my first grow kind of

hi my name is louis aka spliff monkey and about a month ago guy gave my friends acouple of plants they are being grown without hydroponic lighting system or p.h computer gadget shit and they stem and leaves are looking beautiful but some leaves at the bottom are going yellowish brown is this mg deficiency or is this common in plants at this time and because am just using water and that what would be the best type of plant neutrient to use and how much per serving i will keep this updated and that and let you guys know how my plant is and post some picture when she is done by the way i call her the alcoholic plant(i have decided to experiment on a plant it may be my first grow but i want to see what things i can do with a plant and i have a nice plant female going into buddin and i have only being using a bottle and a bit of water