Hypochlorite in Horticulture

Almost everyone has used hypochlorite before at one time or another. Better known as bleach, this common elixir is quite effective as a broad spectrum germicide when used correctly. It is often used in the cleaning of horticulture equipment such as buckets, trays and pruners, yet research has shown that when dosed properly, bleach can also control insect pests and fungal pathogens. Read on to learn more about chlorine chemistry, the mode of action and basic calculations to consider when using hypochlorite in the garden.

Chlorine Chemistry

When added to water, sodium hypochlorite (NaOCl, liquid bleach) disassociates to form a sodium ion (Na+) and a hypochlorite ion (OCl-). The hypochlorite ion will react with a proton (H+) in water to form hypochlorous acid (HOCl), which is regarded as a more effective germicide than hypochlorite.

The ratio of hypochlorous acid to hypochlorite is dictated largely by the pH of the solution. As the pH decreases or becomes more acidic, there is a higher proportion of H+, which means a higher rate of hypochlorous acid relative to hypochlorite. At a pH of 5 to 6, the ratio of hypochlorous acid is roughly 85 to 98%.

The problem is that bleach is basic and has a high pH. Therefore, to have a bleach solution with maximum germicidal action, the pH must be lowered with an acid like hydrochloric or citric acid. Reducing the pH of sodium hypochlorite below 3 favors the formation of chlorine gas.

Not only are you physically removing the germicide by doing this, but chlorine gas is a respiratory irritant and at a higher concentration it is toxic to your respiratory system. Therefore, you must know what you’re doing and have the appropriate equipment and ventilation in place prior to executing this chemistry experiment. If this is not possible where you are, a properly diluted bleach solution will still work, but it won’t be as effective.

We can measure how much chlorine is in a solution by monitoring both free and total chlorine. Free available chlorine (FAC) represents the disinfectant fraction and includes chlorine gas, hypochlorous acid and hypochlorite ions, whereas total chlorine is the sum of the FAC with other compounds that combine with chlorine.

These compounds that combine with chlorine are highly persistent, reduce the efficacy of the chlorine and can pose a human health risk. Therefore, when making chlorine solutions, use filtered water such as deionized or reverse osmosis treated water since regular tap water will reduce the effectiveness of the free chlorine.

Likewise, chlorine in a solution will readily react with debris, including stems, leaves and soil, which also reduces the efficacy. Minimizing exposure to organic compounds will prolong the effectiveness of a bleach solution. Also avoid sunlight. Chlorine is readily degraded by ultraviolet radiation.

On contact, free chlorine causes degradation to cell membranes, proteins and nucleic acids. What does this mean? Basically, free chlorine breaks apart chemical bonds necessary for normal functioning, which leads to cellular death.

While bleach is a broad spectrum germicide, there is high variability in both plant and microbe sensitivity. That is to say, what might kill a fungal spore may also damage the plant, which defeats the initial purpose. For instance, most bacteria are quite sensitive to bleach while fungal spores are not. Lastly, chlorine will only kill microbes or certain insects that come into direct contact with the active chemical. To this end, concentrations ranging from 50 to 200 ppm NaOCl have proved effective.

Hypochlorous acid to hypochlorite ratio

To maximize the ratio of hypochlorous acid to hypochlorite, the ideal pH should be between 5 and 6. At this pH, you will minimize the formation of chlorine gas that is financially wasteful and dangerous to your health.

For disinfection purposes, you need to maintain a free chlorine concentration of 2 ppm or above. I personally like to maintain 5 ppm, but the former concentration is sufficient. Higher levels of 100 to 300 ppm sodium hypochlorite have been used to control fungal pathogens; however, I would advise caution as tissue death (spotting) can occur depending on the sensitivity of the plant. Always apply a small amount of bleach to the target area prior to broadcasting over the whole plant.

How can we obtain this small concentration with a regular 5.25% bleach solution? As a general rule, 1 tsp. per gallon or 1 tbsp. per gallon will yield a total chlorine concentration of 65 and 200 ppm, respectively. This concentration should be suitable for most applications of bleach.

Bleach (sodium or calcium hypochlorite) is an effective, cheap, broad spectrum germicide and sanitizing agent. When used properly and changed frequently, the spread of diseases and insects can be minimized.

Even though bleach is a better germicide at a low pH, caution should be exhibited prior to this manipulation as too much acid will produce chlorine gas, which is wasteful and harmful to you. Bleach is no longer just for white clothes anymore!