"
lol where did you get PPM of 1600 from? I use the EC option on my meter."
In the first posting, you wrote "Starting EC was 1691. Checked around 6:00 p.m. tonight And pH was up at 6.5 and EC was 1630."
EC cannot be that high. You can get a reading of 1600± from an EC meter because it's calculating the PPM and displaying the calculated value.
"
Not sure I understand. My understanding is that the EC of a solution will measure how much "stuff" is in it. My tap water has an EC of .2. After adding my nutes, let's say the EC is now 2.0. After 1 day in the reservoir I will measure and the probe says the EC is now 1.5. This tells me the plant is consuming more nutes than water (and I should make my nutrient mixes stronger). If it goes up to 2.5 EC this would tell me the plant is drinking more water than nutrients (making the concentration stronger - which would tell me to make the mixture weaker). Is this the right way to think about this?"
EC is an abbreviation for "electrical conductivity" and the unit of measure is microsiemens per whatchawhosit. All that EC tells you is how much current passes between two nodes in a give time (I haven't learned the details of the unit of measure).
An EC meter determines the EC and then converts that value to the equivalent concentration of NaCl or KCl that is needed to pass that same amount of electricity (500 and 700PPM) respectively. That's all the PPM is - a calculation based on the current being passed between two electrodes. It does not indicate what's in the nutrient solution and the concentration of the nutrient solution is reduced when nutrients are taken up even though the EC change is not measured by an EC meter. The reason for that is that N, P, and K are taken up very quickly but they don't cause much of a change in EC. Ca, taken up more slowly, has a greater impact on EC.
I understand the expression "consuming more nutes that water". It's very popular among cannabos growers and it's based on observation and inference but it's not correct. The underlying assumptions are, apparently, that nutrients are taken up at the same rate and, also, that the individual chemicals have the same impact on EC. Both of those assumptions are incorrect.
Plants require higher concentrations of nutrients at different stages of their growth but they don't require a consistently increasing concentration of nutrients. Nutrients are taken up as plans transpire and in response to chemicals that the plant exudes. As plants grow larger, they have more leaf surface area and, as a result, they will transpire more, assuming that the rest of the grow environment remains the same. As a result of increased transpiration, they will absorb more nutrients. They don't need a higher concentration of nutrients as a matter of course, they just need to have the nutrient solution replenished.
If EC is increasing, the plant is still exchanging nutrients with the root zone and the nutrients that are in the root zone have a high electrical conductivity than the nutrients that are being taken up. That does not mean that the plant needs a higher concentration of nutrients.
Again, I know that's conventional wisdom but that's because many growers think that EC tells you that your nutes are "strong". It just means that they have a high concentration of the chemicals in nutrient solutions that are highly conductive. It does not tell you what the mix of chemicals is. It just means that they carry more electricity.
The only way to know which chemicals are in the nutrient solution or the plant is to do a chemical anaylsis of the nutrient solution or the plant.
You can keep increasing the concentration of your nute mix and giving that to your plants and you will, eventually, move the plant into the toxicity range. Once you've done that, you'll have to flush out your soil and, hopefully, you'll reduce the EC of your new solution and the plants will recover. That practice is why industry consultants state that cannabis growers use to much fertilizer. It's a practice that's not uncommon at the commercial level and it's not uncommon at the personal grower level.
It's very rare to see a plant having issues because nutrient levels are too low. What's far more common is to see growers raising continuing to raise the EC of their nutrients or not keeping their temps+RH in range. Way down the list are making mistakes in mixing the nutes, followed by poor watering practices.
"
I guess the big question here (which I haven't heard Bugbee answer yet) is whether plants will keep taking nutes to the point of toxicity or if they will "take what they need". What you're saying is that they can take too much and become toxic, which seems to be the case with my plants and is logical to me. But why doesn't this occur outdoors where the soil has much more nutrients than any one plant can consume?"
If you listen to Bugbee speak re. nutrients you will find that he recommends replenishing the reservoir with "balanced" nutrients by which he means the nutrient solution that your using for the grow.
He addresses the issue in this video:
and he discussed the law of electroequivalency, the concept of mass balance, and the speed of uptake of nutrients in the attached paper. He also talks about nutrient uptake speed in the above video but not in much detail.
My mixture is 5 gallons. I will replace the res once a week. However, in peak feeding where the plant can drink a gallon a day, I let the res get down to 1-2 gallons before swapping the res, which is about every 3 days.
In the Bugbee paper, he recommends replenishing with "⅓ strength Hoagland solution", Hoagland's being EC 1.8 but in the video he says to just replenish with a "balanced solution".
In the CannaStats paper, the author describes using just RO water and replacing the res when pH becomes unstable. I've used that approach for a few grows but, for my current grow, am switching to replenishing rather than topping off with RO.
Doing hydro with a 5 gallon bucket looks challenging. I went the opposite route, using a res that holds 28 gallons so I do three res changes during the course of a grow.
Re. changing concentration - my grow is at day 29 and my EC is 0.8. I'm getting some dark spots and I think it's because my EC is too low so I'm going to bump EC to 550-600 tomorrow. I'm growing two plants this time (I usually just grow one because my tent is only 2' x 4') but I can't quite fill a tent with one plant. The plants are undergoing a signficant growth spurt because PPM has dropped by 30 points (out of 500) in two days. If you grind your way through the Bugbee paper, you'll see which chemicals are being taken up that are causing EC/PPM to drop. Much of the N, P, K, and Mg are already in the plant but it's the "slow uptake" nutrients are are now being depleted that are causing EC to drop. Look for the section on speed of uptake and it will list out what's taken up when. That paper was an eye opener for me.
