Dear Ahssan

 Phosphorus is absorbed by plants in the orthophosphate form, generally as H2PO4- or HPO42-. The amounts of these ions in the soil solution are determined by soil pH (Attached Fig). At pH 7.2, there are approximately equal amounts of these two forms in solution. Maximum solubility of calcium phosphate minerals occurs at about the same pH, therefore maximum plant available P occurs at approximately pH 7. As pH changes in either direction, P availability is decreased. You may notice that other forms of phosphate are equally prevalent at the extreme ends of the pH scale, but this occurs outside the range of normal soil pH. While the portion of P in these forms is high, plants cannot survive the other conditions resulting from these extreme pH levels.

The form of P fertilizer added to soil can affect soil acidity, principally through the release or gain of H+ ions by the phosphate molecule depending on soil pH (Attached Figure ). If phosphoric acid (PA) is added to soil, the molecule will always acidify soil as H+ ions will be released - one H+ ion if the soil pH is less than ~6.2 and two H+ ions is the soil pH is above 8.2. Monoammonium phosphate (MAP), single superphosphate (SSP) and triple superphosphate (TSP) all add P to soil in the form of the H2PO4- ion, which can acidify soil with a pH greater than 7.2 but has no effect on soil pH in acidic soils. The form of P in diammonium phosphate (DAP) is HPO42-which can make acidic soils (pH7.2. The hydrolysis of ammonium polyphosphate (APP), where the P present as the P2O74- molecule converts to HPO4 2-, is pH neutral and hence any acidification due to adding P can be regarded as similar to DAP. SSP or TSP are sometimes declared to cause soil acidification due to reaction products being very acidic;

Ca(H2PO4)2+ 2H2O →CaHPO4 + H+ + H2PO4-

but in soils with pH values less than 7.7 the following reaction neutralizes the acidity produced so that there is no net acidification;

CaHPO4 + H2O → Ca2+ + H2PO4- + OH-

In high pH soils (pH >7.2), dissociation of H+ ion from the H2PO4- molecule will generate some acidity.

Crop uptake of P has little effect on soil acidity due to the small amounts of fertilizer P taken up in any one year – hence fertilizer chemistry dominates pH changes and significant differences in rhizosphere pH have not been observed for uptake of different orthophosphate ions,

Regards,

Prem Baboo

As Prem so ably explained pH will determine the solubility of Phosphates and this is optimized at between pH 6 to 7. In addition the use of humic materials can be useful for Phosphate colubilization. Phosphorous acid can be injected into trees and stimulates improved rooting and pathogen resistance. When plants are colonized with mycorrhizal fungi the ability of plants to utilize soil Phosphorus is increased.

Dear Ahssan

In practical sense, we can not apply H3PO4 directly in soil as its strength is 45N, which is very strong, and will lower the soil pH to a value of 1.1-1.5. So at that much low pH dissociation of H+ ion from H3PO4 will be very minute to form H2PO4- and HPO4=, and if a little amount of this forms are present, it will immediately form insoluble complexes like Strengite and Variscite, leading to a very low PUE., and also other nutrient use will be hampered.

Direct application to plant needs neutralization of acidity prior to application. 

But you can apply H3PO4 in soil after mixing it with a super absorbent  organic gel, the acid will remain in the capillary of the gel, and microbial decomposition will ensure slow release of H2PO4- ions for plant uptake.

@Avijit Ghosh: 

Sir, i was talking about fertigation.

Mixing H3PO4 in water and then fertigate it (using irrigation water).

What 'll be reaction then? in soil and water?

For a given total acid concentration [A] = [H3PO4] + [H2PO4−] + [HPO42−] + [PO43−] ([A] is the total number of moles of pure H3PO4 which have been used to prepare 1 liter of solution), the composition of an aqueous solution of phosphoric acid can be calculated using the equilibrium equations associated with the three reactions described above together with the [H+] [OH−] = 10^−14 relation and the electrical neutrality equation. Possible concentrations of polyphosphoric molecules and ions is neglected. 

