Hi Mike,

Having a pH around 7.1 isn't ideal but it's not an immediate death sentence for plants either. My educated guess for the reason your pH keeps creeping higher is that the plants are absorbing nitrates from the nutrient solution, which growing plants require in relative quantity. Basically the roots make a deal with the nutrient solution where they trade a hydroxide ion for a nitrate. When it's all said the roots absorb the nitrate and in return the nutrient solution receives a hydroxide (chemical responsible for a high pH) so the pH will tend to increase.

This happens somewhat gradually and if problems result from the increasing pH you should see an iron deficiency first since that's the first nutrient to become unavailable. Some pictures and more info about iron deficiency here:

http://bexar-tx.tamu.edu/HomeHort/F1Column/2003Articles/MAY25.htm

Sounds like the PPM of your nutrient solution is reasonable. I would take a closer look at the nutrient solution. Make sure the temperature isn't much more than 75 degrees F (24 degrees C) and you are getting adequate circulation and turnover in the reservoir. If you don't have an air pump look into getting one. Having a tightly sealed reservoir without an air pump can cause problems. If you are going to start over clean down everything with some bleach, fresh hydrogen peroxide, iodine or something similar. It's possible you don't have enough dissolved oxygen in your nutrient solution and the roots need it for cellular respiration plus some bad pathogens like to grow in warm water with low concentrations of oxygen. If your room is considerably warmer during the summer and that's contributing to a higher reservoir temperature what you might have had in the winter you could consider freezing some plastic bottles of water and putting them in your reservoir during the day.

It sounds like you didn't experience this problem with the climbing pH earlier so I'm guessing what you are growing currently has a much higher metabolism, thus increased nitrate uptake. Plus the warmer summertime weather could be contributing if that room was particularly chilly during the winter and spring. Because of this I would suspect that once you isolate and eliminate the source of your problems you will be very productive.

bilberrybrian,

Thanks for the comprehensive reply. You may have found the cause. It isn't the plants taking in too much nutrient or lack of aeration - the plants are new seedlings, some with only 4-six leaves. I also have an air pump supplying plenty or oxygen. But you mentioned water and having a temp of 24C. Mine is 30C. Would that cause the pH to rise as well as the plants to succumb after just 2-3 days?

Mike

Mike,

That's actually getting pretty warm. Pure water will hold at most about 9 ppm of dissolved oxygen at 20˚C and 7 ppm at 30˚C. That doesn't mean you actually have that much oxygen, just that's the most possible given the best case scenario for everything else. That may not sound like a huge difference but those plant roots really need every ppm they can get at that level. Also, these sort of conditions can encourage certain nasty microorganisms to grow that love to attack plant roots. This is one of the reasons you would want to methodically clean all your equipment before the next run, to make sure you kill all the bad stuff that might have become established. The numbers above came from the formula:

14.652 - 0.41022*T + 0.0079910*T^2 - 0.000077774*T^3

Where T is the temperature in ˚C.

That formula is from page 175 of the McGraw-Hill Handbook of Environmental Engineering Calculations. You can plug it into a spread sheet application and whip up a graph if you like.

What hydroponic method are you using? Deep water culture?

Insufficient oxygen at the root level should be fully capable of killing some plants within a couple days. When there's a heavy rain and the ground becomes water-logged that's about how long we would expect certain plants to live before succumbing to the flooded soil. I'm not 100% certain about this, so don't quote me, but I think the following scenario plays out at the biochemical level. Ideally plants can provide themselves with energy during respiration by basically burning up sugars (glucose and fructose primarily) which mostly is turned into carbon dioxide. This is a chemical process that can only proceed so far without oxygen. In a situation where oxygen is limited the plant will do the best it can. A plant cell will make it roughly half way through this process before it can't proceed any further without oxygen and at that point the plant has no other choice but to take a biochemical detour and start turning these partially used up sugars into alcohol. This wouldn't be a problem if it happened briefly but if the plant is stuck doing this it would, I reason, build up enough alcohol to kill itself within a short time. I was told by a professor of biochemistry once that the reason wine is at most ~14% alcohol is that the yeast producing the alcohol will start to poison itself at that point. We would hope that the plant could transport enough oxygen from the leaves down into the roots to prevent this but if that were the case in every type of plant we probably wouldn't have to be cautious about killing so many different types of plants by over watering them. It's possible that there are several other reasons that all contribute to a lack of oxygen being bad, so take what I say with a grain of salt. I'm going to research this topic a little more and see what I can find.

What are you doing to start your young plants? It sounds like you start from cuttings and then them into your hydroponic system?

If you want to try an experiment run one system as you did previously and in the other system take some steps to cool the water. Using the ice bottle trick, for example. If both systems kill their plants in the same way we know that we are barking up the wrong tree here. If not, then you isolated the problem.

I wouldn't worry about the pH changing so much right now. That's kind of a normal thing to expect and you see it happen in hydroponic systems where the plants aren't dying. I wouldn't dismiss it all together though either.

bilberrybrian,

Yes, it is DWC. I'm starting the plants two different ways: potting mix and Rock Wool. Neither has made a difference.

From doing more research, I'm wondering if it may be a couple of culprits. My wife buys kitty litter in five gallon buckets and that is what I have been using. As said, I've used them (or different ones) in the past without any problems. I'm wondering if the higher temps (albeit them on a couple degrees C, might be causing the bucket to decompose. I do notice that the solution in the buckets where the plants die tend to turn a greenish blue rather quickly - in a couple of days, while the other water stays sky blue. I know they are getting enough air - 900 gallons per hour among the five buckets now, a lot less before (but with the same result).

I'm going to try to pick up some food grade buckets Tuesday and try those.

I do plan on cloning some plants so that I have a decent start. I figure I will have toms through no later than mid-October from my garden. Even if I count starting plants in water now as transplanting, it's still going to be 75 days before any from the GH are ripe, and that is being optimistic!

Mike

Yesterday, I went out and bought a bunch of food-grade 5-gallon buckets with lids. $3 each so not a bad price. Threw all the old water and nuit solution out, used new water and ferts. Adjusted the pH to 6.5-6.7 and the ppm from 750-850. Added some seedlings growing in rockwool and placed them under "shop lights" that are within 1-2" of the top leaves. I cannot get them any closer - the lights are practically laying on the hydro buckets now.

It's only been a day, but all six plants look healthy. Another report in a day or two.

Mike