4 weeks old plants, most leafs turning yellow, help please :)

My friends having a problem, hes got 8 Dr underground king kongs that are about 4 or 5 weeks in veg, hes reused his soil from a previous grow and all or most of his leafs are yellow, im thinking this is something to do with reusing the soil? Its something I'd never do but each to there own I guess.
Im trying to help him solve this problem and this is the best place to ask online, or could it be a zink, iron deficiency? I think he said he hasnt used any nutes or hes just ran out, maybe theres no nutrition in the soil and they are starving? Any suggestions?

Thanks

Is 'your friend' feeding them anything?
Does 'your friend' have a camera so we can see what it looks like?
Most likely it's hungry, but could be overwatered, or something else, so pics would help 'your friend' get an accurate diagnosis.

i did the king kong and crystal m.e.t.h. in miracle grow in 10g containers using humboldt nutes. they were both topped multiple times creating good sized "shrubs". they both needed nitrogen throught flower, the king kong needing more than the recommended dosage. i normally use only half dosage unless asked for more. sounds like they are hungry but i would give them a good flush before adding anything.