PH Level and Crop?

[Harold Sukhbir]

Hi All,

In another discussion, some of us are trying to determine the ranges of PH for best growth of different crop grown. This can become a general guide for maintaining specific PH levels to suit the crop you choose to grow.

So, what is your average system PH and what plant (plant doing the best) do you notice thriving at this PH level?

[Alex Veidel]

Okay, well here's an excerpt from Wikipedia:

"Nutrient uptake in the soil is achieved by cation exchange, where root hairs pump hydrogen ions (H⁺) into the soil through proton pumps. These hydrogen ions displace cations attached to negatively charged soil particles so that the cations are available for uptake by the root."

But since acidic solutions have more hydrogen ions, wouldn't plants pumping hydrogen ions into the soil lower pH? If we could get a hydroponic expert over here, that would be cool...

Alex Veidel said:

Ooh, ooh, I feel like the answer to this is in my head somewhere....did it have something to do with cation exchange? I'll go refresh my memory when I have time and get back to you.

[TCLynx]

Well Robert, this discussion will also get more complex seeing as this is an Aquaponics Site and the bacteria in the biofiltration tends to use up alkalinity or water hardness which will cause the pH to go down over time.  In a Hydroponic system the plants are using up nutrients that may may otherwise be tieing up or counteracting that alkalinity faster than they are using up the alkaline nutrients (I know I'm terrible at the technical side of chemistry so some one with a better grasp on it please correct me!!!!!!)

I don't think that "thinking" of it as a cloud of hydrogen dissolving nutrients is really the right way to see it.  The nutrients are dissolved in water not a cloud of gas.  pH is power of Hydrogen but has more to do with ions and charge than it has to do with the actual amount of Hydrogen.  Because you can take a given volume of water H2O and by adding different things to it you can change the pH or power of hydrogen without having the Hydrogen escape or increase (if the hydrogen were to be escaping as in splitting the water you have Hydrogen and Oxygen, not water)  Now there are lots of things that might dissolve into the water and bond with it in different ways to change the pH.  Hydrochloric acid (HCl) will lower pH while Calcium Hydroxide will raise pH (CaOH) each of them have one Hydrogen, the trick is in how the other elements interact with the other Hydrogen already there and what the Hydrogen atom in the added substance gets to do with itself. The HCl has a hydrogen atom that wants to bond with something since it only has one electron in it's outer shell if the water happens to have a bunch of calcium carbonate in it CaCO3 then the HCl can break it up letting the Ca and Cl join CaCl2 and some CO2 Leaving free O to Join with H to get more H2O.  As the CO2 escapes from the water or as the plants use it up from the water, the pH will rise because CO2 dissolved in water is a weak acid.  So by adding an acid to the water in this case the pH comes down because you neutralize some calcium carbonate but then the pH will come back up a bit as the CO2 will escape.

These interactions can be very complex and I didn't manage to learn much chemistry back in school.  I'm learning far more chemistry now that I've been doing aquaponics.

[TCLynx]

In soil the plants might have to do that work for the citation exchange but since he was talking about hydroponics where the nutrients are in solution I think the answer is more complex.  In soil the plants I believe do cause some acidity increase but depending on what the irrigation water is, there might be plenty of alkalinity to counteract the action of the plants and soil bacteria, on the other hand, there is a reason many farms need to lime the fields regularly.

In Hydroponics I think part of the reason the pH tends to rise in the solution is that what the plants take up and what evaporates and what gets concentrated into the solution tends to leave behind more alkalinity (hard water minerals) than the acidic ones and there for as nutrients get out of balance the pH rises and especially if you are topping up with hard tap water, the pH will rise in hydroponics hence part of why the solution is usually discarded and replaced every few weeks or so.

In Aquaponics, our bio-filtration tends to take care of using up or bringing down the alkalinity so if we can manage to keep our systems balanced enough between the fish and plants, we may not need to dump and replace water to keep the system in the "zone" for pH and nutrients levels.  This can be a rather tricky balancing act though.

[Robert Chlebowski]

Thanks for the fast replies.

