Totaly fine to put a chunk of that stem under the soil. I would actually suggest just the for your girl. Since she's already a mother why don't you take acouple clones off of her and then just put her to flower? I've realized with my girls that the lower buds get so little light I shouldve just cloned the heck out of the lower branches and then put her into flower. Blue dream looks amazing, it may even be my next canidate depending on how this log looks in another thread. The soil you purchased I'm not going to hate on because it can really get the job done. However I will jus throw the heads up at ya that sometimes the moiture controls and things of that nature make things kinda just pop up outta no where because you can't control when some things are being released. Ofcoarse this is not your first grow so you should have it all figured out. Keep a ph pen handy when using the MG.