Hydroponics 101 Continued
By Steve Berlow
Well class, welcome back. Recess is over (two months) and we are on to our second lesson in hydroponics 101. This may be the single most important lesson in your hydroponic career as it covers two interrelated topics that will have the most effect over whether you have a successful crop or an outright failure; pH and conductivity. Some of this may seem a trifle boring at times but believe me when I say that it will pay off big time in the end. So class, pens ready and off we go!
What is pH?
The term pH is a notation for the term potential of hydrogen and is logged on a scale of one to 14 to represent the concentration of hydrogen or hydroxyl ions in a solution or compound.
These hydrogen or hydroxyl ions determine the acidity or alkalinity of your hydroponic nutrient solution, whereby pH one is the most acidic and pH 14 is the most alkaline.
Each whole number on the scale represents a 10-fold increase in the acidity or alkalinity of a solution. At pH 7.0 the hydrogen and hydroxyl ions are in equal numbers therefore the solution is neither acidic nor alkaline.
Broadly speaking pH one to three is very acidic, pH three to five is moderately acidic and pH five to 6.8 is slightly acidic.
The same holds true for the alkalinity side of the scale.
Why is pH Important?
pH is important in hydroponics because it directly affects the availability and uptake potential of the mineral elements we feed to our plants via nutrient solution. Remember that the only source of nutrition your plants have is this solution.
A common misconception, and probably the worst, is using a soil based pH mineral availability chart. This chart does not represent the way mineral elements are available in an aqueous solution. Many people using this chart for mineral absorption manipulation have run themselves into very real trouble occasionally resulting in plant death. It is important to know how pH affects nutrient availability. If you look at figures one, you will see the different availability of the elements for hydroponics and soil. From experience, you see that in hydroponics it is preferable to have your pH in the range of 5.3 to 6.3 for maximal nutrient availability and that 5.5 is the theoretical ideal.
pH Up and Down: What They Are and How They Affect Your Working Solution
pH up and pH down are acids and alkalis that you add to adjust or correct the pH of your working solution. The most common acid for pH down is phosphoric acid and the most common alkali is potassium hydroxide.
When adding these elements to your solution they liberate either hydrogen or hydroxyl ions that either raise or lower your pH. Care should be taken with either as they can or do cause serious burns!
These elements can also affect the dynamics or effectiveness of your solution independent of the pH. In the case of phosphoric acid or pH down there are a number of things to be concerned with.
Firstly, phosphoric acid has a tendency to blow apart chelates reducing the effectiveness of your micronutrients and in some cases causing deficiencies.
You should also be concerned with using flowering additives such as potash or phosphorus boosters, etc.
These additives dramatically increase the levels of phosphorous in your solution and adding too much pH down can increase the phosphorous levels to the point of causing element precipitation (no, you can’t always see it) and more commonly by causing element lockout, usually zinc.This is called phosphorous induced zinc deficiency.
With potassium hydroxide things aren’t quite as bad, but it can cause some minor precipitation and lock out of other elements if too much is added.
A good rule of thumb is to use the least amount of these solutions as possible.
When Should I Adjust My Nutrient Solultion pH?
If we look at figure one, we can see that the ideal pH range for maximal nutrient availability is between 5.3 and 6.3 with 5.5 being the theoretical ideal. This doesn’t mean you should add pH up or down if your solution moves off this mark but rather you only need adjust the pH of your solution if it moves outside of this ideal range; lower than 5.3 and higher than 6.3. If you test your solution regularly, as you should be, you will see that the pH of the solution naturally rises and falls within this range as the plants draw upon different ratios of elements. My advice here and the tendancy used by many commercial growers is to not adjust your pH unless it is higher than 6.3 or lower than 5.3. Nice and Easy.
Which pH Meter Should I Use?
(Single vs Multipoint Calibration)
Now that we have established the importance of checking your pH, what type of meter should you buy to measure it?
When using a pH meter the question of accuracy is often raised and the two types of pH meters commonly sold at your local hydroponics store are single (cheaper) and multi-point (more expensive) pH meters.
The terms single and multi-point are terms used to describe at how many points on the pH scale the meter is calibrated at. This is important because the error margin, when deviating from the calibrated point on a single point meter can be great and when running a fine line on pH management can be a critical factor in your crop’s success.
