I am using FFOF. I transplanted almost 2 weeks ago and have used ro h2o ph up to 7.5-7.6 the last 3 watering. I have let the water set out over night I have used liquid compost. It will not move from the 5.2.. Should I buy cal/mag and water again today?
Plain R/O water isn't going to do much in terms of leveling out your pH, seeing as it's inert. What'll likely end up happening is whatever the medium's pH is will remain fairly stable. In order to fix this you can flush the plants with R/O water with a small amount of grow big. That should bring the pH to low 5~ roughly, and from there add a little bit of baking soda to raise the pH. Since you are growing in soil your goal should be to maintain between 6.5-6.8 pH. Depending on how far along the plants are in growth (pics would help), you might be able to replace the baking soda with calmag seeing as that tends to bring the pH up a bit. However if you go easy with the baking soda it works great, I've used it to balance R/O water for seeds to 5.5 and I have pretty much 100% germination rate. EDIT: that's a huge dose of grow big by the way, it converts to a little over 6 tsp. worth. IIRC 1 tsp is like 250-300 PPM or so? if this is the case you are certainly going to run into issues with pH and nute burn unless you have absolutely massive plants that can take all that food. Even then that's a high dose. 30 ml/gallon? HUGE dose.
I am not sure of the ppm.. I have not worked up to buying a tester for that yet. FF suggest 6 tsp which is about 28ml. I have read that baking soda is a temp fix and will have to be flushed or I can have a sodium build up. Does cal mag do that? I want to figure a permant fix. Not sure if this matters but it have just noticed my roots are starting to come out of my drain holes. On a side note I'm trying the LST.
calmag will raise your pH a bit if you have some sort of food mixed in besides just water. For example grow big should leave it on the acidic side and calmag would raise it, but calmag in water wouldn't bring you where you want to go by itself. Also I'm really surprised your plant can take that strong of a dose. I use an oral syringe to measure my nutrients and it goes up to 10mL all the way full, and my fully flowering largest plants only use like 7.5-8mL at the most, and I end up at around 1100-1200 PPM. (EDIT: I don't use FF nutes anymore, but the concentration is roughly the same as what I do use, General Hydroponics floranova) About the baking soda adding sodium, yeah it can do that if used in large amounts, however I've found that the amount needed to make a difference in the pH is really very small. Also, in general I think it's good practice to flush the plants every few weeks in my opinion, however I do grow in soil-less medium, slightly different approach to it.
If you flush the root system out real well with it yea it should do the trick if it removes the previous salt buildup, starting clean.
That's too much Grow Big nutrient. The chart calls for 2 to 3 tsp per gallon. Big Bloom can be fed at a higher rate, but I stay with 2 tsp per gallon every other watering. Make sure to check the water ph after nutrients have been added. Foxfarm nutrients are on the acid end of the scale. I always adjust my ph to about 6.3.