1st time grow. natural light. browning of bottom leaves

this is one of 3 that our housemates have started growing. The ends of the bottom leaves have turned a pale shade of brown and have gone slightly crispy. its happened to all 3 plants, this one however has been introduced to baby bio. the ph is unknown. the soil is ok, bought from a garden centre.

the plants are really good, this one in particular is a lot bushier and shorter than the other 2, however is it common for the leaves to die around the bottom of the plant?

im slightly worried, a 1st timer and i have no clue what to do if anything is seriously wrong!! eek!

is this to be expected for natural light grown plants? or is it something more serious? the other plant has been plucked of its bottom leaves but still seems to be progressing up the plant.

thankyou to anyone who can help me, it would be greatly appreciated =)

Plant as it stands: pretty i think hehe!
http://img284.imageshack.us/img284/6796/image0193ze.jpg

bottom leaves:
http://img341.imageshack.us/img341/888/image0210xa.jpg

Looks like you might have a PH problem. I personally grow hydro so Im not sure what you should be keeping the ph for soil, but I think its low to mid 6's....Check the PH asap, if nothing else you can just rule out one more possiblity. Best of luck man

pik too dark, but I can see possible ph problem..,
plant is pretty...If you bought soil that has nutes in it already...(macro nutrients), then you could be over ferting...need more info and lighter piks...size is perfect.
lw

2nd time I have heard of baby bio...neat stuff. I think you could use baby bio, super thrive and a flower enhancer to grow all day long...but I ahve never bought a bottle and tried...already have great nutrient regimens in place...hard to change