For strong acid concentrations, the solution is mainly composed of H3PO4. For [A] = 10−2, the pH is close to pKa1, giving an equimolar mixture of H3PO4 and H2PO4−. For [A] below 10−3, the solution is mainly composed of H2PO4− with [HPO42−] becoming non-negligible for very dilute solutions. [PO43−] is always negligible. Since this analysis does not take into account ion activity coefficients, the pH and molarity of a real phosphoric acid solution may deviate substantially from the above values.

 Probable reaction in water:

H3PO4(aq) + H2O(l) --> H3O+(aq) + H2PO4-(aq)

Reaction in soil I think will remain same as earlier I explained.

Dear Ahssan,

Apply the H3PO4 can be a fertilizer of different cultures. If the acid is concentrated, the dosage should be no more than 18 g per 10 liters of water. It is important, what the soil in your yard (field). If the pH is 6,5 -7, then everything is fine, but if pH < 6 (acidic soil), the phosphoric acid will increase the acidication of soil, which has a negative impact on most crops. In this case it is possible to add sodium carbonate or potassium carbonate.

In Ukraine for fertigate, as  phosphate fertilizers, used of phosphoric acid and ammophos, which are used by turn. Phosphoric acid also performs the function of cleansing of the drip irrigation system from  foreign matter.

When fertilizer input with irrigation water, maximum allowable concentration of nutrients in the water should not exceed to 0.5% of Nitrogen fertilizer, for Phosphate - 2%  and  Potash - 3%. For  complex solutions content of nutrients - less than 1%.

The mineralization of the fertilizer solution, which is fed into the soil, should not exceed 3 g / l  at the introduction of a single component, and 8 g / l at the introduction of two or three-component fertilizers. The total amount should not exceed 1-1.2 kg fertilizer per 1000 liters of water.

Successes

Dear Zenon Hamkalo,

The pH of soil is 8.5 and thats the reason i want to use acidic form of phosphate. we dont have trusted source of SSP in our country and other low ph phosphate fertilizers are expensive too... So, if Phosphoric acid can be uptaken by plants then it 'll be a very goood alternative. but plants uptake phospgorous as H2PO4 or HPO4-2.

If its a horticultural system, it stands good and also explained nicely by abhijit Ghosh. You have talked about soil ph but not crops.

Dear Ahssan,

Dissociation constant of weak electrolyte depends on the temperature, the nature of the electrolyte, a solvent and is independent of concentration.

H3PO4 = H+ +H2PO4–  (k1= 7,6•10–3)

H2PO4– = H+ +HPO42– (k2= 5,9•10–8)

HPO42– = H+ +PO43–   (k3 = 3,5•10–13)

As you can see, H3PO4 most dissociates the first degree  (10-3) and in this case is a strong acid. At each subsequent stage of dissociation occurs to a lesser extent, and so are the solutions of phosphoric acid molecules H3PO4, cations (H+) and anions H2PO4-  >>>HPO42- >> PO43-. Number each subsequent component gradually decreases.

Sincerelly

Dear Ahssan, first of all I want to congratulate you for your imagination.......

Now come to your question, which is about the utility of H3PO4 as alternative P fertilizer during fertigation, and you also mentioned the pH (8.5) of soil, but you did not mention the CaCO3 content in soil,....... which is also a big factor about the availability of P to plants.

As par my knowledge Zenon, Avijit and Prem Baboo analysed and discussed the question well accordingly. But I think when you apply dilute H3PO4 during fertigation it will significantly reduce the soil pH and cause greater solublization of Ca-phosphates, which simultaneously increase activity of Ca2+ in soil solution also may cause formation of Fe and Al - phosphates and reduce plant available H2PO4-. On the other hand, the higher activity Ca2+ in solution, self function of soil and excessive dissociation of H2O will cause further rise in pH with passing time which might cause availability of HPO4=. But, formation and stability of Ca-phosphates should also be considered. 

Another thing, during fertigation H3PO4 may get fully dissociated in its different P components, among them major portion are plant available. Thus, when you will apply H3PO4 along with irrigation water it may cause excess availability of P in plant rhizosphere, which may not synchronize with plant demand. Thus, excess amount of plant available P may get fixed and locked up in soil as recalcitrant pool of P and also may form higher amount of insoluble initial reaction products. So I think there is no extra advantage of using H3PO4 as an alternative P fertilizer.

