i dunno where the yellowing starts bottom/middle..top??..usually a water issue starts at the bottom and works its way up..too much/too little water..if it's in the middle could be a nute issue,or mg,copper,ect..issue..but most times nowadays the soil has all that in it..u mention white??..it's not mold is it??..does your soil have white in it?...it's hard to say without pic's..