further to HPO42-. Both H2PO4and HPO42- ions are basic forms taken up by the plant, but H2PO4is taken up more readily (Street and Kidder 1989) because in most growth conditions, soil solution pH is below 7. Once inside the plant, both ions are mobile.

Phosphoric acid (H3PO4) should not be confused with phosphorous acid (H3PO3). A little difference in the name or formula of a chemical compound can make a dramatic difference in its properties. The former is a fully oxidized and hydrated form of P, whereas the latter is a partially oxidized and hydrated form. Therefore, phosphorous acid is a powerful reducing agent, but phosphoric acid is not. The former (H3PO4) is a diprotic acid (readily ionizes two protons), but the latter (H3PO3) is a triprotic acid (readily ionizes three protons). Phosphorous acid dissociates to form the phosphonate ion (HPO32-), also called phosphite. Phosphorous acid and its ionized compounds are often referred to as phosphonate or phosphonite. Like phosphate, phosphonate is easily taken up (Street and Kidder 1989) and translocated inside the plant.

Fosetyl-Al, which was registered by the EPA in 1983 (EPA 1991), is an aluminum salt of the diethyl ester of phosphorous acid and is sold under the trade name Aliette®. It is a systemic fungicide used to control damping-off and rot of plant roots, stems, and fruit, and it may be taken up by the plant. Inside the plant, fosetyl-Al may ionize into phosphonate, and therefore fosetyl-Al belongs to the group of phosphorous acid compounds (Cohen and Coffey 1986; McGrath 2004).

Phosphorous acid as phosphate fertilizer?

Phosphorous acid is not converted into phosphate, which is the primary nutrient source of P for plants (Ouimette and Coffey 1989b). There are bacteria capable of transforming phosphonate into phosphate, but this process is so slow that it is of no practical relevance (Huang, Su, and Xu 2005; McDonald, Grant, and Plaxton 2001). To date, no plant enzymes described oxidize phosphonate into phosphate. This is consistent with the fact that phosphonate is stable in plants and is not converted into phosphate (Smillie, Grant, and Guest 1989). Since phosphorous acid and its derivatives are not metabolized in plants, claims that phosphonate can contribute to P nutrient requirements for plant growth should be taken with much caution.

Phophorous acid is used in agriculture but for a different purpose than phosphoric acid. Confirming other investigations into the efficacy of phosphorous acid against oomycetes (a group of pathogens that include water molds and downy mildew), Förster et al. (1998) found that phosphite is capable of controlling Phytophthora root and crown rot on tomato and pepper. The authors also tested the ability of phosphorous acid to act as a nutrient source for plant growth and found that P-deficiency symptoms developed when plants were grown hydroponically with phosphorous acid as the sole source of P (without phosphate). This means that although phosphorous acid can control oomycetes in a number of host-parasite systems, it is not a substitute for phosphate fertilization. The inverse is also true: Phosphate is an excellent source of P for plant growth but is unable to control pathogen attack by oomycetes, other than by improving the general health of the crop and, therefore, its natural defense system. At this point, no evidence exists to substantiate the claim that phosphorous acid provides P for plant growth.

Control of Oomycetes

It is well documented that phosphorous acid can control diseases caused by pathogens that belong to the Oomycota phylum (or oomycetes) on agronomical and horticultural crops. Oomycetes (Figure 1) are not fungi but are frequently grouped with fungi because they form structures (filaments) like the ones that fungi make. Oomycetes are fungal-like organisms that differ from fungi in that their cell walls do not contain chitin but rather a mixture of cellulosic compounds and glycan (polymeric carbohydrate). Another difference is that the nuclei in the cells that form the filaments have two sets of genetic information (diploid) in oomycetes instead of just one set (haploid) as in fungi (Waggoner and Speer 1995).

Figure 1. Downy mildew on lettuce
Figure 1.  Downy mildew on lettuce.
Credit: Tyler Harp and Syngenta Crop Protection

For most practical purposes, oomycetes are grouped with fungi. Compounds that control plant pathogens belonging to the oomycetes are often called fungicides. It is important to distinguish between fungi and oomycetes. Chemicals that are used to control one often will not be effective against the other because of biological differences. Several important plant pathogens belong to oomycetes (Table 3), such as Phytophthora infestans, the causal agent of late blight of potato (Figure 2) and the culprit of the Irish Potato Famine between 1845 and 1849; P. ramorum, the causal agent of sudden oak death (Parke and Lucas 2008); and Pythium and Peronospora species, among others (Fry and Grünwald 2010).

