Potassium or light burn ?

[Imagine]

Hey folks, on day 52 with some Fast Buds wedding cheesecakes and starting to see some spotting /edge browning on the tallest colas. I'm thinking the leds were getting to close and I got some burning or possibly to much potassium. What do you think?

Problem: spotting/browning

Medium/grow method: rdwc. 20 gal in res and two buckets and 5 gal top off tank

Feed: and supplements used:Mega Crop 2 part + bennies - schedule for 25 gals
Part A 45g
Epsom 7.5g
K 5g
part B 16.25g
humic acid 150ml
Raw amino acid 3.12g
raw ominA 3.12g

500-550 ppm

water source: filtered tap and ro to 80ppm

Strain/age: wedding cheesecake/52 days

light used:Mars Hydro sp3000 and tsl 2000

Climate: 77-80°, 65% rh, 68° res temp

Additional info: 5x5x8 closet with ac infinity 6" fan and carbon filter. Oscillating fan

 

[namvet25]

Have you used any cal/mag? How close is the light?

 

[Imagine]

Have you used any cal/mag? How close is the light?

Haven't used any cal-mag. All the lower leaves are perfect and green, just the highest leaves are affected. The top had gotten about to about 8-10 inches from the light before I rigged a way to get it higher. Now it's at about 18".

 

[Bruce Campbell]

What is the "K 5g"? Is that just straight potash?

How long does a reservoir last you? How much pH drift are you seeing? What pH up/down do you use (and how much)?

 

[Simplicio]

Hey folks, on day 52 with some Fast Buds wedding cheesecakes and starting to see some spotting /edge browning on the tallest colas. I'm thinking the leds were getting to close and I got some burning or possibly to much potassium. What do you think?

Problem: spotting/browning

Medium/grow method: rdwc. 20 gal in res and two buckets and 5 gal top off tank

Feed: and supplements used:Mega Crop 2 part + bennies - schedule for 25 gals
Part A 45g
Epsom 7.5g
K 5g
part B 16.25g
humic acid 150ml
Raw amino acid 3.12g
raw ominA 3.12g

500-550 ppm

water source: filtered tap and ro to 80ppm

Strain/age: wedding cheesecake/52 days

light used:Mars Hydro sp3000 and tsl 2000

Climate: 77-80°, 65% rh, 68° res temp

Additional info: 5x5x8 closet with ac infinity 6" fan and carbon filter. Oscillating fan

View attachment 1380658View attachment 1380660View attachment 1380661View attachment 1380662View attachment 1380663

Any good fertilizer, correctly used (probably also with calmag), should produce no deficiencies.
I think the light is too close.

My experience is that autos start needing less food and light after mid-flower.
Too much light can produce those spots you are seeing.

You can figure out feed levels by keeping track of ppm from day to day.
If it falls, plants want more food; if it rises they want less.

 

[Imagine]

What is the "K 5g"? Is that just straight potash?

How long does a reservoir last you? How much pH drift are you seeing? What pH up/down do you use (and how much)?

Res gets changed weekly. I'm using GH down and Bloom City up. Don't keep track of how much, just use as needed. When I change the res it generally go's up for a day then holds steady for 3 or 4 days then starts drifting down.

 

[Imagine]

Any good fertilizer, correctly used (probably also with calmag), should produce no deficiencies.
I think the light is too close.

My experience is that autos start needing less food and light after mid-flower.
Too much light can produce those spots you are seeing.

You can figure out feed levels by keeping track of ppm from day to day.
If it falls, plants want more food; if it rises they want less.

I start the res at 500 to 550 ppm. It has been drifting down over the course of a week but leveled out and has been drifting up over the last week. I may need to back off a bit and see what happens.

 

[Bruce Campbell]

Let's tag @Mañ'O'Green for his input.

The leaves look like they could be low Ca (which is aggravated by high K). They also look a bit like low Mn (which is less common). Your drift is going to be a combination of the plants taking nutrients out of the feed, and chemicals in the feed combining over time and coming out of solution and/or becoming unavailable. I also ran across a couple mentions that fulvic acid may be better than humic acid in your application.

1. Mn

You're slightly low on Mn from your MC Part A - I calculate ~0.29 ppm vs. typical ranges of 0.5-1.0 ppm. You can't really boost your Part A that much without putting everything else out of balance. You could try foliar application of mangenese sulfate, but you'd need an insanely small amount, and I doubt this is the issue. So, ignore it for now and re-visit if the items below don't resolve the issue.

