Q: I used half of a box of rock salt two years ago to kill a big pyracantha climbing up my home’s wall. Has there been enough time to plant a tree near this same location? How long does it take rock salt to dissipate?
A: It all depends on how much moisture has come and gone from the site. Rainwater dissolves rock salt and moves it in whatever direction the water flows. There is no telling exactly where the salt concentration is now. One way to tell if the soil is still contaminated is to take separate samples of soil from spots near where you want to plant a tree. Put each sample in a small pot and plant a seedling vegetable in each one. If the plant leaves turn yellow after a few days then the concentration of salt is too high for your tree. If you want to be perfectly safe, excavate the soil three feet wide and one foot deep in all directions from where the salt was poured. Replace the soil with high-quality topsoil and you can plant your tree with little worry.