Here's where I'm coming from. My aquaponic pH is always swinging upwards to 7.8, I have stones in there that passed the vinegar test however I'm sure that they are buffering any acid. I'm hoping to break this buffer by moderately adding HCL, I've been doing this for a month now and it still climbs to 7.8 over a few days (note: this upward swing is taking longer which I think is a sign that the buffer is approaching bust). Then I read in the hydroponic literature that nutrient uptake swings pH, but it really didn't say why. I got thinking that maybe adding acid to the water has to be as common as adding nutrients for the plants or fish feed. TCLynx, I'll look into the evaporation + concentrating solution = raise pH bit more carefully. Its interesting that some nutrients are 'more acidic' than others, the literature also said that some nutrient solutions act as a pH enhancer or reducer simply by how they react when mixed with water, but it seems that they still have Acidic or Alkaline tendencies even after they are mixed into water? Can you elaborate... if that even made sense!

Ok, so a byproduct of the bio filter doing its thing is CO2 which when mixed well in water becomes carbonic acid H2CO3, which drops the pH. Am I missing something else? Does the bacteria actually eat the hard water? My tank is only 30 gallons with very small fish and I feed the plants once every half hour to four grow bins (flow and ebb). The bins may be generating CO2 (like a biofilter should), however a) i think that this generation is small because i'm only feeding twice an hour and b) if CO2 is generated its almost immediately lost to the atmosphere as opposed to mixing with water because it all gets returned to the tank! I'm considering adding more fish (despite my girlfriends wishes) to boost my ammonia which will give me a reason to add more filters which will hopefully get me a larger bio-filter/bio-buffer.

[TCLynx]

Almost no hydroponic nutrient salts are in a "pure" form.  When they mix up nutrients they are not only trying to get the right combo of nutrients but trying to do it in a way that will react with the water to give an appropriate pH range.  For instance, phosphoric acid is an acid while potassium hydroxide is a strong base and I know a lot of the ways of adding potassium, magnesium and calcium are likely to elevate pH while certain other things may tend to lower pH.

In Aquaponics it is tricky too since you don't want to go adding things willy nilly to your AP system like some kids chemistry set since you could easily kill your fish.

In a really new AP system the most common cause of rising pH is the source water, second most common is accidentally using limestone because so and so said pea gravel was fine.  Other things that can cause pH to rise include anaerobic zones and Algae.  Finally, if the bio-filter isn't cycled up yet, there isn't anything going on to reduce the pH enough to counteract any hardness in the tap water.

Here is a blog post about pH and tap water before you insist that your tap water is neutral.

pH and tap water

CO2 doesn't outgas instantly, If you see strange pH swings between dawn and late afternoon, it is probably because of algae and the CO2 it gives off at night will lower the pH overnight and in the daylight the algae will use up the CO2 and the pH will rise.  Algae can also make thick matts that can become anaerobic and create zones that can elevate pH as well as become a stinking nasty mess.

[Harold Sukhbir]

"  are likely to elevate pH while certain other things may tend to lower pH."

Thanks TC, as I've been finding this out recently!

[Alex Veidel]

Yeah, checking the source water is usually a good idea. Plus I would point out that since his system is on the smalll side of the aquaponic scale, it's going to be a little less stable when it comes to pH.

TCLynx said:

Almost no hydroponic nutrient salts are in a "pure" form.  When they mix up nutrients they are not only trying to get the right combo of nutrients but trying to do it in a way that will react with the water to give an appropriate pH range.  For instance, phosphoric acid is an acid while potassium hydroxide is a strong base and I know a lot of the ways of adding potassium, magnesium and calcium are likely to elevate pH while certain other things may tend to lower pH.

In Aquaponics it is tricky too since you don't want to go adding things willy nilly to your AP system like some kids chemistry set since you could easily kill your fish.

In a really new AP system the most common cause of rising pH is the source water, second most common is accidentally using limestone because so and so said pea gravel was fine.  Other things that can cause pH to rise include anaerobic zones and Algae.  Finally, if the bio-filter isn't cycled up yet, there isn't anything going on to reduce the pH enough to counteract any hardness in the tap water.

Here is a blog post about pH and tap water before you insist that your tap water is neutral.

pH and tap water

CO2 doesn't outgas instantly, If you see strange pH swings between dawn and late afternoon, it is probably because of algae and the CO2 it gives off at night will lower the pH overnight and in the daylight the algae will use up the CO2 and the pH will rise.  Algae can also make thick matts that can become anaerobic and create zones that can elevate pH as well as become a stinking nasty mess.