For you newer guys out there, copy this below into a txt file and save it on your desktop, it will prove invaluable to you.
This is not my work, it is copied from a book so I cannot take any credit. It works though.
For these leaves that are curling up, the suggestion is Mg deficiency so a little epsom salts should help. Or calmag if you have it.
1) a) If the problem affects only the bottom or middle of the plant go to #2.
b) If it affects only the top of the plant or the growing tips, skip to #10. If the problem seems to affect the entire plant equally, skip to #6.
2) a) Leaves are a uniform yellow or light green; leaves die & drop; growth is slow. Leaf margins are not curled-up noticeably. >> Nitrogen (N) deficiency.
b) If not, go to #3.
3) a) Margins of the leaves are turned up, and the tips may be twisted. Leaves are yellowing (and may turn brown), but the veins remain somewhat green. >> Magnesium (Mg) deficiency.
b) If not, go to #4.
4) a) Leaves are browning or yellowing. Yellow, brown, or necrotic (dead) patches, especially around the edges of the leaf, which may be curled. Plant may be too tall. >> Potassium (K) deficiency.
b) If not, keep reading…
5) a) Leaves are dark green or red/purple. Stems and petioles may have purple & red on them. Leaves may turn yellow or curl under. Leaf may drop easily. Growth may be slow and
leaves may be small. >> Phosphorous (P) deficiency.
b) If not, go to #6.
6) a) Tips of leaves are yellow, brown, or dead. Plant otherwise looks healthy & green. Stems may be soft >> Over-fertilization (especially N), over-watering, damaged roots, or
insufficient soil aeration (use more sand or perlite. Occasionally due to not enough N, P, or K.
b) If not, go to #7.
7) a) Leaves are curled under like a ram's horn, and are dark green, gray,
brown, or gold. >> Over-fertilization (too much N).
b) If not, go to #8…
a) The plant is wilted, even though the soil is moist. >>Over-fertilization, soggy soil, damaged roots, disease; copper deficiency (very unlikely).
b) If not, go to #9.
9) a) Plants won't flower, even though they get 12 hours of darkness for over 2 weeks. >> The night period is not completely dark. Too much nitrogen. Too much pruning or cloning.
b) If not, go to #10...
10) a) Leaves are yellow or white, but the veins are mostly green. >> Iron (Fe) deficiency.
b) If not, #11.
11) a) Leaves are light green or yellow beginning at the base, while the leaf
margins remain green. Necrotic spots may be between veins. Leaves are not twisted. >> Manganese (Mn) deficiency.
b) If not, #12.
12) a) Leaves are twisted. Otherwise, pretty much like #11. >> Zinc (Zn)
deficiency.
b) If not, #13.
13) a) Leaves twist, then turn brown or die. >> The lights are too close to the plant. Rarely, a Calcium (Ca) or Boron (B) deficiency.
b) If not… You may just have a weak plant.