Issuu on Google+

pH Management

for Optimal Results by Bob Taylor

This article explains how to keep the pH of nutrient solutions between 5.0 and 6.5. This helps make sure all nutrients are available for root up-take, and minimizes the risk of plumbing blockages.

Optimum pH for Nutrient Solutions For nutrients to remain dissolved and, therefore, available for uptake by roots, it is critical to maintain the pH between 5.0 and 6.0 - with an absolute maximum of 6.5 (figure one). High pH values, or those above 6.0, are to be avoided more than low values of 4.5 – 5.0. The effect of low pH upon the stability of nutrients is relatively insignificant. The precise pH at which precipitation of macro-nutrients starts is determined by the combined concentrations of calcium and sulphate. Except for fertilizers low in calcium and sulphate this problem commonly occurs at pH 6.5 where the net* EC is 2.5 mS, or pH 7.0 for 1.5 mS solutions. Hence, to avoid precipitation, higher nutrient concentrations generally must be held at lower pH values. *Assume make-up water has nil EC. 148

Maximum Yield USA | April 2011

In spite of this precipitation problem, some references advocate pH values well above 6.5 for some plant varieties - conditions which risk depleted concentrations of the above mentioned elements.This is incorrectly justified by quoting the chart in figure two as proof.

pH recommendation of 6.2 - 6.3? Although this is a popular recommendation, it has no scientific basis. It appears to have gained mythological status from the early days of hydroponics when the only cheap means of measuring pH was the common ‘bromothymol blue’ pH indicator used for testing fish tank water. Interestingly, the lowest pH value able to be determined by that indicator is about 6.2. Hence, this value has unfortunately become an entrenched recommendation in some sections of the hydroponic industry.


ph management for optimal results

Figure one: This is what can happen to a working nutrient solution when pH is above 7.0: Calcium, sulphate (and the trace elements copper, iron, manganese and zinc) can precepitate and become unavailable to the roots, and cause plumbing blockages. This freshly made ‘bloom’ nutrient solution (EC 2.5mS) was at pH 7.5 for less than one hour. To help prevent this, use a nutrient that possesses a high ph buffering capacity.

Adjusting Nutrient pH The working nutrient pH should be checked at the following times: 1. When working nutrient solutions are first made. 2. After the addition of top-up water or additives - especially if they are highly alkaline. 3. In re-circulating systems, pH should be checked on a daily basis because the uptake of water and nutrients causes pH to change. It is best to adopt a pH maintenance regime that prevents pH from getting too high. If pH is too high for a long enough period of time, the resultant precipitate usually cannot be redissolved (figure one).

TIP

If your pH tends to continuously rise (the most common trend), then at each adjustment reduce the pH to about 5.0 using a pH down product. This will give you a much larger pH “safety” margin than if adjusting to, for example, 5.8. If pH tends to continuously fall, at each adjustment increase the pH to about 6.0 using a pH up product.

How to Minimize pH Fluctuation 1. U  se a nutrient brand that is highly pH buffered, particularly when using highly alkaline water. 2. Supply at least two gallons of nutrient for each large plant. Failure to do this will magnify pH (and EC) fluctuations, especially during hot and dry weather where water uptake and evaporation are excessive. Note, to avoid excess water uptake and evaporation; keep air temperature below 86°F and relative humidity above 50 per cent.

How to Adjust pH Step 1. Measure the pH: Use either a liquid pH indicator or an electronic pH meter. Before measuring the pH, ensure that the nutrient is well stirred and that the sampling container is clean. Step 2. Choosing a target pH: Note that it is inconvenient and unnecessary to hold pH at a single point value. Therefore, choose a target pH that minimizes the amount of pH maintenance: Step 3. Adjusting the pH: Add a small amount of pH down or up product*. Then stir well and check pH. Repeat this process 150

Maximum Yield USA | April 2011


until the target pH is achieved. *Important: Pre-dilute the dose into one quart (or at least 100 fold) of water before adding to nutrient, then rapidly stir the nutrient as you add this mixture. Failure to do this may cause permanent precipitation of essential nutrients. Also, if accidental overdosing to above 6.5 occurs, reduce the pH back to below 6.0 as quickly as possible using pH down.

Handy Hints for Adjusting Nutrient pH

1

Add “high pH” (alkaline) additives before adding nutrient: Most additives will affect nutrient pH at least slightly. The best technique to adopt with those that elevate pH significantly is to add them to the water and adjust the pH down to 6.0 prior to adding the nutrient. The less preferred but simplest alternative is to pre-dilute the additive in a separate volume of raw water. Then once this solution is added to the nutrient solution, quickly lower the pH to below 6.5. Note that a white cloudy precipitate (calcium sulphate) may form when the pre-diluted additive initially merges with the nutrient solution. However, because the initial particle size of the precipitate is small, it will usually re-dissolve if the pH is immediately re-adjusted.

