Measuring PPMs in Hydro
This is how I do PPMS in hydro if anyone is interested
- Mix nutrients according to schedule at half strength and adjust the pH (most nutrient companies give way too high levels of nutrients for a plant like cannabis, but their ratios are good)
- Check the PPM and then give the water to the plant.
- Pay attention to the plant to determine if PPM needs to be adjusted. At that point I can use the PPM measurement as a reference to see how the plant responds.
- If plant is getting overall pale or lime green over time, I know it needs increased PPMs
- If plant is getting tip burn on the leaves, or the leaves are getting dark green, I know it needs lower PPMs
- Maintain PPM of the reservoir. If the PPMs of the reservoir are dropping as the plant drinks, it means I should be topping off with nutrient water to replenish the nutrients to keep it at the "correct" PPM. If the PPMs are climbing as the plant drinks, I know I should only top off with plain water to lower the PPMs.
- Next time I mix up nutrients, I'll make it stronger or weaker as needed. So maybe the plant got nutrient burn when I gave it nutrients at half strength. I'll mix my next set of nutrients at 40% strength according to the schedule. Then I'll measure the PPMs and that will be my new "base" number to compare to. I've found that optimal PPM numbers change over the plant's life, but I don't worry about that. At least for me, every time it's about measuring the nutrients out according to the schedule, then seeing what the PPM is. Not the other way around of trying to aim for a specific PPM.
Example: So say I mix up my General Hydroponics Flora trio nutrients at half strength. I add Hydroguard and Cal-Mag. I adjust the pH. When I measure the PPM it comes out as 600 PPM. So that's my "starting" PPM. I pour it into my plant's reservoir. Over the next few days I notice that the plant appears to be getting more pale. When I measure the PPM of the reservoir I see that the PPMs are dropping. In that case, I'll top off the tank with full strength nutrient water until I get the PPM to, say 650 PPM, and then see how the plant responds. If it starts gaining back color but doesn't get nutrient burn, I know I've hit a sweet spot.
With GH nutrients, you can mix up nutrient water up to two weeks ahead of time. So I'll usually mix up a jug of full-strength nutrients (stronger than what I would normally give the plant) and another jug of pH'ed plain water. I use these to top off the plant, so I can use them to raise or lower the PPM as needed. However, when PPMs get used up, the plant isn't taking everything equally. It's taking more of some nutrients than others. So over time the nutrient ratios get messed up even if the PPM measurement is staying the same.
I've found that in the vegetative stage, you can go weeks without a full reservoir change if you follow that method of topping off. However, I've found that in the flowering stage you really have to completely change the reservoir once every week or two even if you're topping off that way to keep the ratios just right. Cannabis plants seem to be a lot more finicky about nutrients and pH in the flowering stage.
Each time I mix water, I forget about the PPM from before because that was from a different part of the plant's life. So from the above example, I know that half-strength nutrients was too weak. I'll measure the next set of nutrients (from the schedule for this week of the plant's life) at 3/4 strength. Say I measure the PPM and it's 670 PPM. Now that's my new "base" number and I compare my new observations from that until the next reservoir change. You'll notice that at different points in the schedule, your PPMs will be different after nutrients are measured out.
In Summary:
Basically I let the nutrient company determine the ratios, and the plants determine the strength. The PPMs are just a way to help measure the "strength" as experienced by the plant.