I make a dry mixture of ash and another ingredient and put it in the hole. The potatoes grow like crazy and are not bothered by pests

Sounds like an interesting trick! The combination of ash and another ingredient can really work wonders in potato growing. Here are some possible explanations and more information:

Wood ash as a fertilizer:
Ash is rich in potassium, which is an important nutrient for potatoes. Potassium promotes the growth of tubers and strengthens plants.
In addition, wood ash contains trace elements that are beneficial for plant growth.
However, it is important to note that wood ash has a high pH. Therefore, it should be used sparingly, especially on already alkaline soils.

The second ingredient:
The second ingredient is likely to be lime. A mixture of ash and lime can regulate the pH of the soil and thus create optimal conditions for potato growth.
Lime improves soil structure and helps plants absorb nutrients.

Pest control:
Ash can act as a deterrent to certain pests, such as wireworms or Colorado potato beetles.
A mixture of ash and lime can also form a protective barrier against pests.
Additional tips for growing potatoes:

Crop rotation: Avoid growing potatoes in areas where other nightshades (such as tomatoes or peppers) have previously been grown.
Soil quality: Potatoes prefer loose, humus-rich soils.
Watering: Ensure adequate watering, especially during flowering.
Hilling: Hill the potato plants regularly to promote tuber formation and protect them from sunlight.
Important instructions:

Only use ash from untreated wood.
Use ash and lime sparingly to avoid overly altering the soil pH.
Consider your local soil conditions.
I hope this information helps you achieve a bountiful potato harvest!