Figure 2. Potato late blight caused by Phytophthora infestans.
Figure 2.  Potato late blight caused by Phytophthora infestans.
Credit: Tyler Harp and Syngenta Crop Protection

Phosphorous acid has both direct and indirect effects on oomycetes. It directly inhibits a particular process (oxidative phosphorylation) in the metabolism of oomycetes (McGrath 2004). An indirect effect is the stimulation of the plant's natural defense response against pathogen attack (Biagro Western Sales, Inc. 2003; Smillie, Grant, and Guest 1989). It should be noted, however, that phosphonate-resistant oomycetes have been reported (Ouimette and Coffey 1989a). In addition, some evidence suggests that phosphorous acid has an indirect effect by stimulating the plant's natural defense response against pathogen attack (Biagro Western Sales, Inc. 2003; Smillie, Grant, and Guest 1989).


A major factor in the ability of phosphorous acid to control oomycetes appears to be its chemical stability in the plant (Smillie, Grant, and Guest 1989). Phosphorous acid does not convert into phosphate and is not easily metabolized (Ouimette and Coffey 1989b). The stability of different phosphonate-related compounds may depend on environmental factors, such as climate or crop type. Because phosphonate is systemic and stable in plants, it should be applied infrequently to avoid accumulation problems. Plant species may differ in phosphonate uptake and translocation (Cooke and Little 2001), and individual P. infestans isolates show great variation in sensitivity (BashanLevy, and Cohen 1990; Coffey and Bower 1984) to phosphonate compounds, which may impact the effectiveness of phosphonate.

Table 4 summarizes some of the phosphorous acid-related compounds and research on their efficacy against potato late blight. In most cases, phosphorous acid is applied to the foliage. The compound is translocated from the shoots to the roots and can, therefore, also control oomycetes that affect roots. Phosphorous acid was shown to be effective when applied as a root drench against P. cinnamomi, P. nicotianae, and P. palmivora in lupin, tobacco, and papaya, respectively (Smillie, Grant, and Guest 1989). The efficacy of different phosphonate compounds against nine Phytophthora spp. that cause stem rot of Persea indica L. and pepper was tested both as a curative and preventive method of control by Ouimette and Coffey (1989a). Although there were notable differences in the sensitivity of the Phytophthora spp. in their experiments (Table 4), there was little variation in the ability of phosphonates to control stem rot of pepper, regardless of its use as a curative or a preventive agent in pots. A greater level of control was obtained for Persea indica L. than for pepper (Ouimette and Coffey 1989a).

Like other phosphonate-based systemic fungicides, fosetyl-Al is often used to treat plants infected by root pathogens because it is mobile in the plant and is transferred to the roots (Cohen and Coffey 1986). Cooke and Little (2001) found, however, that foliar application of fosetyl-Al did not reduce tuber blight on potato caused by P. infestans, while foliar sprays with phosphonate reduced the number of symptomatic tubers. This result implies that different host plants may take up, transport, and metabolize fosetyl-Al differently.

In general, potassium phosphonate negatively affected mycelial growth more than phosphonates that had alkyl groups, with some exceptions (Ouimette and Coffey 1989a). None of the compounds used by Ouimette and Coffey (1989a) were able to control infections by Phytophthora spp. completely when they were used as a curative or protective agent. All the compounds were equally effective when used as a protective agent (by root dip). Potassium phosphite controlled strawberry leather rot caused by P. cactorum (Rebollar-Alviter, Madden, and Ellis 2005). It also controlled downy mildew of basil in its early stages (Roberts et al. 2009). Phosphonate was shown to be effective when applied to potato foliage against P. infestans and P. erytrhoseptica (causal agent of pink rot) but not against Pythium ultimum (causal agent of Pythium leak) (Johnson, Inglis, and Miller 2004; Fenn and Coffey 1984). Phosphorous acid is also effective against downy mildew on grapes and against Phytophthora root and crown rot on tomato and green pepper in hydroponic culture (Förster et al. 1998). Studies have shown that phosphonate can control the sudden oak death pathogen in vitro and in vivo (Garbelotto, Harnik, and Schmidt 2009; Garbelotto and Schmidt 2009).