2. Low Ca / High K

I grow in soil/peat, but normally aim for a 4:2:1 ratio of K:Ca:Mg. If you're filtering your tap water, you may be filtering out some of the Ca. If you're using the RAW brand humic acid, it looks like it has some additional K in it. And, your pH up is potassium based.

At first, I'd try reducing / eliminating the potash to reduce K and improve the K/Ca ratio. But, I think you may also have some Ca coming out of solution over the week - which could be a large part of the drift you're seeing.

3. Drift

MC Part B is just calcium nitrate (cal-mag minus the mag), which can form precipitates with phosphates and sulfates over time / at high pH. Here's a chart from a google search of "calcium nitrate compatibility" (source):

Not sure exactly what "incompatible" means - i.e., forms precipitates at the very start, eventually over time, is pH dependent, etc. But, it seems to indicate that the sulfur from the epsom salts may be reducing the Ca in solution. You could try reducing / eliminating the epsom salts from the reservoir (and apply foliar later if a deficiency develops), or try increasing the MC Part B to give you more starting Ca over the course of the week.

4. Fulvic vs. Humic acid

Fulvic acid is humic acid in the next stage of decomposition. Both provide chelation, which helps to keep metal ions in solution and available for plants (and, should lessen drift to a degree). I ran across a couple mentions that humic acid starts to precipitate / become less effective at pH < 6.0 and, therefore, fulvic acid (which can tolerate a broader pH range) is better for hydroponic applications.

That may be more important over time in RDWC than DWC. Increasing the amount of humic acid in your reservoir may help reduce drift, but fulvic acid may be better for RDWC in general.

 

[Imagine]

Let's tag @Mañ'O'Green for his input.

The leaves look like they could be low Ca (which is aggravated by high K). They also look a bit like low Mn (which is less common). Your drift is going to be a combination of the plants taking nutrients out of the feed, and chemicals in the feed combining over time and coming out of solution and/or becoming unavailable. I also ran across a couple mentions that fulvic acid may be better than humic acid in your application.

1. Mn

You're slightly low on Mn from your MC Part A - I calculate ~0.29 ppm vs. typical ranges of 0.5-1.0 ppm. You can't really boost your Part A that much without putting everything else out of balance. You could try foliar application of mangenese sulfate, but you'd need an insanely small amount, and I doubt this is the issue. So, ignore it for now and re-visit if the items below don't resolve the issue.

2. Low Ca / High K

I grow in soil/peat, but normally aim for a 4:2:1 ratio of K:Ca:Mg. If you're filtering your tap water, you may be filtering out some of the Ca. If you're using the RAW brand humic acid, it looks like it has some additional K in it. And, your pH up is potassium based.

At first, I'd try reducing / eliminating the potash to reduce K and improve the K/Ca ratio. But, I think you may also have some Ca coming out of solution over the week - which could be a large part of the drift you're seeing.

3. Drift

MC Part B is just calcium nitrate (cal-mag minus the mag), which can form precipitates with phosphates and sulfates over time / at high pH. Here's a chart from a google search of "calcium nitrate compatibility" (source):

View attachment 1381079

Not sure exactly what "incompatible" means - i.e., forms precipitates at the very start, eventually over time, is pH dependent, etc. But, it seems to indicate that the sulfur from the epsom salts may be reducing the Ca in solution. You could try reducing / eliminating the epsom salts from the reservoir (and apply foliar later if a deficiency develops), or try increasing the MC Part B to give you more starting Ca over the course of the week.

4. Fulvic vs. Humic acid

Fulvic acid is humic acid in the next stage of decomposition. Both provide chelation, which helps to keep metal ions in solution and available for plants (and, should lessen drift to a degree). I ran across a couple mentions that humic acid starts to precipitate / become less effective at pH < 6.0 and, therefore, fulvic acid (which can tolerate a broader pH range) is better for hydroponic applications.

That may be more important over time in RDWC than DWC. Increasing the amount of humic acid in your reservoir may help reduce drift, but fulvic acid may be better for RDWC in general.

Thanks KDawg, I appreciate the insight. This is why I love this site. Folks are willing to help you through troubles, which in turn helps to make better growers. Now if I can just retain a little of it....

 

[Imagine]