Combine ginger, 2 cups water, turbinado sugar, and salt in a medium saucepan over medium-high heat. Bring to a boil, stirring to dissolve sugar. Reduce heat, and simmer for 45 minutes. Cool completely; strain ginger syrup through a fine sieve. For each serving of ginger beer, combine 2 ounces ginger syrup, 1 tablespoon fresh lemon juice, and 8 ounces chilled soda water in a tall glass.