Remove the kohlrabi leaves and cut them into thin strips. Set aside. Peel the kohlrabi and carrot, cut into small pieces and chop in a blender to the desired fineness.
Sauté onion and garlic in butter for 2 - 3 minutes until soft.
Add the chopped kohlrabi and carrot, stir and sauté briefly.
Add water and crumbled vegetable cube; bring to the boil.
Stir in chopped kohlrabi leaves. Cook the soup over the low heat for 15 minutes. Remove from the heat and stir in soy sauce.