I am going to throw my 2 cents in here and say it sounds like root rot...
Especially if the same soil keeps being re-used without being sterilized.
By sterilized I mean baked for several hours at a low temperature to kill off any harmful bacteria and fungus.
This is one of the reasons why soil sold at hydroponics stores costs so much more.
Can you please tell us a little more about the 70 watt LED bulb you are using?
I will be honest, this is not enough…
You will need at least 100+ watts spread out over each plant and the spectrum will have to be in the right range for the plants to get any real benefits from it.
We can talk more about this later…
I am assuming the manure is added to the soil rather than made into a solution that is poured into the soil when watering.
Is this correct?
If this is correct, then you need to do a few things, first don’t add the manure to the soil, doing this you have no real idea of how much fertilizer is in the soil and basically when you water you are just flushing what fertilizer that is in the soil, out of the soil.
The name of the game here is to reduce the number of variables.
Next, I would add plenty of either vermiculite, perlite, hydroton, something to lighten the soil and make it so more air can get into it.
From the sounds of things, the soil may be too dense and it sounds like it is causing a pyhtium fungus bloom, which causes root rot.
When the leaves begin to turn brown, which leaves start first?
The top leaves or the bottom leaves?
This is important.
If it is the top leaves this could be caused by not enough light, nutrient lockout, or by nutrient burning, which is related to pH as well as several other variables (but I don’t want to overwhelm you).
If it is the bottom leaves, this could be nutrient relocation from the older leaves to the newer leaves and this only happens with nutrients that are mobile, like nitrogen.
It could also be a sign of root rot.
The thing about manures is that they are very low in nutrients, usually 0.5-1.2% per macronutrient, when most of us are giving our plants something in the range of 10-10-10, but the n-p-k number doesn't matter, it's the EC or ppm that really matters.
That will give you a better idea of what is going on.
Before I start a fight about what ppm is best for cannabis, all I can offer is what works for me and that has been a ppm range of about 700. Though on this last grow I changed things up and I was just doing top-offs and I was adding 400ppm solution all through veg and flower, two to three times a week. I didn’t change a thing and my grow was exactly like the last one.
For clones I use 700ppm as soon as they throw roots and they love it!
This is because the clones are in a Kratky type setup and the entire reservoir gets changed every week.
I also grow cannatonic high CBD plants, which don’t behave like high THC plants and I am not growing for production yields so, I am not looking for flowers that are the size of baseball bats.
Here are a few more suggestions…
Spend $20 and get a cheap pH and TDS meter off eBay and use this to get a rough idea of the pH and ppm's of your source water and fertilizer additions.
Then you can have a basis for what you are working with and what needs to be tweaked, otherwise you are just guessing.
Test your source water, always test your source water, especially because it is spring water, which will probably have a crazy high buffering capacity because of all the calcium dissolved in it.
Then you deduct this number from the number you get when you add the fertilizer, this gives you the true ppm of only your fertilizer and not the source water as well.
The pH of your fertilizer should be between 5.5 and 6.5, otherwise you'll have nutrient lockout.
Because your water is spring water, I suggest that you test the pH, use a pH down product to get the pH in range (5.5-6.5) and then let the water sit for a few hours, or over night, then test it again.
If the pH is back up to where it was (7-ish), then repeat this cycle again. If later that day or the next the pH is back to 7-ish, then you have source water with a high buffering capacity and without knowing how much calcium or magnesium the water contains you could end up having to pour the entire bottle of pH down in the bucket before you exhaust the buffer.
So, that should be your first task, figuring out the pH and buffering capacity of your water.
Once you have established this, there are several solutions that you can use to lower the buffer and I am more than happy to talk to you about that, but right now figure out what’s going with your source.
If you got your water from a municipal supply, I would suggest getting the water companies water report as this would tell you everything you needed to know about your water.
I would also suggest adding in about 2grams per gallon of magnesium sulfate (Epsom salt), this will help strengthen the cell walls and help the plants to assimilate nitrogen, phosphorus, and sulfur.
You can sometimes find this at the local dollar store, just make sure it says USP or that is food grade.
You may want to get some potassium silicate (agsil 16H), this is water soluble silica and it will help with so many issues.
First, it will act as a buffer for both nutrient and temperature stress, it is awesome in the summer time.
Side note, using potassium silicate allows me to grow lettuce all through summer without it bolting.
Second, the potassium silicate will help your plants assimilate other macro and micro nutrients, which will lead to your plants growing stronger, faster.
There are also lots of smaller benefits to using silica as well, but none of that will make sense right now and I don't want to overwhelm you.
Potassium silicate can be purchased in 1 pound bags off the internet for around $20.
Don't let the price scare you.
This is mixed as 7-10% concentration batches (with distilled or plain water) and you add 1 teaspoon of this “concentrated” solution per gallon of water that you put on the plants.
This lasts a long time.
You will also have to get a pH down product, I recommend Mad farmer because they don't use dyes like the General Hydroponics crap. It used to be cheaper, but things change.
The potassium silicate is a strongly alkaline, so you will need to add the pH down to bring the pH back in range.
For my water, I am able to add about 1/4 teaspoon per gallon, but how much you will need to add will depend on your source water.
If you decide to use the potassium silicate, this gets added first to your source water, then the pH down, then you test the ppms, this number gets deducted from the ppm’s once you add your fertilizer.
Then you will know the true ppm’s of your fertilizer.
If you add potassium silicate after the fertilizer has been added it will cause nutrients to fall out of solution and you will end up with water that has macronutrients in it that are not bio-available to your plants.
If you do these things it should give you 180 degrees in terms of results right off the bat.
Also stay away from Miracle-gro, that stuff is junk.
It is formulated like a fertilizer that is meant to be top dressed to soil (which takes time to breakdown) but it is applied like a fertilizer meant for hydroponics. The break down of the urea and the Iron-EDTA can also wreck havoc with your pH…
Before I end this novel sized post, you may also want to consider swapping from soil to a media based hydroponics system (like Dutch buckets), it's so much easier to grow plants using a system like this.
But again that is my opinion…
