Myths may be a polite way of referring to statements or beliefs that are believed true, but are not accurate or totally true. The Internet has made possible the easy proliferation of such misinformation. Two of these myths relating to household products used in landscapes, often in attempts to be sustainable, concern Epsom salts and baking soda.
The common household ingredient Epsom salts (magnesium sulfate) has been widely recommended for many garden uses including to control pests, to deter slugs and voles, to make plants grow bushier, to increase chlorophyll (the green in leaves) production, to help seeds germinate, to produce more flowers, and to improve uptake of some nutrients. Recommendations also are seen on how much to regularly feed plants from trees and shrubs to houseplants, and in particular tomatoes.
The reality, if one examines the research on this chemical back to early in the last century, is that it does correct and has been effectively used for magnesium deficiency in soils or plants. Other than this, it is generally a myth that applying Epsom salts will fulfill the often-seen claims to otherwise healthy plants.
One claim is that Epsom salts won't accumulate in, and so won't hurt, the soil. What isn't mentioned is that it will be leached, and so potentially pollute water, instead. Epsom salts are highly soluble, and if applied in excess can leach into water (49 percent in one study), the pollution negating its benefits. Other less soluble forms of magnesium supplements often are better. This is especially true on soils under high rainfall or irrigation.
Another common household ingredient, baking soda (sodium bicarbonate), is claimed and believed by many to kill fungi on plants and to reduce diseases. The reality is that baking soda (by the way it's not legally labeled for use as a fungicide) helps prevent rather than kill diseases. The baking soda acts by making surfaces more alkaline-- not conducive for growth of fungi. So it is technically fungistatic (prevents fungi growing) rather than fungicidal (kills fungi).