I have a similar problem, cant figure out what it is either, i was thinking N too at first, but not sure.

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Holden4 Reviewed by Holden4 on . NITROGEN DEFICIENCY ?? Would everyone agree that these young plants are showing signs of N deficiency ? They are about a month old from clone, only been in DWC for 4 days under 600w HPS, PH fine (5.8) room temp 27c, nute temp 20c, good air circulation. They only started to yellow last night. Rating: 5