Adjusting Nutrient Concentration

[rbarter]

I have a question concerning the adjustment of my nutrient concentration based on the needs of my plants.

I operate on a 24 hour cycle, I start with a full nutrient tank with known EC and pH and feed for 24 hours. At the end of 24 hours I measure the concentration and pH, top off the tank with distilled water, measure concentration and pH again and adjust nutrients and pH accordingly.

I am making some simplifying assumptions:

1. EC is a good overall indicator nutrient concentration
2. The NPK and Micro nutrients RATIOS as recommended by the nutrient supplier are correct (even if the recommended concentrations are too high).
3. The Hydroton and stainless steel container are not contributing to EC and pH changes (this may be a very bad assumption)
4. Any change in EC (other than adding distilled water) is caused by plant nutrient uptake.

So, as an example, I start with a full nutrient tank and an EC of 1000. 24 hours later the EC is 1200. I add distilled water and the new EC is 600. Time to adjust the nutrient concentration. (I'm also adjusting pH, but not talking about it here for simplification).

My question is this: does the EC changing from 1000 to 1200 indicate that the EC was too high, too low, or does it mean nothing?

I've seen two explanations regarding plant nutrient uptake: One would suggest that an increase in EC indicates that the EC was too low, the other that is was too high.

When I assume an increasing EC is an indication of the EC being too high, and adjust the new EC to be 800 in the above example, then things seem more stable. If I assume the opposite and adjust the new EC to 1200, the things seem to go off the rails, but I'm not sure.

Any thoughts, comments are welcome.

By the way, my nutrient tank is only 1.5 gallons -- that is the reason for the large EC change when I add distilled water. You can see the nutrient tank under the Grow Cabinet in the image below:

 

[Judaz]

The key indicator here is root zone EC and PH. If you can measure the run off numbers for each plant individually then you can see which plants are right and which ones need adjustment. I do this by collecting the runoff for a plant with a cup and then take measurements. When you measure the whole return as a whole you are taking the avg from all the plants using the same reservoir solution. The key to EC management is balance. Meaning that your ppm in is relatively close to the runoff ppm. My rule is if i see the runoff spike more than 200-300 ppm. I'll flush to bring it down if I'm running to waste. If I'm recirculating I'll lower the res strength so that it comes back at the target level that I have set for that strain in that particular week of the flowering cycle.

 

[rbarter]

Thank you. I had not considered the difference between root EC and tank EC, but you are of course correct.

I'm not thrilled about adding a second probe. I will try putting the probe in the discharge stream in such a way that it sees the discharge when the pump is on and sees the tank nutrients when the pump is off. Multiple plants present additional complexity, but I like the opportunity to diagnose individual plants.

I'm using a Dutch Pot type system, I can imagine simple EC probes in the bottom of each pot (pot pot??). Would it be worth $100 per plant (retail) to be able to diagnose individual plants?

 

[MW7945]

I think you need a bigger carbon filter in there man. Doesn't look like it can handle that space ;)

 

[rbarter]

I think you need a bigger carbon filter in there man. Doesn't look like it can handle that space ;)

I think you're right. It's 4" and stubby. It's working OK for now, but may be close to saturation. Just starting to bud.

The next iteration will have the fan and scrubber on the outside back of the cabinet and I'll probably increase to 6". I'll use a motorized damper to selectively send the exhaust back into the cabinet when I need heating.

It's becoming fairly clear that I'm trying to put too much stuff into the cabinet.

 

[Judaz]

Thank you. I had not considered the difference between root EC and tank EC, but you are of course correct.

I'm not thrilled about adding a second probe. I will try putting the probe in the discharge stream in such a way that it sees the discharge when the pump is on and sees the tank nutrients when the pump is off. Multiple plants present additional complexity, but I like the opportunity to diagnose individual plants.

I'm using a Dutch Pot type system, I can imagine simple EC probes in the bottom of each pot (pot pot??). Would it be worth $100 per plant (retail) to be able to diagnose individual plants?

Get yourself a ppm pen by blue lab. You don't have to leave it in the res and can move it. Not sure exactly how this system works but find a way to isolate runoff for each plant. May be a pain in the ass but worth the to study

 

[MW7945]

I was kidding man, assuming your fan can move a decent cfm that filter is plenty big. I didn't even throw a filter on until 4 weeks into flower

 

[rbarter]

Duh! Color me naive. Although I'll probably stay with some version of this scrubber.

My goal is to develop a highly automate system for a novice grower that is compatible with a kitchen environment. The grower purchases a "cartridge" from a dispensary with a plant that has been taken through the veg stage and trained as a Screen of Green.

As the grower gains knowledge, the cabinet can also be converted to support a four-plant grow tent with just the addition of a duct. The system can support larger grow spaces (up to 5,000 square feet) by adding more monitoring and control points.

 

[MW7945]

You could always put in a "dummy" or false wall in the back to hide a lot of things. Venting, piping, eletrical, etc. That's easily accessible. Would really only have to be 6" wide tops. Can have a hole out the bottom for venting.

 

[rbarter]

Get yourself a ppm pen by blue lab. You don't have to leave it in the res and can move it. Not sure exactly how this system works but find a way to isolate runoff for each plant. May be a pain in the ass but worth the to study

I have ppm and pH pens and manually adjust ppm and pH once a day. Starting tomorrow I'll start recording return-EC as well as tank-EC. Ultimately, I want to develop an adaptive dosing system.

 

[rbarter]

You could always put in a "dummy" or false wall in the back to hide a lot of things. Venting, piping, eletrical, etc. That's easily accessible. Would really only have to be 6" wide tops. Can have a hole out the bottom for venting.

The current design has the LED light somewhat thermally isolated at the top of the cabinet and I'm exhausting from the bottom. I cycle the fan to help control temperature.

I think the next iteration will exhaust from the top and dump the hot air either to the outside, or partially back into the cabinet at the bottom. The fan will always be on with this new design.

Fan and scrubber will be on the outside rear of the cabinet.

Thoughts are welcome.

 

[MW7945]

What type of LED are you running?

 

[rbarter]

I'm in the dark phase right now. I'll get the model tomorrow morning. What I need is a light that is suitable for one plant in the cabinet and will also work in a four plant tent -- for when the grower wants to expand. Anything beyond four plants would require the addition of more lights. The system, by the way, can control an almost unlimited number of lights using Wemo WiFi outlets.

 

[MW7945]

I guess a lot of it depends on what you're willing to spend on the lighting aspect.

 

[rbarter]

The LED light is:
DormGrow 240 Watt Full Spectrum G8LED

Using a HydroFarm Quantum PAR Meter, I measured the PAR 6 inches below the light: 1740 micro mol m−2s−1

 

[MW7945]

Expensive little lights. At 6 inches pretty much anything will push out that kind of par. At 2" my smart phone light even puts out almost 500

 

[rbarter]

I knew there were less expensive solutions out there, but I also read about cautions concerning cheap knockoffs.

Next iteration I'll try less expensive models and verify with the PAR meter.

The ideal solution for me would be to mount the printed circuit board directly to the underside of a metal ceiling plate, heat would then flow through the metal ceiling plate to a small plenum between the plate and the underside of the cabinet. The LED electronics will be in the plenum. ( I've read that a significant amount of power is dissipated by the power supply before it even gets to the LEDs.) I'll then exhaust from the plenum. This will also give me maximum headroom in the grow space.