Natural soap bars that are specifically formulated for use on dogs can be safe, as long as they are made with appropriate ingredients and are pH-balanced for dogs.
Many natural soap bars that are labeled as "organic" or "all natural" may not be suitable for use on dogs, as they can contain ingredients that may be irritating or drying to a dog's skin and coat. Therefore, it is important to read the ingredient list carefully and avoid any ingredients that are known to be irritants for dogs.
Also, it's important to consider the pH level of the soap, as dogs' skin pH is slightly acidic and it's important that the soap pH is close to that, to avoid altering the natural pH and to prevent any problems on skin.
It's always a good idea to patch test any new soap bar before using it on your dog, and if you notice any signs of irritation, such as redness, itching, or dryness, discontinue use immediately and consult a veterinarian.
If you have any concerns about the suitability of a natural soap bar for your dog, it's always best to consult with a veterinarian for advice. They may be able to provide specific recommendations based on your dog's individual needs and sensitivities.