Over watering is one of the biggest mistakes new growers make, reason for this is, because they feel the need to give there plant everything and will overdue a lot of things and one of them is over watering. By over watering your plant you soak the roots so much, they can’t get enough oxygen and slowly die. The plant shows signs of over watering by: Wilting, droopy look, yellow and or dead leaves falling off, which includes leaves that don’t look dead falling off. One of the best ways to tell how NOT to over water is by, picking up the pot when it is dry and then picking up the pot after you water. (This is a reason why it’s smart to use light pots.) When you water, you want to water just enough to where you see a bit of water coming out of the bottom, not gushing or pouring out. Just enough to see a little bit, then you know the plant has enough water. To prevent over watering you can either: add more perlite to your soil, add hydrogen peroxide to your water for extra oxygen, all the while killing bacteria if any in the soil. After watering, wait a few days to water. Mj plants like a good watering and then a couple days to dry out in between watering. So it’s very easy to over water. Besides the weight of the pot, another way to test if your plant needs water is to stick your index finger a couple inches into the soil. If the soil at the tip of your finger feels almost dry, then it's time to water again. The top of the soil should be allowed to dry out between waterings if it's still moist the plant does NOT need watered. You can also use a moisture meter which will tell you the level of moisture down in the soil. You can buy them at most garden supplies or hydro shops.
Here is a picture of what over watering looks like:
(Picture Provided by ShopVac)
Under Watering
Under watering can show the exact same symptoms as over watering can, they show the droopiness along with the top soil being hard. To avoid this make sure every other other watering you give your plants a good watering, like use a bit more water then you normally do, because sometimes your plants will need a good quenching once in a while. So doing this will help the bottom roots and root hairs get enough water to produce newer growth. For new growers they tend to do both over watering and under watering.
More over watering then anything else, they tend to be to "generous", and then you have ones that are to afraid to water them to much and then they get the under watering symptom. Under watering seems to droop more than over watering does, the fan leaves will droop closer to the stalk and won't be as yellow as over watering looks like, under watering looks like this picture below, only way to stop under watering is to water when the pot feels lite. Make sure few drips of water come out at the bottom of the holes, that way you know the plant has enough water.
Quick Deficiency Guide
Nitrogen: Entire plant is light green in color; lower leaves are yellow; growth is stunted....
Phosphorous: Entire plant is bluish-green, often developing a red or purpleish cast; lower leaves may be yellow, drying to a greenish-brown to black color; growth may be stunted...
Potassium: Leaves have a papery appearance; dead areas along the edges of leaves; growth is stunted...
Magnesium: Lower leaves turn yellow along the tips and margin and between the veins; the lower leaves wilt...
Calcium: Young stems and new leaves die...
Zinc: Leaf tissue between the veins is lighter in color; yellowed; papery in appearance...
Iron: Leaf tissue appears yellow, while the veins remain green...
Copper: Leaf edges appear dark green or blue; leaf edges curl upward; young leaves permanently wilt...
Sulfur: Young leaves turn pale green, while the older leaves remain green; plant is stunted and spindly...
Mananese: Growth is stunted; lower leaves have a checkered pattern of yellow and green...
Molybdenum: Leaves are stunted, pale green, and malformed...
Boron: Young leaves are scorched at tips and margins...
All reffed from:
http://autoflower.net/forums/showthread.php/11-Basic-Deficinecies-List?highlight=diagnose