What is pH?

In chemistry, pH is a measure of the activity of the (solvated) hydrogen ion, p[H], which measures the hydrogen ion concentration. Pure water has a pH very close to 7 at 25°C. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline [22].

What pH do plant's need?

For Hydroponics, an optimum root zone pH is between 5.5 and 6.5, to ensure better nutrient absorption [19].
In Aeroponics, this can be further refined to within a 5.8 and 6.2 pH range (Closing the gap by 0.3 pH) [29].

pH is important because it affects availability and absorption of several of the 16 atomic elements needed for plant growth. Maximum absorption of these elements is found at pH readings 5.5 to 6.5. When pH falls below this range, many of the macro elements (N, P, K, etc) have less availability, and absorption of the micro nutrients can reach toxic levels [20].

It's a good idea to let the pH cover a range instead of always adjusting to the exact same pH number. Some nutrients are better absorbed at slightly higher pH readings, while others are absorbed better at lower pH readings [19].

How to check the pH level

When growing Hydroponically, checking and adjusting pH is a simple matter. It can be a bit more complicated when growing organically or in soil.

The pH scale is logarithmic [log-a-rith-mic], which means that each unit of change equals a tenfold change in the Hydrogen/ Hydroxyl ion concentration.

pH Indication Paper / Strips:

Paper test strips are probably the most inexpensive way to check the pH of the nutrient solution. These paper strips are impregnated with a pH sensitive dye which changes color when dipped into the nutrient solution.

The paper strip is then compared to a color chart to determine the pH level of the solution being checked. These test strips are inexpensive, but they can be hard to read, because the colors differences are subtle [24].

1. Merck Millipore - pH-Indicator Paper / Strips

• (109556) pH-indicator paper with a pH scale of <5.4 - 5.8 - 6.2 - 6.4 - 6.7 - 7.0> Price: Request a quote
  

• (109542) pH-indicator strips with a pH scale of <4.0 - 4.4 - 4.7 - 5.0 - 5.3 - 5.5 - 5.8 - 6.1 - 6.5 - 7.0> Price: Request a quote

2. PondCare® Wide Range pH Test Kit (U.S.A.) >> Harrietta Hills.com - pH Test Strips: $7.49

3. Seachem pH Alert: $7.86

pH Liquid Test Kits:

Liquid pH test kits are probably the most popular method to check pH. They work by adding a few drops of pH sensitive dye to a small amount of the nutrient solution and then comparing the color of the resulting liquid with a color chart. While slightly more expensive than the paper test strips, they are easier to read and extremely accurate and reliable [24].

1. Instant Ocean - Ocean Master Test Kit (Includes tests for pH, alkalinity, ammonia, nitrite, and nitrate).
Price: $25.50 AUD (9/01/2013)

2. API - "pH Test Kit": $4.41 AUD
3. API - "pH Test & Adjuster Kit": $10.32 AUD (10/01/2013)

4. Elos - The Aquarium Company - "PH freshwater Test Kit": $14.28 AUD (10/01/2013)

5. Sera - Sera Water analysis >> Sera pH Test: $11.18 AUD (10/01/2013)

6. Hagen - Test Kits Products >> Nutrafin pH Wide Range Test (4.5 - 9.0 pH): $8.41


7. Red Sea - Testing Kit's

• Red Sea "pH/Alkalinity Test Kit": Amazon.com: $21.67 / eBay: 18.13 AUD / Aquatic Addict: $26.95 AUD
  

(For macro elements N, P, K)

• N = Red Sea "Nitrate Pro test kit (NO3): $49.95

• P = Red Sea "Phosphate Pro test kit (PO4)": $49.95
  

• K = Red Sea "Potassium Pro test kit (K)": $59.50
  

Electric pH Testing meters:

The Most high-tech way to check pH is to use the digital meters. The most popular type of pH meter for the hobby gardener is the digital pen. These pens are manufactured by several different companies and are very handy and easy to use. You simply dip the electrode into the nutrient solution for a few moments and the pH value is displayed on an LCD screen.

Many of the pH meters on the market have "Automatic Temperature Compensation" (ATC), which corrects the reading with respect to temperature. It is a good idea to keep an emergency backup for checking pH (paper test strips or a liquid pH test kit), just in case [24].

1. Oakton - EcoTestr pH 2 - (A low cost pocket pH meter, with a pH Resolution of 0.1 pH) [21]: $55.80

2. Milwaukee Waterproof Pocket Testers - pH 55: $50.00

3. Hanna PH Checker and Hanna Primo TDS

• Hanna® Instruments PICCOLO® Stick pH Tester (HI 98113): $188.81

• Hanna® Instruments PICCOLO® Stick pH Tester (HI 98111): $165.00

4. ISFET pH Tester H130: $425.74 AUD

5. Testo 206 pH Meter For Liquids - (Accuracy ±0.02 pH): $166.00

6. Green's Horticulture - "pH & EC Control" $50.20 AUD

7. eBay - "Bluelab pH and PPM Pen Meter Combo Kit" $143.49

Note: Before measuring pH, ensure that the nutrient is well stirred and that the sampling container is clean [23]. Also, when trying to adjust the pH of your root zone, make sure to do all the changes slowly [19].


References:

[19] Grow Weed Easy.com - "How Do I Check and Adjust The pH of Water?" ~ www.growweedeasy.com/how-do-i-…
[20] General Hydroponics.com - pH Dynamics And Adjustment ~ generalhydroponics.com/site/in…
[21] 420 Magazine - "How Can I Raise or Lower the pH of my Soil Mix?" ~ www.420magazine.com/forums/how…
[22] Wikipedia - PH ~ en.wikipedia.org/wiki/PH
[23] Flair form.com - "pH" ~ www.flairform.com/hints/ph_adj…
[24] Simply Hydroponics and Organics - "pH" ~ www.simplyhydro.com/ph.htm
[29] Marijuana Growig.eu - "A guide to Aeroponics" ~ www.marijuanagrowing.eu/guide-…