Im growing some autoflowers in an organic soil myself, but mine has nutrients already in it (1.2 0.3 0.05). I think there is enough nitrogen in my soil to last the whole grow so im going to just be top dressing with some high phosphorus bat guano during flowering (1-15-1).
Some strains of Autos are more sensitive to nutes then others but they all need nutrients. If your medium has no nutes then you should start feeding as soon as you start to see deficencies develop. start with a low dosage of nutes and work your way up to the stronger doses if your not seeing any burn. Dont feed every time you water but every second watering is recomended