What is pH all about?

Maintenance of the proper pH in the irrigation stream helps prevent chemical reaction of fertilizers in the irrigation lines. High solution pH can cause line clogging precipitates to form. Correct pH level ensures that phosphates remain in the more soluble hydrogen form and that minor elements are more available for plant uptake. Minor element deficiencies can result from high pH. What is pH? The abbreviation pH stands for "Potential Hydrogen" and refers to the concentration of positively charged hydrogen ions (H+) relative to negatively charged hydroxyl ions (OH-), in a substance. Hydrogen ions are acidic in nature while hydroxyl ions are basic or alkaline in nature. The pH scale, which measures the concentration of hydrogen ions runs from zero to 14 with 7 being neutral (equal number of H+ and OH- ions). Any value below 7.0 indicates acidity and any value above 7.0 indicates basic or alkaline conditions.

An acid such as Nitric Acid (HNO3) or phosphoric acid is a substance which, when added to water, breaks apart or "ionizes" to provide hydrogen (H+) ions. A base ionizes to provide hydroxyl ions (OH-). The terms strong and weak applied to acids indicate the degree of ionization they undergo. A strong acid such as hydrochloric in a dilute solution undergoes 100% ionization whereas a weak acid like acetic exhibits only 4% ionization. In North America most water supplies are alkaline. In addition plants tend to make the root environment more basic. When a plant takes up nitrate ions, which are negatively charged, the roots shed negatively charged hydroxyl ions to maintain electrical balance. This raises the pH of the root environment. When positively charge ammoniumions are taken up, positively charged hydrogen ions are shed, acidifying the root environment.

When deciding on a pH correction program, the pH of the water is not the only thing to consider, Buffering capacity (the ability of the water to resist pH change) has to be taken into account. The buffering capacity of water is related to the amount of bicarbonate (usually calcium bicarbonate) that is present. If there is a lot of bicarbonate present, much more acid is needed because of the reaction that takes place between the bicarbonate and the acid. The acid that Is added initially to water containing bicarbonate is Used up in this reaction. The hydrogen ion from the calcium bicarbonate molecule (above) and the hydrogen ion from the nitric acid molecule (above) combine to form water. This means that the hydrogen ion from the acid is locked up and therefore does not lower the pH. Once sufficient acid has reacted with the bicarbonate present, any additional acid added will contribute hydrogen ions to lower the pH; therefore, the more bicarbonate that is present, the more acid wit) be needed before free hydrogen ions are available to lower the pH.

Many fertilizer components are acidic and lower the pH, while others are alkaline and raise the pH. To achieve the proper pH with fertilizer alone, however, would require water with little to no bicarbonate present, When nitric acid ionizes, it provides nitrogen ions as well as hydrogen ions; phosphoric acid provides phosphorous. Which acid should you use? Consider the following: Plants use more nitrogen than phosphorous. In addition, high phosphorous levels can cause the formation of calcium and magnesium phosphate; a hard, scale-like precipitate than can coat, and eventually plug, feed lines. It may, according to speculation, even coat plant roots, blocking air passages. Further, high phosphorous levels can hinder the uptake of some minor elements, We believe phosphoric acid should be used only if the amount of acid required is constant and the level of phosphorous provided by the phosphoric acid will not create excessive phosphorous levels in the solution. If you are using a "proprietary" blended fertilizer such as 20-20-20, phosphoric acid should likely not be used at all, as the levels of phosphorous in the blend are usually at an optimum level.

It seems nitric acid is the better choice in many instances. Bear in mind that nitric acid is more dangerous to use. Nitric acid is highly corrosive and produces poisonous fumes when exposed to the atmosphere in the concentrated form. A spray mask, eye protection and rubber gloves should be worn when handling it. Nitric acid is much less hazardous once diluted. Concentrated nitric acid should be stored in sealed glass or stainless steal containers.

Potential Hydrogen can be important for more than fertilization and irrigation. It also plays in the use of some pesticides. Alkaline water can break up the molecules of certain pesticides in a process called alkaline hydrolysis, reducing the activity of the chemical. This problem is heightened if the tank mix will be sitting for any length of time prior to application and if ambient temperatures are high. There are other greenhouse compounds rendered more effective if the water added to is pH corrected before hand.

Finally, at the end of a crop, low pH can be used to clean irrigation lines and dripper tubes of any scale that might have formed. This is done by charging the lines with a low pH solution (3-4) and letting it steep overnight. The lines should then be flushed thoroughly before being put back into regular use.