grey, ash like leaves? any ideas

i went to my little plot the other day and i noticed all the vegatating tips are going a grey ashy looking at the ends of each leaf, i was thiinking fertiliser burn but to be honest it doesnt looks so crispy and yello as associated with fert burn burn like an ash colour, i ve never seen this bfore. im using organic fertiliser (chicken poop) the ground is very sandy though have dug out 1 m2 and filled with 120 litres of compost per plant. any ideas people. havent any pics sorry cheers newbie from uk.

ready to be planted outside.

They look great Lemon H! If the edges aren't crispy and they regain their green color then it seems OK. Make sure its not fungi too I would think.

I found this interesting. One of my plants, a very pregnant autoflower that I knocked up for a seed run, got a bad case of spider mites. I only noticed them when I was examining the trichs with the loupe and I was shocked. I hit the little douche bags with some pyrethrins and it killed them. The next day I hosed off their little carcass'. My point is that the leaves on that plant turned gray too, just like you are describing. Have you checked your plants for spider mites under magnification?