Whenever leaves yellow or brown the most common thing to think is “nute burn!” However, there are other reasons for the leaves going yellow in the particular way that yours have. For instance, nitrogen deficiency.
I’m not familiar with the fert that you mention, but your ppm at 350 doesn’t sound too high to me. If anything, it sounds too low. I’m used to my reservoir being at anywhere from 700 to 1200 ppm, more if I’m generating CO2. So 350 ppm could be a sign that the NPK of your basic fert is not high enough, especially in Nitrogen.
I presume you’re in the veg stage, judging from the picture. Flushing your system is always a good idea, but if your plants are Nitrogen deficient, then you have to add a fert that has adequate supplies of Nitrogen, in order to have your plants grow green, luscious leaves.
I personally use the Advanced Nutrients 2-part,
Sensi Grow A&B during my veg stage. In addition to my basic fert, I also mix in root colonizers, vitamins, and other supplements, including Grandma Enggy’s
Humic Acid and Fulvic Acid.
These last two ingredients simulate the rich, black, organic topsoil that has been the secret of horticulturalists for centuries. Derived from a calcified organic substance known as “leonardite,” Humic Acid and
Fulvic Acid supply a whole array of organic substances that aid in the healthy growth of plant tissues.
Another mistaken idea that you have is that the higher your pH goes the more acidic your reservoir gets. It’s exactly the opposite. The lower your pH, the more acidic the solution. You mention that your pH goes from 5.6 to 7, which is neutral. Anything above 7 is too alkaline. 5.6 pH is the desired level you should be aiming at.
How do you correct your pH? Advanced Nutrients makes two excellent products,
pH Up and pH Down, which should be added in minute quantities to your solution, if the number needs correction.
Another participant commented that perhaps you take your readings too fast, and he’s probably right. After you mix all your ingredients, you should take a pH reading every half hour in order to determine when your solution has stabilized. Once it has, then you can feed it to your plants.
So you seem to have a lot of variables to consider before you can diagnose your problem as nute burn. Marijuana requires high levels of Nitrogen during vegetative growth, lower levels during flowering. Nitrogen, being mobile, is easily washed away and must be replaced regularly.
Nitrogen helps your cannabis plants to regulate the manufacture of proteins that are essential for creating new protoplasm in the cells. It is also essential for the production of chlorophyll, enzymes, alkaloids, amino acids, and nucleic acids, which are in turn irreplaceable ingredients in healthy plant cell production.
If you switch to a better fertilizer, such as the ones made by Advanced Nutrients Medical, you can avoid headaches such as the ones you’re having now. Do some research into how much Nitrogen your should be feeding your vegging plants. Start at the
website of the company that makes the best cannabis specific ferts in the world.