Is this heat stress...?

Many of the leaves on my plants have started rolling up lengthwise. Most of it seems to be occurring towards the tops of the plants and its the worst on the tallest of them. However, it does seem to be affecting all the plants to some degree. I have lifted the light to over 25 inches (from about 18-20) from the tops of the plants and am hoping i will see improvements. Any other ideas? This is my first grow and I am afraid of killing my already haggard babies completely. Thanks!

Put some fans in there; more if you already have some. Aim a fan between the tops of the plants and the light.

It's too much salt buildup and overfertilizing, though.

Here you go

Peace

Solving Marijuana Plan Leaf Curl/Cupping Problems

Plenty of first time and experienced marijuana growers will suffer at some point in their cannabis growing career marijuana plants that begin to show leaf damage.
Often they write emails or post on forums.
“Help, my leaves are cupping and the leaf edges are turning brown!”
or
“My plant's leaf tips are curling down and turning black ....what's wrong?”
Unless another marijuana grower inspects the damage a true assessment might not be possible. It's hard to tell "exactly" what the culprit is. Unfortunately the “solution” the marijuana grower chooses many times is not the right one.
A misdiagnosis only serves to make matters worse by promoting further decline.
The ultimate and correct solution is in the hands of the marijuana grower.
Here are some common problems when marijuana leaves are curling.

1. Too much marijuana fertilizer
   The most common cause of marijuana leaf cupping aka leaf margin rolling, leaf margin burn, and leaf tip curl/burn is overzealous use of marijuana plant food. In relationship to factors such as marijuana plant vigor and rate of growth. Leaf burn is often the very first sign of too much marijuana fertilizer.
   A hard, crispy feel to the marijuana leaf frequently occurs as well, as opposed to a soft and cool feel of a happy pot leaf. Back off on the amount and/or frequency of using marijuana fertilizer. Too much marijuana fertilizer can also burn the roots, especially the sensitive root tips, which then creates another set of problems. Note - as soil dries, the concentration of the remaining salts rises further exacerbating the problem.
2. High Heat
   The marijuana plant is losing water via it’s leaves faster than what can be replaced by the root system. The marijuana leaf responds by leaf margin cupping or rolling up or down (most times up) in order to conserve moisture. A good example is reflected by the appearance of broad-bladed turf grass on a hot summer day, high noon, with low soil moisture levels - the leaf blade will roll upward/inward with the grass taking on a dull, greyish-green appearance. Upon sunrise when moisture levels have returned to normal, the leaf blade will be flat. Lower the heat in the marijuana grow-op and concentrate on developing a large robust root system. An efficient and effective root system will go a long way to prevent heat induced pot leaf dessication or marijuana leaf margin curling. One short episode of high heat is enough to permanently disable or destroy leaf tissue and cause a general decline in the leaves affected, which often occurs to leaves found at the top of the cannabis plant. The damaged pot leaf (usually) does not fully recover, no matter what you do. Bummer in the summer. One can only look to new growth for indications that the problem has been corrected.
3. Too much light
   Yes, it’s true, you can give your marijuana plant too much light. Cannabis does not receive full sun from sunrise to sunset in its natural state. It is shaded or given reduced light levels because of adjacent plant material, cloudy conditions, rain, dust, twilight periods in the morning and late afternoon, and light intensity changes caused by a change in the seasons. Too much light mainly serves to bleach out and destroy chlorophyll as opposed to causing marijuana leaf cupping, but it often goes hand-in-hand with high heat for indoor marijuana growers. Turn down the time when the lights on in your marijuana grow room. If you're using a 24 hr cycle, turn it down to 20 hrs. Those on 18 - 6 marijuana growth cycle can turn their lights down two or three hours. Too much light can have many adverse effects on marijuana plants. Concentrate on developing/maintaining an efficient and robust root system.
4. Over Watering
   For marijuana growers using soil, this practice only serves to weaken the root system by depriving the roots of proper gas exchange. The marijuana plants roots are not getting enough oxygen which creates an anerobic condition inducing root rot and root decline with the end result showing up as leaf stress, stunted growth, and in severe cases, death. Over watering creates a perfect environment for damp-off disease, at, or below the soil line. Many times marijuana growers believe their cannabis plant is not getting enough marijuana fertilizers (which it can't under such adverse conditions), so they add more marijuana fertilizers. Making the problem worst. Not better. Often problem 1 and 4 go together. Too much marijuana fertilizer combined with too much water. Creating plenty of marijuana plant problems.
5. Not Enough Water
   Not only is the marijuana plant now stressed due to a low supply of adequate moisture, but carbohydrate production has been greatly compromised (screwed up). Step up the watering frequency, and if need be, organic marijuana growers may need to water from the bottom up until moisture levels reach a norm throughout the medium. One of the best methods in determining whether a marijuana plant requires watering is lifting the pots. The pots should be light to lift before a water session. After watering the marijuana plants lift the pots to get an understanding how heavy they've become fully watered. If the pot feels light to the lift - it’s time to water. Don’t wait until the soil pulls away from the side of the pot before watering. And of course, leach, once in a while to get rid of excess salts. These are the five most common problems marijuana growers encounter when growing cannabis. Correcting the problems early will save the marijuana plants, but may reduce overall yield. With practice and experience these problems are easily overcome which will then enable the marijuana grower to produce fantastic marijuana plants. With heavy yields.

I thought maybe a problem with the nutrients, but since being repotted a week ago they only received one watering with a very mild nutrient solution. I used less than one third of the recommended dosage. So, with fresh medium and only one small application of nutrients, could that really be an issue...? Thanks for the reply.

How was your PH when you fed them?

ph of the water/nutrient solution was 5.7 if i remember correctly , a little lower than ideal but I figured it was close enough.

Eh, PH makes a big difference. And you're growing in soil. You need around 6.5 PH. Lower than that, like yours is, you're locking nutrients out that they need. 6.5 to 5.7 is more of a difference than you'd think.

I'd give them another feed with 6.5 PH and an average amount of nutes and see how they respond.

k, ill raise the ph with the next round and hope for the best , thanks for the advice.

if the tips curl in, that's heat stress for future reference

Np. And it's pretty easy to determine whether your plants will acquire heat stress btw. Just stick your hand where the plant is and see if it's getting hot or feels good, etc. I like to have two fans going. One that circulates the air (I point it away from the plants when they're small and then turn it towards the plants when they can handle it.) and another one that aims between the tops of the plants and the light, or towards your exhaust, wherever it may be.

If your hand feels fine at where the plants are experiencing problems and your pH is dialed in properly, you know it's definitely an overnute problem.