Lower leaves turning yellow and curling under

Thanks for any help in advance.

This picture isn't the best.



These are the very lowest leaves. They are yellowing and curling under.

This is MG organic, perlite and MG potting soil. 600w HPS 2 ft from tops. Temp is 84 max, 67 min.

I haven't added any nutes. I water every three days when the soil has dried a bit. The plants are 4.5 weeks old.

Any help would be much appreciated.

Need more pics showing the whole plant but that could be a slight Phosphorus deficiency.

Thanks,I'll post more pics today

pee in a bottle add 1/10 pee 9/10 water and water it fresh morning pee. I know it's gross and I never done it yet (yet), but I've heard it's really good for the plants...