Combine the sweet potato, carrots, celery, leeks, lentils, ginger, 3/4 teaspoon curry powder and 1 teaspoon salt in a 4-to-6-quart slow cooker. Add 6 cups water and stir, then cover and cook on low, undisturbed, 8 hours.
Stir the soup vigorously with a whisk to make a rough puree. Thin with hot water, if desired.
Melt the butter in a small skillet over medium-high heat. Add the garlic and the remaining 1/4 teaspoon curry powder and cook until the curry powder is slightly toasted, about 1 minute. Stir the curry mixture into the soup and add the lemon juice, cilantro, and salt to taste. Serve with lemon wedges.