Need Help: Seedling Week 2 Bottom Leaves Curling with Brown Spots

I need help determining what problem my plat if having. All leaves look healthy except the bottom two (two additional leaves are starting to show similar symptoms). They started curling down yesterday and today they are starting to form some brown spots. My growing medium is 30% Coco, 30% peat, 30% Perlite. My growing environment is set up with a 4500 Lumens LED grow light (equivalent to 300W light). I will be doubling the light in about a week. The light is about 18 inches (46 cm) away from the top of the plants. Temperature varies between 78 to 83 degrees Fahrenheit with the average about 81 degrees (In Celsius that 26 to 28 with and average of 27). I would like to get the temperatures lower but cannot without major modifications. Humidity varies between 60% to 68% (average around 63%). I have good ventilation with a fan that is circulating air. For the most part the fan does not blow air directly on the plants. I water my plants when the top soil feels mostly dry and the pot feels light. I water the plants until I see runoff. My current nutrient regiment is as follows:

• 1 Gallon of distilled water (starting ppm 0, starting pH 6.1)
• 7 ml Pure Blend Pro Grow (3-2-4)
• 3 ml Cal-Mag Plus (2-0-0)
• 1 ml Rapid Start (1-0.5-1)
• 1 ml Super Thrive (0.5-0-0)
• 8ml Hygrozyme
• pH adjusted to 6.4
• ppm 373

This plant is one week ahead of all the others and the others are not showing any signs yet. I'll be transplanting this plant to a larger pot soon but I want to make sure its happy and healthy before transplanting.

My initial thoughts are either wind burn or something is not right with my nutrients. Even though the fan is not blowing directly on the plants it was on high so I turned the fan to low. Your help is greatly appreciated.

 

Mick,

Thank you for your reply and input on my growing medium. I have read a number of things about watering and knew that it should not get as dry as regular soil but was uncertain about how dry (or not dry at all as you said). You input is of great help. Thank you.

 

Oh I hope not. Since Mo happens at lower ph levels (Below 5.6) I'm thinking I may be safe from that but only time will tell. I think my issue may be that my ph is to high and I need to bring it down to 6.0 like Mick pointed out.

 

Interesting, I'll have to read more on ph fluctuations and how it affects nut uptake. Thank you for you additional input.

 

Here is an update on the status of my plants. By Friday the symptoms became worse but the good thing was that the symptoms became more clear as it progressed. It turns out my pants were having a Magnesium deficiency. Since I mixed in 30% Peat it was causing my soil pH to be too acidic. I started to measure the nutrient runoff after my each watering (which I should have been doing from the beginning). I raised my nutrient pH until I had runoff that was around 5.6 pH. With this adjustment I am already seeing new healthy growth on my plants. Thank you all for your input and help. I'll post another update in about a week with some pictures.