pH problem

[Austin Edwin]

Hi all,

The pH of the raw water that we use in our greenhouse is 7.5.

We prepare the nutrient stock solution in a drum and which is then used for irrigation using the dosing system. We adjust the stock solution pH with Phosphoric acid to get the final pH of 5.8.

Now after adjusting the final pH, we find that the pH value keeps increasing with time. The pH increase to 6.4 after 3 days and 7.2 after a week.

Can someone explain why this happens and advice us, how to prevent the pH from increasing.

 

Thanks in advance

Regards,

Austin

 

 

 

A happy person is not a person in a certain set of circumstances, but rather a person with a certain set of attitudes.  (Hugh Downs)

 

[Bob]

Im no chemist but it sure sounds like you need a better buffered PH
down solution.
Bob

[Rg...@aol.com]

austin,

 

the one thing that comes to mind is that the phosphates in the solution are being depleted.

 

chris.

 

[Austin Edwin]

Hi Bob and Chris,

Thanks for your information regarding the phosphoric acid.

 

We basically use two drums of nutrient solutions. One drum contains the stock solution of  Ca(NO3)2 + Fe DTPA and the other drum contains all the other macro and micronutrients. After determining the amount of Phosphoric acid to be added (by titration) we added the acid into the drum containing the macro & micro nutrients. With this method we faced the problem of increasing pH over the time.

 

Now the question, I would like to ask is, Will the phosphoric acid dissipate or deplete if we diluted it with water and store it in a separate drum and mix it with the nutrient solution only at the time of application?

 

If this can work, we will have three drums with the third drum with only diluted phosphoric acid. After determining the amount of acid to be added by titration, we will dose it separately along with the nutrient solution at the time of irrigation to get the required pH.

 

Any suggestion or advice in this regard would be highly appreciated.

 

Thank you

Austin

 

 

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[Rg...@aol.com]

hi austin,

 

re your ph problem. im new to this  but i believe the phospheric acid will only affect the ph as it is consumed. if you are using gravel culture the gravel will coat with phosphates before stabalising also the gravel if it is other than crushed granite , eg limestone, can contribute to rising ph levels. This is according to Howard Resh in the book Hydroponic Food Production.

he states that granite gravel should be treated at the rate of 5 to 50 pounds of superphosphate per 1000 gallons and soaked for several hours. if the solution goes below 300 ppm then drain and a new solution should be made up. When the solution stays above 100 ppm then flush the beds with clean water.

 

looking at the aztec nobel website

 

http://www.micronutrients.info/Applications/StabilityandpH/

 

 it may suggest that the fe dtpa is at fault and that a change to some other fe chelate is appropriate.

 

im not sure that i even know what i am talking about here but you have to give it your best shot right?

 

let me know what you come up with.

 

chris.

 

 

 

[Austin Edwin]

Hi Chris,

Thanks for that nice piece of information.

We grow Tissue Cultured Potato plants (to produce minitubers) in the hydroponic system with vermiculite as the growing medium.  We grow plants in approximately 1200 sq.m. of greenhouse space. We prepare 1000 litres of nutrient stock in each of the two drums and irrigate it using a dosing system.

 

Earlier we used 60% Nitric acid (HNO3) to control the pH, but later realized that it was significantly increasing the N level in the nutrient solution. Later we changed to 21.8% Phosphoric acid (H3PO4). It only increased the P level by 1.76 ppm, when we added 12L in 500 L of nutrient stock solution. We maintain a safe level of only 34 ppm of P in our final nutrient solution. Phosphorus can induce Zn or Fe deficiency in alkaline conditions.

 

We have been using Fe-DTPA as the Iron source, as the pH of final nutrient solution was always above 6.5. (Fe-EDTA is lost at pH values higher than 6.5) If we are able to maintain the pH around 6, then we will be able to switch back to Fe-EDTA.

 

Now, I will do a smaller scale trial with Phosphoric acid in a separate drum.  And let you know that results as soon as I find a solution to my problem.

 

Thanks & Regards,

Austin

 

 

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From: Greenhouse-...@googlegroups.com [mailto:Greenhouse-...@googlegroups.com] On Behalf Of Rg...@aol.com
Sent: 18 September 2006 20:08
To: Greenhouse-...@googlegroups.com
Subject: Re: pH problem

 

hi austin,