This is a general idea, but to be specific you should conduct a POT CULTURE experiment for 30 days or up to crop establishment stage with different doses of H3PO4. I think that will give you a better result of your quarries. You should try this and once again I want to appreciate your thoughts from my heart......All the best!!

Dear colleagues ,very good discussion on an important topic.I appreciate all the followers of this question for their contributions.Soil application of phosphoric acid is  a difficult proposition,in view of the problems listed by colleagues especially its affects on soil. Fertigation may be tried and tested as per the details given by Dr.Zenon(4 days ago).It appears to me promising alternative(even with some reservations expressed by Dr.Sarkar). We have to generate information on what risks are there and how much P use efficiency we can achieved by phosphoric acid fertigation .Of course economics of application can be seen later.It can be tested with other major nutrients and also be applied alternatively with ammonium phosphate ,giving some gap between applications for the soil to adjust or recover.

Dear Ahssan,

most of the chemical aspects have already been addressed, but I wish to clarify some points here:

1. Concerning the way the original question was put: There is no way to remove or add a proton to the various phosphate forms or the phosphoric acid directly. It’s all related to the surrounding pH, and within the chemistry we can implement there is no way to change the dissociation state of any ion in a given solution without changing the pH.

2. I really don’t understand way many people tend to jump to extremes. Ahssan did never suggest soaking the soil with concentrated phosphoric acid, killing all crops in the shortest possible time, and lowering the pH to ranges where Al- and/or Fe-phosphates may form. I think most of us agree that many soils in the subtropical area and those in the Mediterranean are calcareous and/or saline and suffer from high pH, restricting the availability of P, Fe, Mn, Zn etc. Hence, lowering the alkalinity of these soils to – say – pH 6 would certainly improve availability of these nutrients, making the nutrition of crops more balanced. What would happen below pH 5 we don’t need to discuss.

3. Lowering the pH of calcareous soils would certainly increase the Ca concentration in the soil solution. I don’t consider that a disadvantage. In high bicarbonate soils at high pH the Ca concentration in the soil solution can be very low because of CaCO3 precipitation, rendering the Ca insufficient for optimal growth. Under such conditions Ca-nitrate is often recommended.

4. I also feel such a question may not be adequately placed in this research forum at all. A quick search on Google on “phosphoric acid fertigation” revealed the following from the first hits:

The Tamil Nadu Agritech Portal (http://agritech.tnau.ac.in/agriculture/agri_nutrientmgt_fertigation.html) states „Thus phosphoric acid is more suitable for fertigation as it is available in liquid form“.

On the section „How to Adjust the pH in Fertigation Systems“ Israel-based SMART fertilizer management (from http://www.smart-fertilizer.com/articles/pH-in-fertigation) discuss the question „How Much Acid Should You Add?“ and also state „The most common acids for agricultural use are Sulfuric acid, Phosphoric acid and Nitric acid.“

Australian-based NSW Department of Primary Industries on Water & Irrigation lists in a section regarding Phosphorous fertilisers phosphoric acid as a common fertiliser, and states „The possibility of precipitation of insoluble phosphates is high when using MAP or DAP with irrigation water that is high in calcium or magnesium. This will cause clogging of outlets and pipelines. Phosphoric acid provides the advantage of cleansing lines and sprays and is a concentrated liquid offering ease of use” (http://www.dpi.nsw.gov.au/agriculture/resources/water/irrigation/crops/publications/fertigation).

Combined with what Zenon had written it feels that among the fertigation experts (advisors, fertigation companies) the technology of phosphoric acid application is well established the world over. They don’t discuss whether phosphoric acid addition is possible – that’s taken for granted – they only discuss how to determine the amount to be used, and provide detailed guidelines for this. The chemistry behind it has been worked out decades ago anyhow. Having said that I even see limited scope for new scientific research in that area. No point reinventing the wheel! Some adjustments to local conditions and some experimental work to form the basis for local extension services is of course in high demand in some parts of the world.

