no it's not Mg. It's probaly nitrogen. when ever im not sure i give em 1/2 strength fish emulison. It's organic so its easy on them. you will see visual change in about 24 hours.

nitrogen is the most common in veg stage. as well it makes the new growth a lighter green. Mg will cause the lower leafs to yellow intervenualy
harris7 Reviewed by harris7 on . Deficiency or a burn? This is a picture of my 5 week old white widow growing in organic soil under 400w MH light. The tips have turned yellow and the new growth on top is yellow. She had her last dose of pure blend 3-1.5-4 12 days ago. It was given at a rate of 1 tsp per gallon. Her soil is a pH of 6.9 and the run-off was a pH 6.5. The problem is just at the top half of the plant. There are no brown spots on the leaves or tips. I'm not sure if this a sulfur or Mn deficiency or a nute burn. I have other WW growing Rating: 5