How old are your plants?
If your plants are very young then that could be causing nutrient burn. You should never begin feeding plants nutrients until they reach their 2nd week of pre-veg growth(seedling-sprout).
What light schedule are you using?
18 ON/6 OFF... this simulates the long days/short nights of the warmer/summer months. This schedule is used for optimum vegetive growth.
12 ON/12 OFF... this simulates the shorter days/longer nights of the cooler/fall months. This schedule is used to force flowering(budding).
How close to your plants are your lights?
If using HID(High Pressure Sodium/Metal-Halide) make sure to keep the bottom of the lights no less than 18" - 24" from the canopy(the top) of your plants otherwise you will dehydrate your plants and burn the veg growth. Leaving the lights too close will cause heat stress, which in turn, will lead to the plants leaves to dry up, curl downward and even show brown spots on the leaves.
However if you are using CFL spirals or T5 High Output flouro's to light your girls, you will need to get those lights as close to the plants as possible. General rule of thumb... CFL Sprials = 3" - 5" and T5 HO Flouro's = 4" - 8". This change in light height is due to the simple fact that flourescent lights produce light to no heat. Making these lights better for vegging plants period! Once the plants under these flouro's get bushy increase the amount of room between the bulbs and the top of the plants to make the girls stretch of a few inches then bring the light back to being as close to the tops as they can be without burning the plants.
(I'm sure you understand the lighting concept but its important to fully understand because if the lights you are using now are too close they will cause your plants leaves to curl as you mentioned)
How often are you watering/feeding?
If this is a soil grow....
You should not be feeding nutrients with every watering. Remember soil holds water and nutrients unlike a soil-less medium. And as far as watering goes, unless you have a moisture meter, all you need to do to see if watering is necessary is stick a finger down into the soil about 2 inches. If when you pull your finger out it feels dry, then water. If you pull your finger out and it feels wet then leave em alone. How would you feel if you just drank a half-gallon of water then someone forced you to drink another half-gallon? You'd probably feel like shit as would the plants! Feeding using a soil medium should only be done every 10 - 14 days. If you feel as if you have used to much nutrients when feeding there is a simple solution! Bring your pot outside and hose the shit out of your soil from the top(assuming your grow containers have good drainage) and then let it drain out completely. If your grow container doesnt have any way to drain out excess water then use something with a sharp point to puncture holes in the bottom so it will drain effectively. Also make sure to let the soil dry out before watering again. To know when it is completely dried out use the finger trick i told you about up above. What happens is when your soil cant drain out, the nutrients build up in the soil and cause Salt-Lock/Nutrient-Lock. This will cause your plants to not be able to uptake nutrients that it needs to thrive. It also will stunt your plants growth!
If this is a hydro grow(ebb n flow, wick, drip system, ntf, bubbleponics, aeroponics, DWC)...
Growing in a hydro system of some sort is much different. First... you are using a soil-less medium(ex.. hydroton, lava rock) so drainage isnt much of a concern. The only thing you need to understand about feeding plants in a hydroponic, aeroponics system is that the plants root system sits in a constant flow of nutrients yet these systems include a bubble/air stone to keep the water and plants root system oxygenated. If working with a hydroponics/aeroponics system just remember to clean/sterilize/change your systems resivoir tank every 5-7 days.
(Remember that getting the correct dosage of nutrients is critical when feeding depending on the maturity of the plants!)
Seed/Sprout/Pre-Veg = 1/4 strength than what label says
First 2 weeks of Veg(Starts after first 2 weeks of growth) = 1/2 strength of what label says
3rd week of Veg to 1rst day of Flowering = Full strength of what label says
Pre-Flower - end of 2nd week of Flowering = 1/2 strength of what label says
Flowering(3rd week after switching lights to 12/12) - 3 weeks before harvest = Full strength of what label says
3 weeks prior to harvesting - Harvest = No nutrients, Plain water with a couple teaspoons of molasses to flush your soil of all nutrients to improve taste & aroma
Indoors/Outdoors?
I dont have much to speak of on this topic. Indoors is easier to control your plants growing environment as far as temperature, humidity, co2 and whatever else a plant needs to thrive. Outdoors is a great way to maximize your yield however if you are willing to fight with pests and other issues. I only ask this question because I'm curious as to which you decided to go with.
What is your grow space's temperatures?
Temperature is critical as well! If the grow spaces temperature gets to hot it will stop the plants growth. remember the hotter the space gets the more you will need to water. If the temps are too low it will kill your plant. If it is too hot in your grow space this will also cause your plants to wilt or make the leaves curl downward. Optimum temps are between 65 degrees F - 76 degrees F. Use a fan to intake/outake air and also to strengthen the plants stem.
What is the humidity at?
humidity is critical too! Too humid makes your plants leaves sweat which is how they cool themselves down. but this will stress them out if you cant control the humidity issue. if the grow spaces humidity level is to low then the plants will become dehydrated. Best percentage of humidity in the grow space should be between 40% - 50%. If you run issues in your grow space where your humidity level is too high then you will need a dehumidifier to combat the problem. If you're grow space is not humid enough you could try to put a few cereal bowls filled with water in your grow space. Your plants will take this moisture from the bowls of water through their leaves when the humidity is very low. Just dont put too many bowls of water in there incase it increases your humidity too much!
answer these questions and you'll find your answers... its all about process of elimination!
hoe this helped my friend... good luck on the grow~!
thanks for the help folks!