pH Acidbase

 

Chemical change results in the production of new substances; these substances may be helpful or harmful (and sometimes both!). Two types of substances which are very important in everyday life are acids and bases.

Acids are sour, water-soluble substances which are very useful in industry, household cleaning agents, and cooking; some examples are vinegar, vitamin C tablets, club soda, Aspirin, lemon juice and cream of tartar. Vinegar is a solution of about one part acetic acid to 20 parts water – such a mixture of acid and water is called an acidic solution. Lemons and grapefruit have a tangy flavour, because they contain an acid called citric acid. Baking powder contains a dry acid called tartaric acid. Another very important acid is stomach acid (dilute hydrochloric acid) which aids in the digestion of our food. Acids which are not diluted in water are dangerous – they undergo chemical change so readily that they can react with skin and cause burns.

Bases are bitter, water-soluble substances which are also very useful. Examples of bases are ammonia, baking soda, and drain cleaner. Bases are also used in some batteries. The solutions they form with water are called basic or alkaline solutions. Bases are also highly reactive and must be treated with extreme caution, as they also react readily with skin.

You have probably heard of antacids. These substances are bases which are safe to ingest and with react with stomach acid. A chemical change in which an acid reacts with a base is called neutralization. It is called neutralization because equal amounts of acid and base produce a solution which is neutral – neutral acidic nor basic. Antacids are used when the stomach contains too much acid, which irritates the stomach lining.

The pH scale is a measure of how acidic or basic a solution is. This scale ranges from 0 for an extremely acidic solution to 14 for an extremely basic solution. A neutral solution has pH 7. A decrease of one unit on this scale represents multiplying acidity ten times. Most plants prefer a slightly acidic pH 6.0 – 6.5. Correct pH levels are important for the plant to be able to take up all the nutrient supplied in the solution. A too high or low pH is one of the most common problems associated with home hobby growers. These problems show quickly and can be countered quickly and easily!

Most city tap water has a slightly basic pH 7 – 8, the nutrient we mix into a solution is acidic based and will adjust the pH a point or so lower. We may however have to adjust further down using a stable, usable acid such as a dilute phosphoric acid. This is the most common scenario.

Perhaps the source of water we use is acidic (eg. some well and ground waters) and after mixing the nutrient we need to adjust the pH higher. In this case we would use a stable, usable alkali such as a dilute di-potassium phosphate.

A simple method can be used to detect whether a solution is acidic or basic. An indicator is a substance which changes color, depending on whether it is placed in a an acidic solution or a basic solution. There are also electronic meters available to easily check pH levels in solution. They are simply dipped into the solution and give a digital read out.

Perhaps one of the most overlooked aspects of gardening, pH is very important in hydroponic and organic as well as regular “dirt” gardening. pH is measured on a scale of 1-14 with 7 being “neutral”. Acids are lower than 7 and alkalis (bases) are above 7.

This article deals with the pH of hydroponic gardening and the availability of nutrients at different pH levels in a soiless growing medium. Organic and dirt gardening have different levels, so the following chart doesn’t pertain to them.

To be technical, the term pH refers to the potential hydrogen-hydroxyl ion content of a solution. Solutions ionize into positive and negative ions. If the solution has more hydrogen (positive) ions than hydroxyl (negative) ions then it is an acid (1-6.9 on the pH scale). Conversely if the solution has more hydroxyl ions than hydrogen it is alkaline (or base), with a range of 7.1-14 on the pH scale.

Pure water has a balance of  hydrogen (H+) and hydroxyl (OH-) ions and is therefore pH neutral (pH 7). When the water is less than pure it can have a pH either higher or lower than 7.

The pH scale is logarithmic, which means that each unit of change equals a ten fold change in the hydrogen/hydroxyl ion concentration. To put it another way, a solution with a pH of 6.0 is 10 times more acidic than a solution with a value of pH 7.0, and a solution with a pH value of 5.0 would be 10 times more acidic than the solution of 6.0 pH and 100 times more acidic than the solution with a 7.0 pH. This means that when you are adjusting the pH of your nutrient solution and you need to move it 2 points (example: 7.5 to 5.5) you would have to use 10 times more adjuster than if you were moving the pH value just 1 point (7.5 to 6.5).

WHY IS pH IMPORTANT?

When the pH is not at the proper level the plant will lose it’s ability to absorb some of the essential elements required for healthy growth. For all plants there is a particular pH level that will produce optimum results (see chart 1 below). This pH level will vary from plant to plant, but in general most plants prefer a slightly acid growing environment (between 6.0 – 6.5), although most plants can still survive in an environment with a pH of between 5.0 and 7.5.