5. Finally I just want to mention that in more temperate regions and in inert systems substrate pH adjustment/control is done using the ratio of ammonium to nitrate – but that’s another topic…

Regards, Jóska Gerendás

Thank you Dr.Gerendas for sharing/providing  a lot of information on fertigation with phosphoric acid as a source of P.Can any body  inform the extent of use of phosphoric acid in fertigation in a country like India and other countries?

Dear Ahssan,

Phosphorus (P) is one of the 17 nutrient elements essential for plant growth and development 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. The percentage of phosphorus in a fertilizer, as phosphorus pent oxide (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 P2O5 unit used to represent P content in fertilizer is a conventional unit (in reality, there is little or no P in the form of P2O5 in fertilizer).Compared with either H2PO4- or HPO4 2-, it is only one out of 100,000 because it always dissociates to H2PO4- and further to HPO4 2-. Both H2PO4- and HPO4 2- ions are basic forms taken up by the plant, but H2PO4- is taken up more readily  because in most growth conditions, soil solution pH is below 7. Once inside the plant, both ions are mobile.

Regards,

Prem Baboo

Article Are Phosphorous and Phosphoric Acids Equal Phosphorous Sourc...

 Dr.Baboo,as I understand ,we are not discussing the phosphoric  acid(H3PO4) and phosphorous acid(H3PO3) as sources of P in the question asked by Mr. Manan Khan. The phosphorous acid dissociates to form the phosphonate /phosponite/phosphite ion,HPO3. The phosphonite though taken up by plants,it is not used by plants as nutrient source.The phosphonite serves as good controlling agent of plant pathogens,oomycetes. The reference attached is interesting.

Some good points Joska , worth appreciating  them . In citrus like perennial crops , we very often use phosphoric acid with an added advantage to dissolve the turbidity created out of dissolution of urea and muriat of potash . And , it works well 

That is right Dr.Gerendas, there is no way to remove or add a proton to the various phosphate forms or the phosphoric acid directly. It’s all related to the surrounding pH

 In relation to use of Phosphoric Acid this can be used as Anoop points out for drip irrigation systems to clean out clogged drip orifices. This acid can reduce the Cationic sediments usually from Calcium which clog the drip lines. As Dr. Rao points out that Phosphorous Acid can have efficacy as as Oomycete fungicide.

The use of Phosphoric Acid can contribute to Phosphorus needs of plants it may not be the most economic or efficacious source however. The use of Phosphorus would best be applied based on the soil capacity and targeted for seedling stages to stimulate early aggressive root growth. Under alkaline soil conditions P can be largely unavailable in the soil. In the alkaline soil condition the addition of Phosphorus can make Zinc deficiencies worse. The reaction to Phosphorus can be dependent on simultaneously applying Zinc and checking that both nutrients are in optimized range. 

Although much focus has been focused on Nitrogen for crop systems when Phosphorus is optimized the robust Biological Nitrogen Fixation of legume crops can supply the total crop system need for Nitrogen from Rhizobium symbiosis that takes the Nitrogen from the atmosphere. 

Some good points Paul, appreciate your informative feedback . while implementing fertigation of phosphorous , we can also assign some credit to PSBs as well ...

Ahssan... The answers and discussion given above are scientific and fruitful. First of all, phosphoric acid is the most available resource of P. However, The basic consideration, is the soil reaction. In many countries, soils are acidic or alkaline. Plants need almost a neutral environment. That's why the P is introduced as phosphate or hydrogen phosphate salt in the form of NH4 to utilize both the P and N content as nutrients. Moreover, the handling of acid (H3PO4) is more difficult that handling Mono-ammonium phosphate (MAP) or Di-ammonium phosphate (DAP) as neutral and slightly acidic materials is much favored.. Good luck.

I am very much aligned with Mr. Mahmood. If you will see the pH of phosphoric acid is less than 2. In case of neutral or slightly alkaline soil it may disturb the root system due to oscillating movement( drastic reduction of pH and recovery of pH) that may result into root burning. Due to oscillating movement of pH around the rhizosphere there will be impact on nutrient uptake also

My view point is same