Wash the apples, celery, cucumber and kale well, especially the celery, as dirt hides in the cracks. Chop the apples, celery and cucumber into bite-sized cubes. Put the kale at the bottom of the blender, then add the apples, celery, cucumber, lemon juice, salt and water. Start the blender on low and slowly ramp up to medium-high. Blend for 3 minutes until smooth, adding an extra cup of water if needed. Pour the juice into a nut milk bag and strain it over a glass bowl or large jar. Transfer the strained juice to a glass jar and chill in the fridge. It keeps up to 5 days; shake well before serving if it separates.