i use mg potting soil as well as nutes/what i do is take a whole bag of the MG and mix it with one bag of regular potting soil with no nutes,and add a half a bag of perlite.Mix this together well and then i use it as is through vegging until the 2nd week of flowering when i begin giving MG ferts at half strength 1 watering per week-ph adjusted of coarse-the main diff is i use the time release that everyone hates,i think it works for me because i dont use it at full strength i dilute it with the added soil and perlite-good luck with your grow
