The most effective salt for killing trees is road salt or deicing salt. Trucks spread this kind of salt in the winter to remove ice from roads and walkways, and it will passively kill nearby small trees. The salt mixes with water and enters the soil, where the chlorine from the salt causes leaf scald and the sodium prevents the tree from absorbing water. You can use the mixture yourself to kill small, unwanted trees in your yard.
Fill a 5-gallon bucket halfway with deicing salt. Slowly add water as you stir the mixture with a stick. Continue adding water and stirring until the mixture has the consistency of a briny sludge.
Pull up any weeds, grass or other plants in the soil around the base of the tree so the soil is totally bare. This will allow the salt to more easily soak into the soil.
Pour the salt sludge on the soil around the base of the tree in the early spring, just as the soil begins to warm up. As spring progresses and the tree develops leaf buds, the chlorine will move to the new leaves, causing them to scorch and die.
Repeat the application once a week until the trees are dead.