Phosphoric acid (and how to use it successfully on your plants)


The fertilizer in agriculture is the main source of food, because as we have realized, water is insufficient to grow a plant. Although we do not provide anything other than irrigation, the soil is constantly supplying these nutrients to the root of the plant.

Of course, everything has a limit and that is where phosphoric acid comes into play as a highly enriched source of nitrogen

One of the most used fertilizers to incorporate phosphorus into the crop is phosphoric acid .

A fertilizer that comes in liquid form, convenient to use (but dangerous), with a generic contribution of 52% w / w of phosphorus, with acidic pH (ideal for alkaline soils), and with greater control of the contribution that is made to fertigation.

In addition, it also manages to eliminate organic debris and salts that clog the droppers .


Surely you have heard of the importance of phosphorus in the early stages of plant development, when proper rooting of the crop is important. You may have also heard about the role of phosphorus in flowering.

All of these things are true, up to a point, as there is a popular belief advocating the high phosphorus needs in early fertilizers to promote rooting.

This can be denied, since at higher levels of phosphorus there will be no further root development  ( John P. Hammond and Philip J. White ). Phosphorus is needed by the plant, but not in such high amounts as is added early in the crop.

In fact, when the plant observes that there is a lack of phosphorus in the soil (or there is sufficient quantity, but immobilized by calcium or alkaline pH), it produces organic acids that mobilize said retained phosphorus, such as malate and citrate.

Somehow there is a movement of “energy” from the stem to the root, to promote its growth, so that the lower the phosphorus, the greater the development of the root.

However, this has its connotations and its twists, since it is a closed circle of energy that is transported from one side to another. In this case, as there is a movement of sugars (energy, basically) towards the root, there will be a decrease in the development of the stem and leaves, which we do not want either.

As a curiosity, comment that it is the same term to mention phosphoric acid and orthophosphoric acid . Simply that the first term is more accepted and is used in a common way.

Another, even less common, way of naming this product would also be ‘tetraoxyphosphoric acid’.


Currently, there are 2 manufacturing methods for this component.

Dry way

From solid white phosphorus (very hygroscopic), it is burned in a combustion chamber, enriching it with oxygen (P2O5). From here, the element dissolves, forming phosphorous pentoxide.

Wet route

From calcium phosphate rock phosphate, it reacts with mineral acids such as sulfuric acid . From the mixture, phosphoric acid and sulfate of lime (gypsum) are created, used as a by-product.

Typical symptom of phosphorus deficiency. Source: Wikimedia commons


Phosphorus is essential for photosynthesis and to form organic compounds . It is involved in cellular respiration and in the transport and storage of energy. It also intervenes in the formation of flowers and the advancement of maturation. At low levels of phosphorus, the plant reduces the level of flower production and, therefore, fruit set.

Phosphorus is a mobile element inside the plant.

Therefore, the first deficiencies of this element will appear in the adult leaves.

It is typical to see a reddish or violet color in a leaf with low levels of phosphorus, as well as a higher concentration of carbohydrates and reserve substances (which will later be directed towards the roots to compensate for their lack of development).


To provide you with data on the characteristics and properties of phosphoric acid, here is an example of a standard solution of this liquid fertilizer.

Phosphoric acid 72%. To say, and it is important, that because it contains 72% phosphoric acid does not mean that the phosphorus content, in fertilizer units, is 0.72 UF for each liter of solution.

The actual declared phosphorus content (measured in orthophosphoric acid, P2O5), is 52%. This means that for every liter of phosphoric acid that we apply to our irrigation solution + fertilizer, we will be applying 0.52 UF to the crop.


Rather, we could say that we are contributing 0.52 UF to the ground. Another thing is the actual percentage that the plant is capable of assimilating depending on a large number of factors.

[alert style = »green»] An important part of the phosphorus that we apply to a soil rich in calcium, passes into insoluble and immobile states such as dicalcium phosphate.

However, it must also be said that we not only find this example with phosphoric solution , with a richness of 72%. There are also other mixtures, such as 75% phosphoric acid and with a contribution of 0.543 fertilizer units (UF) for each liter of liquid fertilizer, or 85%.

The pH of the concentrate is usually around 1 or lower than 1, so the amounts added to irrigation must be measured with caution. In addition, we must not fail to mention that it is a product that, due to its pH, is highly corrosive and causes burns. Special care must be taken when handling it.

The phosphoric acid can also be used to perform a cleaning droppers , especially when we do not incorporate nitrogen cultivation (as nitric acid).

In this case, we could use a volume of 250-500 ml per 1,000 liters of water , for a 75% phosphoric acid mixture. Of course, you must be careful to use low doses in those waters that have high levels of calcium or magnesium.

This happens due to the known antagonism between phosphorus and calcium and the possible precipitates of dicalcium phosphates that may appear.

[alert style = »green»] Not only did we not correct the clogging of the drippers, but we made it worse. [/ alert]


The label and the pictograms that appear on it make it clear to us. In fact, with the latest changes in regulations, the size of these images has been increased, which warn of the damage that phosphoric acid can cause us , in case there is someone confused.

It is a corrosive product in contact with the skin, causing irritation, redness and burns .

The problem also comes when it spills to the ground and the mist it produces. These gases are also irritating when inhaled, causing eye and throat irritation.


However, there are farmers who prefer to use other types of solutions rich in phosphorus, or others who prefer to use solid fertilizers rather than liquid ones. For tastes, the colors.

In the market we can also find many crystalline fertilizers and compound fertilizers , such as monoammonium phosphate (12-61-0) or monopotassium phosphate (0-52-34).


The latter is interesting when we do not want to stimulate the development of the vegetative part of the crop, promoting the absorption of nitrogen by the plant.

Apart from this, you will find many brands that work with tailored mixtures, not always complexes that carry NPK, but binary fertilizers, such as DAP (18-46-0) , when you do not want to make a contribution of potassium due to the needs of the crop or because there are adequate levels in the ground.


  • It is acidic and can be used in a fertilizer plan without using nitric acid to clean pipes
  • It has a high concentration of water-soluble phosphorus (between 52 and 54% w / w).
  • It is liquid, so it is comfortable to use


  • It is acidic and requires special transport and use measures
  • Sometimes the quality of the phosphoric acid is not quite good, leaving traces at the bottom of the containers. Be careful when choosing the quality of the product.
  • It only contains phosphorus, which is an element that is not usually used as a unit, but mixed with other nutrients (nitrogen or potassium).

In short, phosphoric acid is an effective alternative when you want to prepare acidic liquid solutions, without the need to dilute a solid and without the risk of clogging irrigation systems.

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