I now know that high levels of copper can adversely affect plant growth. However I did not know that when I was 25.
By the time I was 26 I began to suspect the old copper washing-pot-cum-planter was leaching something into the soil as most plants that were grown into this container had a quick death. A few remained – i think they were succulents.
Some plants are susceptible to certain chemicals. Substances that are poisonous to plants are called Phytotoxic. But what is phytotoxic to one plant may not necessarily affect another. For example small amounts of salt can adversely effect citrus trees and have minimal affect on say, eucalyptus trees (this can be a huge issue for fruit growers in arid countries such as Spain, Italy and Australia and is referred to as salinity).
There are numerous variables that can affect plant growth and soil chemistry is just one of the many. If you suspect something is affecting your plants other than the normal suspects (ie sun, water, light) do consider whether there are any contaminants affecting the chemical composition of the soil.
You may want to simply change planter pots, do a ph test or if you are really concerned about potential contamination you can always undertake detailed soil investigation by a soil scientist.