Cathy Kavassalis: Halton Master Gardener
(editor’s note: This is another very informative post from Cathy Kavassalis on the Master Gardeners of Ontario Facebook page, in response to a question from an Ontario gardener)
The Ontario Ministry of the Environment, have asked that Master Gardeners NOT recommend home made solutions. These products have not been tested for safety and impact on non-target species. They can do more harm than good.C. Kavassalis
Let’s consider soaps. The problem is ‘dish soaps’ are not designed to be used on plants. Some of what folks call ‘dish soaps’ are soaps, and others are actually detergents. Many have additives that provide fragrance, soften hands, improve rinsing and or disinfect. Some are formulated as salts, using sodium that can be harmful to plants; commercial insecticidal soaps use potassium, which is less disruptive to salt balances and the movement of water from the roots to the leaves.
Some background: A soap is made from the action of an alkali such as sodium hydroxide or potassium hydroxide on a fat. Fats consist mainly of fatty acids of varying lengths. These are chains of carbon and hydrogen atoms with a reactive oxygen tail. Short chain fatty acids tend to be damaging to plants (phytotoxic). For example, acetic acid or vinegar (C2H4O2) is a very short chain fatty acid that is used as an herbicide. Insecticidal soaps are based on long-chain fatty acids (10–18 carbon atoms) of potassium (rather than sodium) salts. An example would be potassium laurate C12H23KO2. Some dish soaps are made with sodium palmitate: C16H31NaO2. It has the right fatty acid, but the sodium is not good. Excess sodium in the soil causes salt stress (some plants are more sensitive than others to sodium). In addition, it is likely to have other additives that have not been tested for safety on plants or their impacts on non-target species. I would be particularly cautious around vegetables that can absorb these compounds with unknown consequences.
A detergent is more problematic. It is an ammonium or sulphonate salt of long-chain fatty acid like sodium lauryl sulfate: CH₃(CH₂)₁₂-OS(O)₂-O⁻ Na⁺. Detergents are more likely to be phytotoxic and there is some research suggesting detergents can harm plants when absorbed through roots (e.g. corn shows impairment to “light-harvesting pigments and cell viability.” Environ Monit Assess. 2018 Oct 18;190(11):651). All this is to say, understanding the chemistry and composition of a product is important.
Both soaps and detergents can damage the leaf cuticle (outer coating of the leaf). They strip naturally occurring protective oils and waxes from leaves. For some plants, loosing this layer can be deadly. For others, thinning the layers can simply make them more susceptible to fungal diseases and herbivory.
Prevention is key.
The bottom line is that any pesticide should only be used if the level of pest or disease threatens the survival of a plant. They should not be used where cultural or physical or mechanical measures can be applied.
There are now a plethora of naturally occurring bio-pesticides like Btk available to home gardeners. These can be used when alternative strategies (i.e., row covers, hand-picking etc.) and disease levels merit their use. However, all pesticides have the capacity to impact non-target species and great care should be taken to limit their use and target the problem.
With the plunging numbers of insects around the globe, this is an issue that is of great concern to me.