Plant stopped uptaking nutes in flowering: DWC

[TheMoz4rt]

Hello everyone, first time grower here.
I'm growing an autoflower in DWC and she's 63 days from sprout, 3 weeks into flowering stage.
I've switched from Atami line to CANNA Aqua Flores as I couldn't use organic boosters since I had problems with roots early on, and I wanted to feed her with something more specific for this blooming stage.
I kept the same EC during the switch, 1,65 mS/cm (825 ppm - Hanna). While I was giving her 1,65 EC, I was experiencing a 0.1 (50ppm) daily drop.
Increasing it didn't work as N toxicity signs were showing up (tips pointing down).

For the last two weeks, approximately from the moment I switched nutrients, she's drinking almost 3L/day probably due to the growth stage but EC is always higher now, every time I check it before the daily refill.

After a deeper look at the new leaves, I noticed some overfeeding signs (of course..) so I dropped the EC down to 1.4 (700ppm) even though it keep rising

What's happening? Isn't she supposed to eat more at this stage? I'm scared to overfeed her as well as to start seeing deficiencies soon. I should give her some PK 13/14 soon and I can't figure out how to avoid more problems when that time comes.

Never had problems till now. pH always in range. 20/4 light schedule, 27-28°C (82F) at lights on and 21°C (70F) off. I could drop the day temperature to 24°C (75F) but I'm waiting as I guess the flowers are not ripening yet.

Any help would be super appreciated, thanks in advance.

Here some pics: you can see those burned/curled leaves tips if watching carefully.

 

[indica vet]

When in doubt , flush it out .
Seeing as the nutrient change was indicative of your issue starting I would flush it out and try again on weaker dose to start with

 

[indica vet]

Also with dwc the walls above the water line of tub/bucket get splashed with nutes which then evaporates leaving dry mineral salt residue then when refilling etc this dry salts rehydrate and add to your ec.
Best to clean tubs/bucket periodically to avoid ec rises

 

[TheMoz4rt]

Thanks man, that's a useful input. Indeed I notice dry mireals in the buckets. I'll clean everything and see if that helps.

 

[KingJohnC]

The component for stability is pH stability of the nutrient solution in a hydroponic system.

You mention a Deep Water Culture hydroponic system but not the reservoir volume size of the container. Can you please list the equipment and volume size of the grow containers and reservoir including the specific type of DWC system?

Can you please post some photographs of the hydroponic system and equipment in use?

 

[TheMoz4rt]

Hello KingJohnC,
I know pH makes nutrients available to be uptaken by the plant. I never let it make big jumps, and I keep it between 5.8 and 6.2, if that's the stability you're talking about.

While I'm not that expert to know how many specific types of DWC systems exist, I can tell you I have a signle standard black 5 gal (20L) bucket and two airpumps (4.2L/min with a 10cm ceramic disk airstone, and a weaker 1.5L/min with a 5cm ball-shape airstone). The bucket, when completely filled, holds 13L of nutrients, leaving a 1.5" air gap for the roots.

I got a 150W LED light (TS1000), 80x80cm growbox and ph/EC readers are from Adwa.
I can surely upload more pictures when lights turn on, just let me know what you want to see in specific. I'm confident you don't care to see the bottom of the bucket to check how many times the plastic was recycled or which country it was made in. Don't get me wrong, I'm just not a native English speaker and a newbie in growing, simply seeking advices from anyone who has more experience than me.

 

[KingJohnC]

Ideally in hydroponics you want the pH to remain stable at pH 5.8 and do not want pH to rise ever. There is a narrow pH band where nutrient have the highest usability. A pH difference from 5.8 pH to 6.2 pH is actually a difference of 40x times greater change in the nutrient mixture. The reason why the pH increases is due to at least 3 or more major factors which alter pH and cause the pH of the nutrient solution to ever rise out of optimal range.

a) As nutrients are used the pH of the nutrient solution is changed by the removal of non proportional nutrient use from the nutrient solution.

b) Cannabis plants may use more water than nutrients from the nutrient solution and increase the PPM strength and alter the pH of the nutrient solution.

c) Mostly during the dark cycle cannabis plants release metabolic wastes exudates back into the nutrient solution and alter nutrient composition, PPM strength and pH of the nutrient solution.

d) When either caused by over abundance of any single elements at relative pH the actual preferred form of the nutrient available reforms to more complex bonds which are not as easily used by the plants for growth an require much more energy to convert the elements back to preferred easily used simple bonds most easily used by the cannabis plants requirements.

We usually see abnormal slow growth or damaged leaves from nutrient deficiency showing the plants requirements are out of ideal ranges.

The usual issue with DWC single bucket systems is the low volume available nutrient mixture does not support large vegetative mass easily and remain stable. The 5 gallon dry 4.2 - 4.5 actual wet gallon buckets are too small to buffer any pH changes.

Beer and wine making suppliers sometimes carry larger volume bucket containers.

Do you calibrate both the pH meter and EC or PPM meter before use?

There are 3 methods commonly used to adjust the pH of small volume single bucket containers with DWC hydroponic systems.

a) The add back method is where pH adjusted water is added back to the DWC buckets to counter the pH rise and bring the nutrient solution to 5.8 pH allowing the same nutrient solution to be available for the plant to use in optimal range.

b) pH adjusting solution is added to the nutrient solution to adjust the nutrient solution back to 5.8 pH.

c) The nutrient solution is changed every 2nd day adjusted.

Please do not add pH adjusting solution directly to an area where cannabis plants roots are exposed.

I prefer the add back method to adjust the pH of the nutrient reservoir as the plants are able to continue using nutrients from solution in a predictable manner allowing a predictable quantity of pH adjusted water to be added until the entire reservoir is changed every 4-5 days. The smaller volume of the nutrient reservoir the shorter period the same nutrient solution may be used before being discarded. The expense of the nutrients is a major factor to grow cannabis plants to harvest and prolonging the length of time that a nutrient solution may be viable and the nutrients available for the cannabis plants to access greatly reduces the expense of chemical nutrients and the $ per gram costs for high quality cannabis medicine.

 

[TheMoz4rt]

Thank you, I will work on keeping pH steady as much as I can. I've looked for informations before opening this thread buy they're are quite deceiving, like that pH should drift to keep the plant healthy.
I'm gonna follow your advices as you gave me great explanations.

I calibrate pH meter once a week, and EC meter once every two weeks.

My plant is definitely drinking more water than nutrients.
Do you think I should lower the lights on temperatures? Maybe she's transpiring more than it should due to high temps. Gonna stick to 5.8 anyway

 

[KingJohnC]

If you check a chart for reference and see where the pH for greatest assimilation is possible for cannabis plants grown in hydroponics 5.8 pH is ideal. It is a great miss understanding that pH should increase. As explained the pH does rise but through undesirable metabolic waste processes which create more stable bonds to reduce harm from active forms which are much more energy expensive for the plant to reduce back into the simple forms the plant is most easily able to assimilate.

The ideal temperature range is also relative humidity dependent. Below 16 Celsius or 62 Fahrenheit many plants metabolisms are not able to sufficiently assimilate nutrients for growth. Allow the plant to remain above 19 Celsius or 65 Fahrenheit. 23 - 27 Celsius or 73 - 80 Fahrenheit and 55% - 65% humidity is ideal for vegetative growth and 30% - 50% humidity in flower. In vegetative growth 65% humidity is preferred to reduce the occurrence of powdery mildew.

There is a better understanding on the effects of humidity and growing cannabis plants Vapor Pressure Deficit

The Downlow On Vapor Pressure Density - How Does VPD Affect Cannabis Plants?