Ah. I believe you’re missing a critical detail - when that type of conditions occur, its because the soil dried out and blew away - there’s usually only only about 12 in of topsoil on any average bit of land.
Thia is what happened with the American Dust Bowl in the SW. The dirt blew away as dust.
After the defiler ash blows away, that’s exactly what you’d get, 100%.
How and when are you imaging this transition occurs? Does the ash blow away in minutes, weeks, years? Or not at all - its just black hardpan left behind by defiling?
Dust bowl or sub-saharan Africa desertification is exactly what I’m imagining. In the real world, organic matter is what holds topsoil together. Even overly-aggressive plowing tears apart the topsoil to the extent that it blows away under fairly mild weather conditions. It is very difficult to reverse. In these real world conditions, the soil is still full of dead organic matter that is simply cut up too much, and even this is enough for it to blow away. It becomes extremely difficult to revive this soil. The scenario posed by defiling magic is significantly more violent to the topsoil. It is stripping all of the organic matter out of the soil.
So I’m simply proposing using 100% real world science, destroy all the organic matter in a circle of land, dust this land with materials that you yourself propose – inorganic salts, clays, and silica. With this as your starting point, you will be left with something that is extremely far away from becoming nutrient rich soil.
There is a specific example of how long it takes an area to recover from being defiled. In the Ivory Triangle Box set they list a number of historical conflicts with dates attached. One of them is the Clearing War, in which at the end of the conflict Nibenay himself razes a mile wide section of the Crescent Forest. The date they list works out to -414 on the FY calendar with the current date of the release likely being about FY3. They conclude the entry saying: " This clearing remained ashen for centuries and has only in the past few decades begun to take seed."