1 Tablespoon Epsom Salt 1 gallon water A watering can 1. Combine the Epsom salt and water. 2. Use the solution to water your plants. 3. Repeat once a month. Why This Works--Epsom salt is made up of magnesium and sulfate – both vital plant nutrients. Some magnesium-loving plants to try it on: houseplants, roses, peppers, tomatoes etc.