I rephrase sorry it would probably be good for seedlings not plants.
Burnt plant matter turns to exttemely solubal potash.
You are correct about field burning, that is one of the reasons for it. Also helps to control pests.
Funny thing is "environmentalists" have passed legislature against it and it is almost non existant anymore... so it takes thousands of gallons of diesel fuel and emissions to choo the straw fine enough to be able to be tilled into the soil.
(Still decomposes and provides nutrients, but takes longer to become soluble)
A mix of ash and composted plant material would be very benificial, especially with fungal and bacterial inoculation.
Just take care of your compost