A

B

Figure four: This is what can happen when an undiluted high pH additive is added to the working nutrient solution (left). Unless pH is quickly corrected to below 6.0-6.5 the precipitate will remain (right).

working nutrient. Step 2. Add a few drops of pH indicator (figure five ‘a’). Step 3. While stirring this solution, measure the volume of acid required to turn this solution yellow – figure five ‘b’ (Yellow indicates a pH of 6.0 with most broad range liquid indicators). Step 4. Multiply the volume of acid by the volume of nutrient in your reservoir. That calculation will give you the volume of acid required to adjust the entire volume down to pH 6.0, for example.

Measuring pH with ‘Indicators’ pH indicators are undoubtedly the simplest and most reliable method of measuring nutrient pH. Although they will

not distinguish between, for example, a pH of 5.2 and 5.3, wide range indicators with good colour resolution can be: •  fast and user friendly •  extremely accurate and reliable •  economical In comparison, pH meters require constant up-keep (i.e. cleaning, calibrating and correct storage), but even then may not give reliable readings. pH indicators work on the principle that the color produced by the particular dye used in the indicator formulation is dependant on the pH of the solution (figure six). Experience shows if you are aiming to adjust pH to 5.5 (orange) then an accuracy of +/- 0.2 is achievable. Because of their fundamental accuracy, reliability

2

Do not pre-adjust pH of raw water: Note that the pH values being discussed here are the values of the working nutrient solution - not your make-up water. Unless your make-up water has a high alkalinity, do not bother attempting to adjust its pH prior to the nutrient being added. If you attempt this procedure you will typically get wild pH swings either side of the pH target without ever landing on the target value.

3

Estimating the volume of acid (especially for larger systems): Step 1. Take a one quart subsample (or known volume) of Maximum Yield USA | April 2011

151


ph management for optimal results

Figure five: pH indicator’s are useful for determining how much acid needs to be added to the nutrient reservoir.

and easy of use, wide range pH indicators are the preferred method for measurement of pH in nutrient solutions. Note that pool and aquarium pH indicators are usually not suitable because unlike broad range indicators, they do not operate below pH 6.0.

Taking pH Readings

Total volume of nutrient ÷ Volume of sub-sample x volume of acid

TIP If this volume is very small (most likely if a highly concentrated acid is used), to ensure accuracy you will require the use of a finely graded pipette. A better method is to allocate a portion of acid specifically for conducting this calculation and dilute it by a known amount - for example 10-fold. Ensure to compensate for this dilution when calculating how much of the concentrated acid to add to the reservoir.

Step 1. Before measuring the pH ensure that the nutrient is well stirred, especially after pH up or down products are used. This is one of the most common mistakes made when testing pH (or conductivity). Also, ensure that the sampling container is clean. Step 2. Using the sampling vial, remove a small sample of nutrient from the nutrient reservoir, add a drop of the indicator, mix, and then compare the final solution colour with those on the coloured reference chart (figure six). Step 3. If the pH is not between 5.0 and 6.5, adjust it immediately.

Measuring pH with pH Meters

Figure six: This is the colour range produced by a wide range pH indicator within the optimum pH range 5.0 to 6.5. Note the ease with which pH change can be detected.

152

Maximum Yield USA | April 2011

pH meters employing a glass electrode are useful for precise pH measurement in nutrient solutions but require frequent calibration, proper storage and handling to ensure accuracy and reliability. The principle on which such meters operate is based on the fact that when glass of a certain composition separates two aqueous solutions having different hydrogen ion concentrations, a voltage is developed between the two faces of the glass. The electronic meter is simply a very sensitive voltmeter which measures that voltage but is calibrated in terms of pH units instead of volts.


Obtaining pH Readings Step 1. Make sure the meter is calibrated. Step 2. Remove a ‘representative’ sample from the nutrient reservoir (figure seven): • Stir the nutrient thoroughly prior to sampling. • Ensure the sampling container is clean. Step 3. Rinse electrode in distilled water before immersing in the sample.Wait a few minutes before switching the meter on and recording the pH.Wait longer if the sample’s temperature is significantly different from 77°F. Step 4. If the pH is not between 5.0 and 6.5, adjust it immediately. Step 5. When complete, rinse the electrode

with distilled water. Store the electrode in a proper storage solution when not in use. MY

Figure seven: Thoroughly stir nutrient reservoir before sampling. Then leave the electrode in the sample for a few minutes before switching the metre on and taking the measurement. Do not immerse the electrode deeper than ~20mm.

Maximum Yield USA | April 2011

153


PH Managment