Why is My pH Doing That - And Does it Matter?

 

Perhaps my main motivation for writing about issues in hydroponics is to develop my own systematic practical knowledge from all the information that’s out there. In trying to put things in a form someone else can use, whether I succeed or not, I have to try to understand it myself in a way that I can recall it when needed. So now I take up pH and E.C. and their very dynamic interactions with the plants and the nutrient solutions.

 

pH, the measure of acidity or alkalinity, is much talked about in hydroponic circles. There are many who worry about keeping pH numbers stable and as close to some ideal they have chosen as possible. But that is not necessary nor necessarily productive. As always, one has to be alert when searching the Internet for answers to questions, because of the fact that plants in soil and plants grown hydroponically have different ph needs.

How strictly must pH be managed? Research has found that as pH decreased (more acidic nutrient), Phosphorus, calcium, magnesium, sulfur, boron, manganese, and zinc concentrations in leaves declined, while potassium and aluminum increased. But in the plants studied, at least in the short term of 20-28 days, these changes had no effect on plant growth and no apparent ill effects at all. In fact, it was found that holding nutrient pH at 4.0 could suppress severity of root rot when investigators inoculate basil with P. aphanidermatum spores. P. aphanidermatum causes significant loses in tomato seedling nurseries.

And while it’s generally understood that pH effects nutrient uptake, many do not realize different pH ranges act differently upon different nutrients.

Nitrogen, calcium and magnesium are taken up most readily from about pH 6.0 to 8.5.

But sulfur, potassium, phosphorus and molybdenum are also taken up readily in that range but also all the way off scale at pH 10.

At the same time, iron, manganese, boron, copper and zinc are best taken up in more acid conditions centered on the classic 5.0 to 6.0 . Boron has an additional peak above 8.8 .

So there’s the first lesson:

Stringent efforts to check and adjust often to any one pH value is actually counter-productive.

When pH is allowed to fluctuate both high and low, all nutrients get time in their own sweet spots.

We know it often does fluctuate. Why does that happen? Feeding habits of plants play a role. Most nutrients are slightly acidic. As plants absorb them, they are taking those acidic compounds out of the solution, leaving it more alkaline. If you just let it go on, the plants will keep taking up acidic nutrients and raising the pH until it runs out of nutrients and you change solutions or add more nutrient and start over.

When the solution warms, the water slowly releases CO2, which also increases pH. Nutrient solutions often rise as they absorb heat from hot air delivered by air pumps that usually run very hot indeed.

We are now going to consider how E.C. works into all this. I suggest you read over the three situations that follow The Rules, until you have a good feel for the interactions, rather than depending on charts. It will begin to make sense.

 

Remember some important points. These are the rules of the game.

 

Rule #1 - Nutrients are acidic. Take nutrients out, and pH will rise. Put them in, and pH will fall.

 

Rule #2 - Plants will take the nutrients in the amounts they want, no matter what it does to the solution. Plants don’t know how to budget. “Feed me” means NOW!

 

Rule #3 - Plants are active agents in manipulating pH to their own advantage. They learned how, because their natural environments can vary so much.

 

Rule #4 - Plants can take water out of a solution at about four times the rate they take nutrients.

 

Rule #5 - Plants are subject to the same rule as our own membranes; water follows salt by osmosis (nutrients are salts), but but osmosis does not move salts. See Rule #6.

 

Rule #6 - Plants manage nutrient uptake, moving sales, by producing sugars in their roots that allow it to balance nutrient movement in (and out).

 

Rule #7 – Rule 5 and 6 reiterated, because this is important. Plants take in water through osmosis, water following salt concentration; they take up nutrients through separate diffusion and transport mechanisms using modulated by sugar concentration.

 

Because our exciting moments are when we look twice at our meters and wonder what’s happening to make E.C. and pH misbehave, I will discuss these issues from that point of view.

 

·       E.C. High and Rising/pH Falling - When the nutrient E.C. starts high or becomes high, plants cannot keep up with the amount of sugar needed to balance nutrient movement and will start putting nutrient into the solution, rather than taking it out. More nutrient means even higher E.C. And the higher concentration of salts in the solution means osmosis draws water out of the plant. The result is “nute burn,” dry leaves, and death. But it’s really lack of water in the plant. The two processes for plant management of nutrients and water can be in conflict, resulting in fluctuating E.C. and pH values. Strong nutrient solutions cause all manner of difficulties.

 

·       E.C. Low and Falling/pH Rising - Strength of the nutrient solution affects how rapidly pH changes. If the solution is weak, with a low E.C., plants will still take nutrient at the same rate, the rate they want it, but it will be proportionally more, and (because nutrients are acidic) pH will rise rapidly with loss of a relatively high percentage of the already low nutrient. Plus (there’s always more, isn’t there), when E.C. is low, plants initially take up too much water by osmosis because of higher salt concentration in the plant tissues, so the extra water dilutes the concentration of sugar in their roots until it is higher than the concentration of salts in the solution, and they try to balance it through the diffusion mechanism. The change accelerates, because with every unit of nutrient lost, every next nutrient unit lost to the plant is an even greater part of the total remaining.

