u can pick up a cheap (less then $10) ph soil meter from lowes, home depot, or the like; knowing ur soil ph is essential to growing & guaging the nutrients in ur soil.
i am not familiar with scott's but i am pretty sure they have some nutrients in them; check the BAG! having said that, most commercial soil needs to be supplemented after the 3rd week. tomatoe plants r the closest to mj as far as nutrient requirements, so while not ideal, that might be fine to use. But, again, make sure to use no more then 1/4 (u might even want to start with 1/8 of the suggested amount it states on the package. And again, only every 3rd watering for now. u will be able to tell the effects, be it positive or negative from the nutes u have given in approximately 3-5 days after u have fed it. Do urself a favor & get a ph guage, & do a little research on how to use it & how the ph of ur soil affects the entire growth & health of ur plant