pH for the Gardener.

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Two Buffer solutions and pH meter.
Click the image to enlarge.


On this page is a simplified explanation of pH for the Gardener, whether gardening in soil or gardening hydroponically. The pH of your soil or nutrient solution governs how well your plants will grow. If you cannot effectively control your pH, plant growth is likely to be poor.

The pH scale runs from 0 (highly acidic) to 14 (highly alkaline), with 7 being the mid point which is considered to be neither acidic nor alkaline.

In discussions on pH, it is useful to divide both acids and alkalies into three classes: Strong, Medium strong and Weak.

(1) The pH scale can measure only low concentrations of Strong acids and alkalies. Higher concentrations are way off the pH scale.
Examples of strong acids are: Sulphuric Acid, Hydrochloric Acid and Nitric Acid.
Examples of strong alkalies are: Sodium Hydroxide and Potassium Hydroxide.

(2) The pH scale can measure medium concentrations of Medium strong acids and alkalies.
Examples of medium strong acids are Phosphoric Acid and Sulphamic Acid (a powder).
Examples of medium strong alkalies are Sodium Carbonate, Sodium Metasilicate and Ammonium Hydroxide.

(3) The pH scale can measure high concentrations of Weak acids and alkalies.
Examples of weak acids are Boric Acid and Carbonic Acid.
Examples of weak alkalies are Sodium Bicarbonate and Milk of Magnesia.

Some examples will clarify the situation somewhat. The pH figures are only approximate.

Pepsi Cola is said to contain about 2% Food Grade Phosphoric Acid, as well as the Carbonic Acid common to soft drinks and can be expected to have a pH as low as 3.0 to 3.5. Being a cola, it contains Caffeine as well, which has no effect on pH.

Regular soft drinks rely on Carbonic Acid for their fizz, and can be expected to have a pH around 4.5. This is not as acidic as Pepsi above which uses Phosphoric Acid for an extra "kick".

Milk is a mild alkali and can be expected to have a pH about 8.0. Milk can be used to settle an upset stomach. Milk of Magnesia can do the same job and has a pH of about 8.0 to 8.5

Clothes washing detergents in washing machines have a pH ranging from about 9.0 to 10.0, and this alkalinity is typically supplied by Sodium Carbonate with or without Sodium Metasilicate.

The pH Scale

The pH scale itself is logarithmic, which means that each full point on the scale has a ten times relationship with points either side of it. Common logarithms are always taken to have a base of 10, which explains this ten times relationship.

For example, on the acidic side (pH 7.0 down to 0.0) a pH of 4.0 is ten times as acidic as a pH of 5.0, and a pH of 3.0 is 100 times more acidic than a pH of 5.0, and a pH of 2.0 is 1000 times more acidic than a pH of 5.0.

On the alkaline side (pH 7.0 up to 14.0) a pH of 9.0 is ten times more alkaline than a pH of 8.0, and a pH of 11.0 is 1000 times more alkaline than a pH of 8.0

Measuring pH

For the Hydroponic gardener, it is recommended that you purchase a pocket type pH meter as shown on the left. You will need two Buffer Solutions for calibration, one at pH = 4.0 (coloured Red) and one at pH = 7.0 (coloured Green). Calibration instructions are included with the pH meter. To use, collect a sample of nutrient solution in a plastic or glass jar and turn the pH meter ON, then dip it in the jar and stir slowly. The pH is read when there is a steady reading. Turn the pH meter OFF after use.

For the soil gardener, it is recommended that you purchase a pH test kit from a plant nursery or garden centre. This is much cheaper than buying a pH meter. It contains directions for use. Usually, a sample of soil is placed on a white plate and a white powder is sprinkled on to the soil, then a buffer solution containing a colour indicator is dribbled on to the white powder and the colour compared to the colours on a colour chart in the kit. Select the colour that corresponds to your sample and read off the pH directly.

If the soil gardener already has a pH meter, here is how to measure the pH of a soil sample. Place a sample of soil in a glass tumbler and add an equal volume of water to make it disperse well. Crush any lumps and stir well. Take a pocket handkerchief or a similar fine fabric and cover the end of the pH meter with it, turn the pH meter ON, then carefully dip it into the soil dispersion and stir slowly. The pH is read when there is a steady reading. Remove the handkerchief from the pH meter, turn it OFF, and wash the pH meter under running water until it is clean. Wash the handkerchief as well.

Buying a pH meter

For a wide range of pH meters suitable for the home hydroponic grower and the commercial hydroponic grower, click on the link below for a very reliable supplier situated in Brisbane, Australia. When you order your pH meter, be sure to order two fresh buffer solutions ... pH 4 (coloured red), and pH 7 (coloured green).

pH meters, buffer solutions and conductivity meters for both home and commercial hydroponic growers. This company stocks a wide range of both pH and conductivity meters and will deliver your order anywhere in the world.


The Importance of pH to the Gardener

Many fertilizers are salts of the acids mentioned above, and their pH values in solution vary considerably. The pH of the soil or nutrient solution governs the availability of the Trace Elements such as Iron, Manganese, Boron, Copper, Zinc and Molybdenum. For example, Iron from Ferrous Sulphate becomes unavailable when the pH goes higher than about 7.5 .... it turns to rust, which is not soluble, and is unavailable for plant use. For hydroponic gardeners the Author recommends that the nutrient solution be kept at a pH of about 6.4 and the use of Iron Chelate (the Iron salt of Ethylene diamine tetra-acetic acid) as the sole source of Iron. If you must use Ferrous Sulphate, you would need to add some every two or three days to keep some Iron in solution. The rationale is that Ferrous Sulphate oxidises to rust in two or three days, depending on the pH.

Notes on solubilities.

The solubilities in water of fertilizers vary considerably. For example, Boric Acid is not very soluble in water and its concentrated solution has a pH of about 4.5 to 5.0, which is quite weak. Because of the poor solubility of Boric Acid in water, we use its Sodium salt known as Borax (Sodium Tetraborate), which is far more soluble.

Both Calcium Nitrate and Potassium Nitrate are very soluble in water and are endothermic (they take heat out of the atmosphere and are therefore cool to touch), and dissolve much better in hot water than in cold water. As a matter of fact, most soluble solids dissolve better in hot water than they do in cold water. Gases that are soluble in water dissolve much better in cool water than in warm or hot water. If you are aerating your hydroponic system you must keep it as cool as possible to retain dissolved Oxygen.

Mono Ammonium Phosphate (MAP) is not as soluble in water as Calcium Nitrate or Potassium Nitrate, so relatively more water must be used to get it into solution.

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