Chlorine

Chlorine in your plants? Yes. Well, sort of.

​Before you go grab a jug of bleach, you need to know that laundry bleach most commonly refers to a dilute solution of sodium hypochlorite. This is NOT something you want anywhere near your plants. In fact, high concentrations of chlorine are fatal to all living things. It was even used in World War I as the first chemical warfare agent. 

We are not quite ready to throw the book at chlorine, however. We need to know that chlorine is an element, much like copper or nitrogen, used by plants as food. You don’t hear much about it because plants only need it in tiny amounts. Once called trace elements, minerals used in such small amounts are now referred to as micronutrients. The form of chlorine used by plants is called chloride (Cl-).

Forms of chlorine

Chlorine is a highly reactive element. As such, it rarely occurs naturally by itself. Instead, it binds to other, nearby elements. In fact, chlorine will pair with practically every other element in the Periodic Table. Those parings occur because chlorine most commonly exists as an anion, or negatively charged, somewhat unstable atom, called chloride. To stabilize its outer electron field, chloride shares electrons with other elements, creating molecules. Some of these more familiar ‘binary chlorides’ include:

• calcium chloride - a common liming agent
• magnesium chloride - a de-icing agent
• potassium chloride - a common fertilizer ingredient [Berries such as currants are sensitive to this ingredient]
• sodium chloride - common table salt

We all know ‘salting your fields’ ends badly for plants. Unfortunately, it can be difficult to know just how much chlorine is in your soil. Most soil tests do not include chlorine results. If your soil test indicates excessive levels of other anions, such as sulfur and boron, it may be difficult for your plants to absorb the chlorine they need. Only a lab-based soil test can tell you what those levels are and how they are changing over time. If you see signs of chlorine toxicity, you may want to limit the use of calcium chloride and potassium chloride.

How plants use chlorine

Chlorine aids plant metabolism during photosynthesis. It is necessary for osmosis and fluid balance within plants, working in tandem with potassium ions to open and close the stoma. As an anion, chlorine binds with many cations, or positively changed ions, helping to transport them throughout a plant. Chlorine also appears to have antifungal properties which are currently being explored.

Chlorine toxicities and deficiencies

Chlorine is a relatively mobile nutrient, which means it moves around freely within a plant, going wherever it is needed. This means that deficiencies are most often seen in older growth. Chlorine deficiencies appear as wilting, leaf mottling, and a highly branched but stubby root system. [Cabbages that are grown in chlorine deficient soils do not smell like cabbages.]

More often, chlorine toxicities occur close to swimming pools and in areas with hard water. [San Jose tap water ranges in pH from 7.0 to 8.7.] Symptoms of chlorine toxicity appear as scorched leaf margins, excessive leaf drop, reduced leaf size, and reduced overall growth. Too much chlorine can also interfere with nitrogen absorption, causing chlorosis, or yellowing, but that might not always be a bad thing.

We know that new growth tends to be more susceptible to disease than older growth. It ends up that chlorine’s interference with nitrogen uptake may be a method of reducing disease severity. As a disease occurs, plants absorb more chloride anions, blocking nitrogen uptake, and reducing the amount of vulnerable new growth being produced.

Now you know.