It’s one of two things
Nutrient Burn is one of the MOST common mistakes a new grower makes, reason for this is, because a newer grower will use a chemical nutrient most of the time and listen to the directions on the box. This is a NO NO! Depending on the age of the plant, size, strain and soil mixture you are using also has a factor. There is no set guideline when using nutrients, but I can give you a good example to start out with so you will not burn your plants. It’s always good to start out light, rather than feed heavy. Remember you can always add more later, but can not take out when you added to much. Chemical and Organic nutrients differ. Chemical nutrients are more readily available and can burn way easier than organics can. Organics are easier for a newer grower to use, most of the time, and lessen your chances of burning your plants.
I recommend not using more than ½ teaspoon of chemical nutrients per gallon of water. Unless the plants are very big 5 feet+, then it’s safe to use 1 teaspoon per gallon of water. When your plants first emerge you want to wait at least 2 weeks before feeding your plants, unless your plants are in a soiless mixture, like pro mix. The cotyledons (its first set of round looking leaves) are what give the plant its food until they get the first 2 or 3 sets of leaves. If your plants are in a soiless mixture and are over the first week of age; you can feed a weak amount of nutrients, like ¼ teaspoon per gallon of water. Soiless mixtures are different from soil plants and soiless plants need to be fed more when using this mixture.
I also recommend not feeding more than 1 time a week if using ½ teaspoon per gallon of water for chemical nutrients. You can feed every other day,( this goes for chemical and organics) at very weak amounts, but doing this may contribute to over watering, and for that I do not recommend feeding more than once a week. Some people feed 2 times a week using like ¼ teaspoon per gallon of water… Use 1/4 strength for first feeding and then go up to 1/2 strength from the 2nd feeding when using chemical nutrients. It’s very easy to overdo it. When using organics, depending on which one you’re using, I recommend using 1 teaspoon per gallon of water. When the plant gets bigger you can work your way up to using more nutrients when the plants get bigger.
As for soil mixtures, there are a lot of different kinds of soil’s out there. Using a rich soil mixture is not recommended for seedlings. Seedlings that are under 2 weeks of age you do not want to start them in rich soil, using a seed starter mixture is one of the safer ways. Seed starter mixtures are weak in nutrients, so it will not burn the seedlings but will provide them enough to get past seedling stage, but the downside is you have to transplant into a better soil mixture after 2 weeks of age. If you decide to start with this mixture, do not put your seedlings into a big pot. Start them out with using a cup or a small pot.
Nutrient Burn causes leaf tips to appear yellow or burnt. They can also be brown and twisted and crispy looking. Depending on the severity it can show many different symptoms and shows on lower part of the plant when its young, at older stages it can move anywhere on the plant.
To fix the problem when you have Nutrient burn, you want to flush out the plants with lots of water.
Soil should be flushed with lots of water, Use 3 gallons of water per one gallon of soil. Flush very thoroughly, after plant recovers usually after a week, you can resume using nutrients after a week or a week 1/2. When you flush your soil, you flush everything out, a lot of nutrients go with it, including the soil nutrients.
Change out the reservoir, flush out any lines and clean out the entire system and replace with plain water for the first hour, then start out with lower parts per million (PPM)
Its good to clean out your system every 2 weeks and replace with fresh water and nutrients. Some people change everything every week!
Ahh, nute burn! Stop this by not adding to much chemical/organic nutrients to your water,foliar feeding. DONT feed more than 1 time a week unless using weak amount, use 1/4 strength for first feeding and then go up to 1/2 strength from then on when using chemical nutrients. Its very easy to overdo it. Causes leaf tips to appear yellow or burnt. NEVER give nutrients to plants that are under 2 weeks of age, at this age the soil nutrients are enough to suppliment them untill 2 weeks of age or more depending on how good your soil is. Using ferts before 2 weeks will almost likley kill your plants.
These pictures is what heat stress looks like.
Only way to fix this is to Move the lights away from the plant!
And get better ventalation, and or add more fans! Heat - If the lights are too close to the plant, the tops may be curled,
dry, and look burnt, mimicking a
nutrient problem. Your hand should not feel hot after a minute when you hold it at the top of the plants.