For control of oomycetes on turfgrass, Riverdale Magellan (a mixture of phosphorous acid compounds) and Chipco® Signature™ (Aluminum tris [O-ethyl phosphonate]) were found to be equally effective against Pythium blight development on perennial ryegrass (Lolium perenne) (Datnoff et al. 2003). Similarly, different commercial formulations of phosphorous acid suppressed Pythium blight on rough bluegrass (Poa trivialis) during the 2004 growing season (Datnoff et al. 2005).

The existence of Phytophthora spp. that are resistant to phosphonate has been reported (Brown et al. 2004; Dolan and Coffey 1988; Fenn and Coffey 1985, 1989; Griffith, Coffey, and Grant 1993; Nelson et al. 2004; Ouimette and Coffey 1989a). Hence, care should be taken to alternate phosphonates with other effective compounds to prevent a buildup of resistant Phytophtora spp. in the field.


Both phosphoric acid (H3PO4) and phosphorous acid (H3PO3) are agrochemicals essential for crop production. Under normal plant growth conditions, both dissociate and exist as corresponding anions, phosphate and phosphite. A clear distinction exists between the two agrochemical compounds: The former is a nutrient source of P essential for plants, and the latter helps control agricultural epidemics of oomycetes. Phosphate and phosphite are not equivalent inside the plant. Phosphoric acid or phosphate cannot function as phosphorous acid or phosphite and vice versa. Since phosphites are systemic and very stable in plants, they should not be applied frequently. To help delay the development of phosphite-resistant oomycetes, care should be taken to alternate or mix phosphite with other effective fungicides.


Association of American Plant Food Control Officials. 2005. "The Model for Fertilizer Regulation in North America." Accessed on February 5, 2024. 

Bai, C., C. C. Reilly, and B. W. Wood. 2006. "Nickel Deficiency Disrupts Metabolism of Ureides, Amino Acids, and Organic Acids of Young Pecan Foliage." Plant Physiology 140 (2): 433–443.

Bashan, B., Y. Levy, and Y. Cohen. 1990. "Variation in Sensitivity of Phytophthora infestans to Fosetyl-Al." Plant Pathology 39 (2): 134–140.

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Cooke, L. R. and G. Little. 2002. "The Effect of Foliar Application of Phosphonate Formulations on the Susceptibility of Potato Tubers to Late Blight." Pest Management Science 58 (1): 17–25. doi: 10.1002/ps.408 Accessed on February 5, 2024.

Datnoff, L., J. Cisar, B. Rutherford, K. Williams, and D. Park. 2003. "Effect of Riverdale Magellan and Chipco Signature on Pythium Blight Development on Lolium perenne, 2001–2002." F&N Tests 58 (T041): 1–2. Accessed on February 5, 2024.

———. 2005. "Effect of Fungicides and Other Prophylactic Treatments on Pythium Blight Development on Poa trivialis, 2004." F&N Tests 60 (T033): 1. Accessed on February 5, 2024.

Dolan, T. E. and M. D. Coffey. 1988. "Correlative In vitro and In vivo Behavior of Mutant Strains of Phytophthora palmivora Expressing Different Resistances to Phosphorous Acid and Fosetyl-Na." Phytopathology 78 (7): 974–978.

Environmental Protection Agency (EPA). 1991. "R.E.D. Facts: Fosetyl-Al (Aliette)." Accessed October 19, August 24, 2021. Accessed on February 5, 2024.

Fenn, M. E. and M. D. Coffey. 1984. "Studies on the In vitro and In vivo Antifungal Activity of Fosetyl-Al and Phosphorous Acid." Phytopathology 74 (5): 606–611.

———. 1985. "Further Evidence for the Direct Mode of Action of Fosetyl-Al and Phosphorous Acid." Phytopathology 75 (9): 1064–1068.

———. 1989. "Quantification of Phosphonate and Ethyl Phosphonate in Tobacco and Tomato Tissues and Significance for the Mode of Action of Two Phosphonate Fungicides." Phytopathology 79 (1): 76–82.

