pH is the term used to assess the acidity or alkalinity of a solution. This acidity or alkalinity is determined by measuring the concentration of Hydrogen ions in the solution. This normally falls between 10 to the power 0 and 10 to the power 14, gram-equivalents per litre. In order to simplify this, a scale of values between 0 and 14 has been adopted.
This scale is a measurement of how strongly the electrical charges hold the atoms and molecules of substances within the solution together. The higher the concentration of positively charged ions, the lower the value is on the scale and the higher the concentration of negatively charged ions the greater the pH value.
Hydrogen (H+) has a positive charge while the hydroxides (OH-) have a negative charge. Pure water has a value of 7.0 so it is easy to see that water must be H-OH or more commonly H2O.
The decimal points on the scale are very important because each whole number is approximately 10 times greater (or less) than the next whole number. So pH 2.0 is 1000 times stronger than pH 5.0. The greater the concentration of Hydrogen ions in the solution the more acidic it is said to be and the lower its value on the scale. The greater the number of hydroxide ions the greater the alkalinity (or basicity) it is said to have and the higher its value.
If we look at the structure of the pH scale, we can see that it goes from 0 (very strong acid) to 7 (neutral) and then to 14.0 (very strong Base or Alkali). If we mix an acidic solution with an alkaline one, providing that the positions on the scale are equidistant from the neutral value of 7.0, we will end up with a neutral solution. This is because the positive charges will be cancelled out by the negative ones. If however we use differing positions on the scale, then the resulting imbalance will give us a solution with either acidic or basic properties depending on which side had the greater distance from the neutral value.
But Why Is This So Important?
All of the chemicals in the solution have differing electrical charges because each of them is made up of different combinations of elements and ionic values. As they are all competing for the exchange of charged particles, a huge electrical battle is constantly raging within the solution. This constant exchange of positive and negative charges surrounds the plantís root system and it is this that allows it to absorb the vital nutrients needed for its growth.
You can think about the chemical battle being a bit like a moving, 3D, jigsaw puzzle, with the positive and negative charges all having to combine in the correct shape and order. The plant can only absorb those bits that fit into its own bits, like a lock and key. As the Hydrogen ion concentration changes, so the jigsaw bits alter and no longer open the lock. In fact the plant itself will, at times, alter its own internal cellular pH in order to either slow down or speed up certain enzyme reactions.
It is vitally important that the Hydrogen ion concentration of the nutrient used in the hydroponic system matches the plantís own internal concentration as closely as possible, otherwise this chemical exchange cannot take place. The main chemicals within the solution, Sodium (Na+), Phosphorous (P+), Calcium (Ca+) and Potassium (K+), together with all the other elements will affect the efficiency of each nutrientís absorption through the root walls.
Different species of plant prefer different pH values. The three main things that affect the Hydrogen ion concentration a plant prefers are:
1 The pH of the water used.
Your water will not be pure and so will contain charged ions either from deliberately introduced contaminants or from environmentally absorbed ones like Calcium Carbonate from passing through limestone and Sulphurs from acid rain.
2 The growing medium that you are using.
Rockwool is over pH 7.0, Peat Moss below 6.0 and hardened expanded clay is 7.59.
3 The nutrient you are using.
Nutrients can be mixed in lots of different ways, forming various combinations of elements and so giving a wide variation in pH. Because these chemical combinations behave in different ways they give up their elements to the plant at differing pH values. Therefore the nutrient preferred by the plant determines which the best pH value for that species is.
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