By using a multi-point meter and calibrating at two or more points, you are correcting the pH slope on the meter, ensuring it is accurate across the range you are using it on.
For example, if you are dealing with pH solutions of around five to 6.5 you would calibrate at pH 4.0 and pH 7.0 ensuring complete accuracy across this range.
The other thing to be aware of, when purchasing a pH meter, is whether it is waterproof or not. Waterproof meters are more expensive but believe me they are worth the extra money. Just drop a non-waterproof meter in your nutrient solution to see how worth it they are!
The Importance of Correct
Calibration of pH Meters
When calibrating a pH meter it is important to ensure that it is calibrated correctly and that the meter is in fact functioning correctly.
When calibrating a multi-point meter and using the multi calibration points, this check is automatic as a by process of the two point calibration sequence.
However, when calibrating a single point meter or only using a single calibration point on a multi-point meter the following checking procedure should be used.
First calibrate to 7.0 or 4.0 depending on which end of this part scale you work in. For example: If you were running your solution at pH 5.0 then you would calibrate at pH 4.0 as this is the closest standard to your working pH.
After calibration at your desired standard or calibration point you then need to check two things; a) that the slope is correct as it could be significantly out and, b) that the pH electrode and microprocessor are functioning properly.
We do this by first calibrating at the desired calibration point then checking in the other calibration standard to ensure that our two checks are correct.
In the case of our working solution of pH 5.0, we would first calibrate at pH 4.0 then check at pH 7.0
When we check at 7.0 it is important to realize that in most instances the pH meter will not read exactly at pH 7.0 but will read between pH 6.7 and pH 7.3. This is fine and means that everything is as it should be.
If the meter reads outside this range then what we would do is first recalibrate to pH 7.0 then calibrate at pH 4.0 then recheck at pH 7. In most cases this corrects the slope and the meter is fine, if you find however that it is still outside the desired check range then immediately take this meter to your local store and have them return the meter to the manufacturer or service center for a detailed check of it’s electrode and circuitry. It is important that you don’t use the meter as it will not be accurate and you can very easily kill your plants! Use a cheap and inexpensive drop tester or paper strips while you wait for your meter to be repaired.
Now that we have covered the basics of pH, lets look at conductivity or nutrient strength.
Exactly What is Conductivity?
Conductivity, as it relates to hydroponics, is the measurement of the electro conductivity of a nutrient solution or how strong or weak the solution is in terms of nutritional content.
Conductivity cannot determine the status of individual elements but can give us an overall picture of element depletion or accumulation. Conductivity is important because it gives us an overall picture of how fast or slow the plant is using gross amounts of mineral elements and can alert us to potential problems or stress that a plant is experiencing. For example; plants take up mainly water when they are stressed, so a rapid increase in conductivity can be an early indication that something is wrong such as insects, fungal attacks etc.
Factors Affecting Correct Conductivity Readings (Or is My Meter Really Broken)
There are normally four things that affect accurate readings on your conductivity meter.
- The batteries are flat or are going flat.
If you need to adjust up your meter more than twice consecutively then the batteries are starting to go flat and need to be replaced. This happens when the voltage curve of the battery first begins to fall (see figure two), not when the batteries are flat and in fact, this starts to happen many months before the batteries actually go flat! (Many months of inaccurate readings!). I have seen more people do damage to their crops this way than any other, as they continually add more and more nutrients to their solution until one of two things happen. They either, a) Come into the store saying that someone is watering the nutrients down as their nutrient is not as strong as before (the lucky ones) or b) They come in complaining that the nutrient has damaged or killed their plants (the unlucky ones).
In both cases this is a cause of not replacing your batteries when you should.
- The electrodes need cleaning. All conductivity meters need regular electrode cleaning to ensure that the readings are accurate because when placed into solution they act as a cathode and an anode whereby one collects particles and one loses them. Regular cleaning ensures the maximum life from your meter.
- The incorrect calibrating standard has been used. (Did you use a PPM standard for a EC meter or vise versa?)
- The meter is indeed broken! If you have checked all the above points and the meter still does not read correctly or will not calibrate then the chances are that meter is malfunctioning.
Well class, the lunch bell has gone and I will see you next issue when we will really start to put our newly found knowledge of pH and conductivity to practical use.