Förster, H., J. E. Adaskaveg, D. H. Kim, and M. E. Stanghellini. 1998. "Effect of Phosphite on Tomato and Pepper Plants and on Susceptibility of Pepper to Phytophthora Root and Crown Rot in Hydroponic Culture." Plant Disease 82 (10): 1165–1170.

Fry, W. E. and N. J. Grünwald. 2010. "Introduction to Oomycetes." The Plant Health Instructor. doi:10.1094/PHI-I-2010-1207-01. August 24, 2021. Accessed on February 5, 2024.

Garbelotto, M., T. Y. Harnik, and D. J. Schmidt. 2009. "Efficacy of Phosphonic Acid, Metalaxyl-M and Copper Hydroxide Against Phytophthora ramorum In vitro and In planta." Plant Pathology 58 (1): 111–119.

Garbelotto, M. and D. Schmidt. 2009. "Phosphonate Controls Sudden Oak Death Pathogen for up to 2 Years." California Agriculture 63 (1): 10–17.

Griffith, J. M., M. D. Coffey, and B. R. Grant. 1993. "Phosphonate Inhibition as a Function of Phosphate Concentration in Isolates of Phytophthora palmivora." Journal of General Microbiology 139 (9): 2109–2116.

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Helena Chemical Company. 2002. Helena ProPhyt: A Systemic Fungicide Containing Potassium and Phosphate (promotional brochure). Memphis, TN: Author.

Huang, J., Z. Su, and Y. Xu. 2005. "The Evolution of Microbial Phosphonate Degradative Pathways." Journal of Molecular Evolution 61 (5): 682–690.

Johnson, D. A., D. A. Inglis, and J. S. Miller. 2004. "Control of Potato Tuber Rots Caused by Oomycetes with Foliar Applications of Phosphorous Acid." Plant Disease 88 (10): 1153–1159.

McDonald, A. E., B. R. Grant, and W. C. Plaxton. 2001. "Phosphite (Phosphorous Acid): Its Relevance in the Environment and Agriculture and Influence on Plant Phosphate Starvation Response." Journal of Plant Nutrition 24 (10): 1505–1519.

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———. 1989b. "Phosphonate Levels in Avocado (Persea americana) Seedlings and Soil Following Treatment with Fosethyl-Al or Potassium Phosphonate." Plant Disease 73 (3): 212–215.

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Table 1. 

Agriculturally relevant P-containing compounds.



What is it?



The chemical element indicated with the symbol P is a structural component of many things in plants and other organisms, including biological membranes, DNA, RNA, and ATP, and it is essential for numerous biochemical processes in all organisms. It does not occur as a free element in nature.

Phosphoric acid


Also known as orthophosphoric acid or phosphoric (V) acid, it is a mineral (inorganic) acid. This chemical compound normally does not exist in P fertilizers unless the fertilizer is put in a strong acidic solution. The P form in the fertilizer includes either phosphate salts or esters. Potassium or diammonium phosphate exemplifies the former, whereas phytate is an example of the latter. For acidic soils, phosphate rock can be directly used as a P source.

Dihydrogen phosphate


It is a partially dissociated form of H3PO4 in which P is most readily taken up by the plant. It is the major form of phosphate when pH is greater than 2.1 but lower than 7.2

Hydrogen phosphate


It is a partially dissociated form of H3PO4, in which P can also be taken up by the plant. This form dominates when pH is greater than 7.2 but lower than 12.7. At pH 7, both dihydrogen phosphate and hydrogen phosphate are approximately equal in amount.



It is a completely dissociated form of H3PO4. Under growth conditions, it is present in negligible amounts, less than 1:100,000 that of either dihydrogen phosphate or hydrogen phosphate.

Phosphorus pentoxide


It is a formula used to express the P content of fertilizers. It is a white and anhydride form of phosphoric acid. It is a powerful desiccant and dehydrating agent.

Phosphorous acid


It is a powerful reducing agent used for preparing phosphite salts, such as potassium phosphite. These salts, as well as aqueous solutions of pure phosphorous acid, control a variety of microbial plant diseases caused by oomycota.

Dihydrogen phosphonate


It is a partially dissociated form of H3PO3, the major form of phosphonate at pH >1.3.

Hydrogen phosphonate


A completely dissociated form of H3PO3, it dominates at pH >6.7. The hydrogen in the anion has a covalent bond with phosphorus that cannot be readily dissociated.

Table 2. 

