pH Explained
The dictionary definition of pH is potential of hydrogen, the scientific definition is the negative logarithm to base 10 of the molecular concentration of hydrogen ion i.e. pH = -log[H+]. A more simplistic description is that pH is the acidity of a substance.
The pH of a sand, rootzone or top-dressing is measured using a solution, usually distilled or deionised water. Absolutely clean, pure water should have pH 7, however because water can dissolve gases readily the carbon dioxide, present in the atmosphere, increases this to about pH 6. (Carbon dioxide reacts with water molecules to produce a very weak acid.) Water is made up of Hydrogen [H] and Oxygen [O] these atoms combine to form a molecule. They combine because the hydrogen has a single positive charge [H+ ion] whereas the oxygen has a double negative charge [O-- ion]. They combine in the ratio of two hydrogen atoms to one oxygen atom, hence H2O. The chemical equation is shown as: H+ + OH- g H2O
In pure water the H2O molecules are constantly being broken down into H+ + OH- ions and then joining together again to form H2O this is called dissociating. In neutral solutions the relative rates of these two reactions are equal and so the concentration of H+ and OH- ions must also be equal. When a solution contains substances, which affect this dynamic (moving) equilibrium, the properties of the water are affected. In other words if something else is introduced into pure water then it will affect the reaction of the H+ and OH- ions to dissociate. Some atoms or molecules will have a greater affinity than either the hydrogen or oxygen. When a pH test is carried out on a sand, rootzone or top-dressing it is the molecules, atoms and ions present in the material which affects the pH of the water solution. (pH less than 7 acidic : pH greater than 7 alkaline.)
The importance of a pH test result can be somewhat misleading, as mentioned earlier, it only takes a very small amount of a substance to alter the pH. (Remember that even carbon dioxide contained in air will bring the pH of pure water down to about pH 6.) If a small amount of another substance e.g. sand or soil were to be added, then this would affect the pH. This is due to the fact that there is nothing else in the solution to hold it at that pH. This is called the "Buffering Capacity" hence pure water of pH 6 would have an extremely low buffering capacity. It would almost certainly adopt the pH of whatever substance was introduced to it.
Rufford sands are washed so they do not contain silts or clays, neither do they contain chloride salts or carbonates, therefore when a pH test is carried out on a Rufford sand it is the impurities that alter the pH. However if it rained heavily and the rainwater was of pH 5 or pH 8 then this would alter the pH reading of the sand solution.
When looking at sand, rootzone or top-dressing the pH result should be used in conjunction with other chemical analysis information. For example, take two top-dressings, one with a pH of 7.3 and the other with pH of 7.0 Which one would you pick? To make the right choice you would need to know more about the material being offered. Does it contain lime? (calcium carbonate.) Does it contain salts? etc…
Other questions to ask yourself are: What is the pH of my fertiliser? What is the pH of my existing green or pitch? What material is my green or pitch built on?
The ratio of a top-dressing application to an existing rootzone is so small and the buffering capacity of a sandy top-dressing so low that it will probably assume the pH of the green/pitch anyway, so long as the top-dressing does not contain appreciable amounts of solid carbonates. (Did you know that a rootzone, of 10cm depth containing 1% of carbonate, would contain as much as 15 tonnes/ha which is equivalent to three normal agricultural lime applications!)
Michael J Higgins
Technical Manager
