You've got no soil lighteners... this is going to be difficult.
You need to flush that plant out well using 3x the volume of the pot in water that has been pH adjusted according to the soil pH (determine by runoff test). You want your soil to be at pH 6.5-6.8 after the flush.
Extra holes in the pot will make this process easier. If it has a built-in drip tray, remove it.