Marketing of products with active ingredient phosphorous acid or related compounds.z



Active ingredient

Marketed as


K-Phite® 7LP

Plant Food Systems Inc.

Mono- and di-potassium salts of phosphorous acid


Raid, McAvoy, and Sui 2010

Terronate WDG

Agriliance LLC



Pesticide Action Network 2004


Bayer Cropscience LP



Bayer Cropscience 2004


Biagro Western Sales

Phosphite and organic acids


Biagro Western Sales, Inc. 2003

CP Home and Garden Fungicide

Contract Packaging, Inc.



Pesticide Action Network 2004

Tree Tech® brand Aliette Injectable

Florida Silvics Inc.



Pesticide Action Network 2004


Whippet (Phosphorus Acid) Fungicide, Wedgle Direct-Inject

Phosphorous acid

Fungicide Accessed on February 5, 2024.


Soil Phosphate

Foliar Phosphate

Helena Chemical

Phosphorus acidy

Foliar fertilizer

Helena 2002


Helena Chemical

Potassium phosphite

Systemic fungicide

Helena 2002; Nufarm USA, n.d.


Nufarm America

Phosphorus acid

Biochemical pesticide

Pesticide Action Network 2004


Loveland Products, Inc.

Potassium phosphite

Systemic fungicide Accessed on February 5, 2024.

Riverdale Magellan

Nufarm America

Phosphorous acid


Pesticide Action Network 2004

Plant Synergists Phosphorous Acid Technical

Plant Synergists, Inc.

Phosphorous acid


Pesticide Action Network 2004

z Products and companies are mentioned for educational purposes and are not recommended over similar products in this document.

y It is unclear whether "phosphorus acid" means phosphoric or phosphorous acid. The word "phosphite" in the name implies that phosphorous acid is the active ingredient. However, the fact that the product is marketed as a fertilizer implies that the active ingredient is phosphate.

Table 3. 

Genera of oomycetes that cause disease on horticultural crops and that are likely to be controlled by phosphorous acid (Heffer, Powelson, and Johnson 2002).




Root rot

Bremia, Peronospora, Plasmopara, Pseudoperonospora, Sclerospora

Downy mildew (Figure 2)


Root rot and damping-off


Late blight of potato and tomato, foliar blights on peppers and cucurbits, root and stem rots


White rust on cruciferous plants

Table 4. 

Control of potato late blight by phosphorous acid and related products.






Not good in field

Foliar spray

Cooke and Little 2001


Good in field, variable against oomycetes in the lab

Foliar spray

Cooke and Little 2001

Phosphonate compounds

Good in pots

Root dip

Ouimette and Coffey 1989a


Variable against P. infestans isolates in the lab

Foliar spray to detached leaves

Bashan, Levy, and Cohen 1990

Phosphorous acid

Good against P. infestans in the field

Foliar spray

Johnson, Inglis, and Miller 2004

Potassium phosphite

Effective against Peronospora belbahrii (basil downy mildew) early in the trial

Foliar spray

Raid 2008

Publication #HS1010

Release Date:March 7, 2024

Related Experts

Datnoff, Lawrence E

University of Florida

Simonne, Eric H.


University of Florida

Zhang, Shouan


University of Florida

Liu, Guodong


University of Florida

Li, Yuncong


University of Florida

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition.

All chemicals should be used in accordance with directions on the manufacturer's label.

Use pesticides safely. Read and follow directions on the manufacturer's label.

Fact Sheet

About this Publication

This document is HS1010, one of a series of the Horticultural Sciences Department, UF/IFAS Extension. Original publication date April 2005. Revised March 2012, January 2015, May 2018, September 2021, and February 2024. Visit the EDIS website at for the currently supported version of this publication.

About the Authors

Asha M. Brunings, associate professor at Santa Fe College and former postdoctoral research associate; Guodong Liu, associate professor; and Eric H. Simonne, professor and Director for Northeast District Cooperative Extension Service, Horticultural Sciences Department; Shouan Zhang, professor, Plant Pathology Department; Yuncong Li, professor, Department of Soil, Water, and Ecosystem Sciences, UF/IFAS Tropical Research and Education Center; and Lawrence E. Datnoff, former professor, Plant Pathology Department; UF/IFAS Extension, Gainesville, FL 32611.


  • Guodong Liu
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