The most popular method of expressing acidity and alkalinity is as a mathematical ratio between the amounts of hydrogen ion, or acid element, and the hydroxyl ion, or alkaline element. The ratio is figured on what is known as the pH scale, which runs from an initial point just above o to 14. When the numbers of each ion are equal in the solution, the pH reading is 7,' or neutral. This is the reading for pure water, in which acid and alkaline ions are produced by the dissociation of water molecules.
For every ten-fold increase in the concentration of one or the other, of the acid and alkaline elements, the pH reading will change one number. If the alkaline element is increased, the number will be higher; if the acid is increased, the number will be lower on the scale.
The chemicals used in nutrient solutions are salts, products of reactions between an acid and alkali. The change in reaction brought about by their addition to the liquid depends upon the quantity and the character of their constituents. The major chemicals do not contain hydrogen or hydroxyl ions but cause reaction changes by dissociating water molecules. One gram of hydrogen ions is equal in effect to seventeen grams of the hydroxyl ions. These are the amounts contained in ten million liters of pure water and give a neutral reaction, or reading of 7, on the pH scale. From the table of molecular weights, you can weigh out the mixture which will contain 10 grams of hydrogen ions and 170 of hydroxyl ions. Either of these amounts, if added alone to pure water and completely dissociated, will change the reaction one point. But they will not have the same effect on a more concentrated solution.
This is because, the more concentrated the solution, the smaller is the number of molecules in the added chemicals which will split up into ions. Returning to the example of pure water, if enough acid is added to increase the hydrogen ion content from one to ten grams per ten million liters, the reading will change from 7 to 6 on the pH scale. On the other hand, if enough alkali is added to increase the hydroxyl content from 17 to 170 grams, the pH reading would rise to 8. Thus, as the reading proceeds from 7 downward, the solution becomes increasingly acid, and from 7 upward, increasingly alkaline.
Optimal Range of Ph for Plants & Roots
The most favorable pH reading of the nutrient solution is from 5 to 6.5. This is for plant growth in general. For rooting of most crops, the best range is from 5 to 6. However, if conditions are favorable for good root development, crops will grow well in neutral or even slightly alkaline solutions; also, some plants thrive in more acid solution than pH 5.
Unless the pH is so alkaline or acid that living tissue is affected and the root points damaged, reaction bears principally on the availability of iron. Conditions leading to unavailability of this element are treated fully in the chapter on Symptoms.
It is enough to point out here that, if conditions are favorable to root growth, the plants can absorb needed iron even though the solution is slightly alkaline.
Determining the Ph
The pH of the nutrient solution can be determined by the use of two chemical indicators: brom cresol green and phenol red. These are used as liquids and can be obtained from chemical supply houses. The first tells you if the solution is too acid, and the second if it is too alkaline. To take a reading, fill a glass about one-fourth full of solution. Then add one drop of brom cresol green from an eyedropper. If the indicator turns yellow or brown, the nutrient solution is too acid. If it turns dark blue, it is not too acid but may be too alkaline.
Your next step is to add a drop of phenol red. If the liquid turns yellow with this indicator and blue with brom cresol green, the reaction is favorable. Charts showing the various shades of the colors mentioned will prove useful to you. After you have acquired sufficient skill, phenol red is the only indicator that will be needed, as you can gauge the reaction by the rate of change from red to yellow.
Phenol red gives a red color in neutral and alkaline solutions and yellow in acids. If a drop of phenol red is added to a solution with a pH of 4 or greater acidity, the change of color to yellow is so quick that it cannot be seen. At pH 5 to 6.8 a fringe of red is visible, when the drop first hits the solution, gradually changing to yellow — the greater the acidity, the quicker the change of color. When phenol red turns yellow, the reaction is acid, and if the change from red to yellow can be observed, the reaction is favorable to plant growth.
Indicators are also available as impregnated slips of paper, of which a small piece is dropped into the solution. Indicators that only show whether a solution is acid or alkaline are in- adequate to give all the information needed. You should know how acid or alkaline the solution actually is and at what pH it changes color.
Buffer Action of Carbonates
Character and quantity of the solutes in the water used, will be of importance to reaction. Most of the natural waters have an alkaline reaction due to the presence of carbonates and bi- carbonates. Large amounts of acid will be needed to overcome the effect of these substances, for they have what is known as "high buffer properties"; that is, high resistance to change in reaction. Thus, more acid is needed to change the ptt when alkalinity is due to carbonates and bicarbonates than when it is caused by the presence of other salts. When dilute solutions are being used, the reaction will change more quickly with the addition of acid.
Determination and change of the reaction are often regarded as extremely difficult matters to handle. Actually, if you know how to use your indicators and add the chemicals properly, it is very simple.
Though an important feature, the nutrient solution is not the most important. Many factors remain to be considered and will be treated in the ensuing chapters. Particular attention should be paid to the sections on Aeration and Iron. Remember that natural growth requires the coordination of many factors and that growth in hydroponics is natural growth.