 

·       E.C. Near Normal and Falling Slowly/pH Rising - E.C. dropping and pH rising means plants are feeding and taking nutrient out. The pH rise is a normal sign because nutrients are acidic, and the plants is taking them out of the solution. Unless nutrient is restored, E.C. will fall at an slowly accelerating rate (every unit of nutrient being an ever greater part of the solution), and pH will rise ever faster. Complaining about pH rise does no good. It’s generally expected in hydroponics. But whether the E.C. is changing depends on the rate rater is leaving the solution. Which takes us to the next situation.

 

·       E.C. Rising from Normal/pH Falling - When water is taken up by the plant more rapidly and lost to evaporation in hot conditions, E.C. will of course rise as the concentration increases more rapidly than the plant need to takes up nutrient. pH will drop as acidic nutrients become more concentrated. And as E.C. rises, plant uptake will slow because it’s increasingly and plants start leaching nutrient solution back into the reservoir. Heat and humidity can also hinder transpiration from the leaves, which disrupts the normal nutrient and water transport throughout the plant. The resulting condition is edema in which leaves well, swollen tissues may rupture and nutrient crystal is left on leaves after evaporation.

 

·       If pH and E.C. remain stable, it means we are seeing the agreeable situation where the plants are taking nutrient and water at balanced rates so that the concentration of nutrient remains as it began. This is most likely to occur when temperature and humidity are managed, buffered or neutral medium is used and nutrient is slightly weak to prevent the cascading problems of strong nutrient. All the plant needs is what it needs. It doesn’t do better with stronger nutrient.

 

The safe course is to use slightly weak but normally balanced nutrient. pH will rise as normal, but not precipitously, and reasonable monitoring will detect when it is getting too high and therefore threatening to run away. And pH can then be brought back down to 5.5 . But there is no reason to correct pH before it reaches 6.2, and letting it fluctuate between 5.5 and 6.5 will give all plants good chances at all nutrients as it slowly drifts upward again through the nutrient uptake sweet zones.

If a plant is able to take up all the nutrient it needs because the pH values move among values amenable to taking up different nutrients, but the nutrient solution is too hot, the plant is forced by necessity to take up more water than would be ideal to keep nutrient and water in original balance. E.C. goes high because water is lost, leaving nutrient more concentrated. pH goes low because nutrient is acidic. And you’re back into instability and confused.

And wouldn’t it be nice if it were all that simple. But the plants aren’t just dumb plants; they do more than just take up nutrients when available and take up water as needed. Plants also play the pH game. They secrete acids and bases in their root zone to move the pH there to suit the nutrient they want to take up. Isn’t that clever? Are you impressed?

If it wants more potassium, it can secrete acid, dropping pH to within the good range for potassium uptake. Or it can secrete base buffers to raise pH when it wants more calcium. If we have a single pH we consider ideal and try to be the perfect plant parent by continuously setting pH back there, we may be fighting the plant’s good efforts to get a shot at all the different nutrients.

Better to stay loose, understand that there is more going on than we can know, and not get excited unless things get out of hand, mostly meaning pH moving past 6.2 and more. And even then, giving it some time to correct itself. Mysterious pH fluctuations may be just the plant creating different root zone environments to help it feed. pH should not be adjusted until you see if it is stabilizing high or low and not just in a plant-driven normal fluctuation or if it is clearly running away. But then, it’s time to find out why that’s happening, rather than using more buffer.

We also have to consider some media that can affect pH. Rock wool is one that may raise pH. Natural rock gravel often has a very strong base buffer in the form of limestone. Rock wool can be prebuffered before use with acidic buffers like one liter of vinegar in one gallon of water, until the pH of the soaking solution stabilizes overnight, meaning the rock wool buffer has been exhausted. It’s probably a waste of time to try that with gravel. You may have to dissolve most of the limestone gravel before it stabilizes.

Pathogens can also be responsible for pH swings. Bacteria, algae and other organic matter in the nutrient solution that do not do much harm when alive, eventually die. When they die, they release organic acids and you see an strong pH swing downward. When pH reaches 4.5, it’s likely root disease loose in the reservoir. If hydrogen peroxide doesn’t restore normal pH, removal of all dead and dying root material may be necessary. But at the same time, look into what caused the roots to become vulnerable. It will likely be an aeration issue, which includes possible root matting around air stones.

One last situation that you now would probably automatically suspect to be responsible for pH swings is very low nutrient volume, either because you neglected to keep it up or you are using a lamentably small reservoir to grow in.

When nutrient volume drops below one gallon, the plants go into the kind of rapid changes described above as they attempt to balance their nutrient intake. And if you allow volume to get low, it is likely you have waited so long to intervene that all semblance of the original proportions of nutrients has been lost and antagonistic reactions are setting in between nutrients. The fix is obvious. Replenish nutrient or use a larger reservoir. And remember that the situation can result when circulating an insufficient volume to too many plants.

And a last note. Plants have a remarkable ability to adapt to adverse conditions, meaning conditions other than their ideal. We, in fact, take advantage of that. Unless plants could adapt to slowly flooding by taking dissolved air from a solution, we could not operate Deep Water Culture or any other submerged-root method. And plants can adapt to living with absurdly high E.C. and low pH values. I have known a Chard plant to thrive in an abandoned DWC bucket where the last half inch of nutrient E.C. tested at 8.4 . Of course it tasted quite awful, but it looked great.