First, plants need certain essential nutrients—carbon, hydrogen, oxygen, nitrogen, phosphorus and potassium—in large quantities. Nitrogen, phosphorus and potassium are sold in the market as fertilizers; they make up the N-P-K numbers [grade] on fertilizer bags.

In terms of phosphorus (P), not all forms of phosphorus are available in the soil for plants to use, nor are all of the forms useful to the plant once inside. The main use of phosphorus in plants is for the formation of adenosine triphosphate (ATP). ATP is the energy-storing molecule (or gasoline, if you will) of the plant.

Phosphorus is also used heavily in root growth and other biochemical processes. Phosphorus can come in many forms—each with a different function in the plant—and yet still be sold as fertilizer. It is important to know the differences between these products and their functions so you know what kind of results to expect from them.

In fertilizers, phosphorus is usually in the phosphate form (PO4) and expressed as a percentage of equivalent P2O5.

Phosphate fertilizers are made using phosphoric acid. It makes more sense to list the amount of phosphorus on fertilizer labels (as it is done in Europe), but the P2O5 equivalence requirement stems from an archaic law that is still on the books in the United States. Some common phosphate fertilizers are super phosphate and ammonium phosphate.

Phosphate fertilizers are used in large amounts in horticultural, agricultural and turf applications and they are often blamed for algae blooms in rivers and lakes (water carries phosphates and nitrogen fertilizers to these bodies of water).

The rapidly growing algae blooms suck oxygen from the water, thus resulting in large fish kills. It is for this reason that many states are regulating and limiting the amount of phosphorus products that can be applied to crops and turf as fertilizers.

In fact, the Environmental Protection Agency (EPA) is considering regulating waste water effluent from greenhouses and hydroponics operations in the future to control their levels of nutrients released into municipal sewer systems and the environment.

In other words, growers will have to become more efficient in their use of phosphate, nitrogen and other nutrients, so it will become more important in the future to know the various forms of these materials and how to use them correctly.


Phosphite (PO3) is a phosphorous compound that has one less oxygen molecule than phosphate fertilizer. Phosphite is made using phosphorous acid. The key difference between phosphite and phosphate is that phosphite has very strong fungicidal qualities, particularly against the water mold fungi phytophthora, rhizoctonia, and downy mildew.

This quality makes phosphites particularly attractive for hydroponics applications since the plant’s roots are kept in very moist and wet conditions for prolonged periods of time. Phosphites also have some fungicidal activity against fusarium, apple scab, phomopsis, colletotrichum leaf spot (anthracnose), uromyces, sclerotinia and xanthomonas. Conversely, phosphate fertilizer has no fungicidal activity.

Phosphites are labeled and sold as both fungicides and fertilizers. The reason for the fertilizer label is that the phosphite will be exposed to oxygen and microbial action over time and, thus, transformed into phosphate fertilizer. The problem is that, according to some scientists, this process is too slow to be of any practical value (it takes at least six weeks).

For example, bone meal is a form of phosphorus certified for use in organic food production, but you can imagine how immediately available to plants the phosphorus in bones might be—yet bone meal has no label restriction in this regard.

Nevertheless, it is for this reason that thirteen states—California, Georgia, Indiana, Kentucky, Michigan, Minnesota, Missouri, North Carolina, Oklahoma, Oregon, Pennsylvania, Texas and Washington—do not recognize phosphites as a source of phosphate fertilizer.

In these states, a phosphite with an analysis of 0-28-25 would have to be labeled 0-0-25. In California, it would be labeled as 0-28*-25 (the asterisk is required since a state law was passed in 2006). So, there are often arguments about whether phosphites are really fungicides or fertilizers—sort of like the old Saturday Night Live skit: “it’s a floor wax, no it’s a dessert topping; actually, it’s both!”

The mode of action in which phosphite materials control fungal disease is not clear, but it is thought that there is direct activity against the pathogen and an effect on the crop plant. Phosphites are known to jump-start the plants’ immune system by ramping up the production of phytoalexins.

There is evidence for the existence of other lesser-known modes of action, thus making the formation of resistance to phosphites very unlikely (given that the pathogen has to overcome these multiple systems).

It is for these reasons that phosphites are so effective for disease control and why there are literally dozens of manufacturers worldwide selling them as fertilizer and fungicide. Phosphites are routinely used in fungicide rotations to manage resistance. The phosphites belong to resistance management class FRAC 33.

Phosphites are usually created by combining a strong inorganic acid (low pH solvent) with a strong inorganic base (high pH solvent) to form phosphite salts. Inorganic, in the chemical sense, means there is no carbon in the molecule.

Phosphites have another interesting characteristic: once absorbed into the plant they travel in both the xylem and phloem conductive tissues, meaning that they are transported to all plant tissues in a systemic fashion.

Moreover, phosphites are readily absorbed by root, stem and leaf tissues, making for versatile application procedures (such as drenches, root dips, tree bark treatments, soil application, soil injection, chemigation and foliar and aerial application methods).

And besides being useful during the crop season, many phosphites are labeled for post-harvest applications by either spraying the crop as it enters a storage bin or by using a vaporizing system.

As a rule, phosphites are very safe for humans and the EPA often approves them as reduced-risk pesticides.

One side benefit of using phosphites is that they enhance root growth through root flushes. There is some debate as to whether this is the result of direct growth promotion of the roots, or a suppression of root-rotting fungi, leading to healthier and more extensive root systems. Phosphites have both curative and preventive action, so it is important to apply them preventively to get the full benefit.


In chemistry, any material with a carbon-phosphorus bond is termed a phosphonate. The main beneficial feature of phosphonates is that they decompose or break down to become phosphites.

One particular phosphonate product is made with aluminum, but—true to the phosphonates—this product must undergo a chemical conversion after application to produce phosphite. Ethylene gas is released during the conversion process, which can be a problem at times as ethylene gas controls the ripening of fruits.

Moreover, aluminum can be toxic to plants in acidic soils and it can compete with other micronutrients in the soil, thus causing nutrient deficiencies. Although this type of product has been in the market a long time and is effective, many growers like to buy products where the ratio of ingredients that are usable by plants is maximized (for example, potassium phosphite, calcium phosphite, ammonium phosphite and others are preferred as potassium, calcium and ammonium are all used by plants).

Therefore, the term phosphonates is often used or confused as an umbrella term for all phosphite-related products, but this is a misnomer since inorganic phosphites (phosphite salts) contain no carbon.

So, now that you have a basic understanding of the terminology, you can feel more confident in buying and using phosphorus and phosphorous acid products for your indoor growing operation. The versatility, effectiveness and safety of using phosphorous acid products make them an attractive and useful addition to your fertilizer and fungicide application program.