When pH raises above 6.5 some of the nutrients and micro-nutrients begin to precipitate out of solution and can stick to the walls of the reservoir and growing chambers. For example: Iron will be about half precipitated at the pH level of 7.3 and at about 8.0 there is virtually no iron left in solution at all. In order for your plants to use the nutrients they must be dissolved in the solution. Once the nutrients have precipitated out of solution your plants can no longer absorb them and will suffer (or die). Some nutrients will precipitate out of solution when the pH drops also. Chart 2 (below) will give you an idea of what happens to availability some of the nutrients at different pH levels:

 

Chart 1

pH Values For Different
Hydroponic Crops

(From Hydroponic Food Production
by Howard M. Resh
Woodbridge Press, 1987)

 

Chart 2

Availability Of Nutrients
Available At Different
pH Levels

 pHchartsm1

NOTE:
This chart is for soiless (hydroponic) gardening only and does not apply to organic or dirt gardening.

Plant
Beans
Broccoli
Cabbage
Cantaloupe
Carrots
Chives
Cucumbers
Garlic
Lettuce
Onions
Peas
Pineapple
Pumpkin
Radish
Strawberries
Tomatoes
pH Range
6.0-6.5
6.0-6.5
6.5-7.5
6.5-6.8
5.8-6.4
6.0-6.5
5.8-6.0
6.0-6.5
6.0-6.5
6.5-7.0
6.0-6.8
5.0-5.5
5.0-6.5
6.0-7.0
5.5-6.5
5.5-6.5

 

CHECKING pH

When you are growing hydroponically checking and adjusting pH is a simple matter, it can be a bit more complicated when growing organically or in dirt. There are several ways to check the pH of the nutrient solution in your hydroponic system.

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 sometimes they can be hard to read, because the colors differences can be subtle.

Liquid pH test kits are probably the most popular method to check pH for the hobby gardener. These liquid test kits work by adding a few drops of a pH sensitive dye to a small amount of the nutrient solution and then comparing the color of the resulting liquid with a color chart. The liquid kits are a bit more expensive than the paper test strips but they work very well, and are normally easier to “read” than the paper indicator strips.

The Most high-tech way to check pH is to use the digital meters. These meters come in a huge array of sizes and prices. The most popular type of pH meter for the hobby gardener are the digital “pens”. 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 a LCD display.

The pH meters are very accurate (when properly calibrated) and fast. They need to cared for properly however, or they will quit working. The glass bulb electrode must be kept clean and wet at all times. The pH meters are actually very sensitive volt meters and are susceptible to problems with the electrode.

The pH meters are slightly temperature sensitive. Many of the pH meters on the market have Automatic Temperature Compensation (ATC), which corrects the reading with respect to temperature. On meters without ATC the pH should be checked at the same time of day each time in order to minimize any temperature related fluctuations.

The pH meters usually need to be calibrated frequently, as the meters can “drift” and to insure accuracy you must check calibration often. The tip needs to be stored in a electrode storage solution or in a buffer solution. The tip should never be allowed to dry out.

Due to the fact that pH meters have a reputation of breaking down without warning it is a good idea to keep an emergency back up for checking pH (paper test strips or a liquid pH test kit), just in case.

ADJUSTING pH

There are several chemicals used by the hobby gardener to adjust pH. The most popular are probably phosphoric acid (to lower pH), and potassium hydroxide (to raise pH). Both of these chemicals are relatively safe, although they can cause burns and should never come in contact with the eyes. Most hydroponic supply stores sell pH adjusters that are diluted to a level that is reasonably safe and easy to use. Concentrated adjusters can cause large pH changes and can make adjusting the pH very frustrating.

Several other chemicals can be used to adjust the pH of hydroponic nutrient solutions. Nitric acid and sulfuric acid can be used to lower pH but are much more dangerous than phosphoric acid. Food grade citric acid is sometimes used in organic gardening to lower pH.

Always add the nutrients to the water before checking and adjusting the pH of your nutrient solution. The fertilizer will usually lower the pH of the water due to it’s chemical make up. After adding nutrient and mixing the solution, check the pH using what ever means you have. If the pH needs to be adjusted add the appropriate adjuster. Use small amounts of pH adjuster until you get familiar with the process. Recheck the pH and repeat the above steps until the pH level is where you want it to be.

The pH of the nutrient solution will have a tendency to go up as the plants use the nutrients. As a result the pH needs to be checked periodically (and adjusted if necessary). To start out I suggest that you check pH on a daily basis. Each system will change pH at a different rate depending on a variety of factors. The type of growing medium used, the weather, kind of plants and even the age of the plants all effect the pH variations.