Are you doing it correctly? The proper procedure is to first burn, then prepare the soil (with a shovel), and then plant. The soil preparation has to be performed while the ashes are still on the ground (i.e. a day or two), or you have to go back to the burning stage to provide fresh ashes.
I've found burning a row of 9 tiles (material prepared in advance) allows me to go back to the first tile and preparing it directly after setting fire to the last tile. Since the preparation time varies widely, the number of rows you can prepare in a day varies as well, but if you have to break off for the night you can continue the next day (if you start that day with checking your traps and finding an elk, you're going to lose the burn preparation of the last bit, as it takes a fair bit of time to process an elk, at least if you want to preserve most of the meat). A new burning session doesn't take that long, though.