Delps8
Well-Known Member
You're welcome.
It's a simple concept but it's an abstraction from the values that we deal with so frequently, temperature and RH. We see our environment in those terms so that's where we're most comfortable. In contrast, VPD is how the plant reacts to the environment and how the plant reacts is what counts.
I think I saw that your water supply was at 130PPM. That's fine as long as you know what's in your water, as long as it's consistent, and as long as you account for that when you're mixing your nutes.
Swapping the res is a valid approach. If you've got a small res, it's not a lot of labor but you're throwing out a lot of good nutrients. Topping off with RO or with nutrient solution are less labor intensive and cheaper since you're not tossing out nutrients. I've attached a paper that will give some insight into res management. The "mass balance" concept helps explain why it's perfectly acceptable to not throw out nutes. Jacks Nutrients are a popular dry fertilizer and in one of the videos on their web site, the people who work there explain that they use a res for about a month at a time.
When PPM in a solution drops from, say 800 to 500, the reservoir still has a lot of nutrients left in it. The biggest issue with nutrient levels dropping is that pH starts going out of range. That's one indicator that the res needs to be swapped.
Checking out You Tube for vids on how plants take up nutrient is time well spent. Search on this in YT - "nutrient uptake in hydroponics". Bugbee videos are very helpful and the guy who hosts at scienceinhydroponics.com (check his video
The low temp is hampering growth but, IIRC, it looks like your plants are getting good light. Due to the latter, they're able to make food but the low temps reduce the ability to use that food.
A res at 20 will be OK but, above that, get a product like Hydroguard. That will keep bud rot at bay but 22° ambient temp is meat locker cold.
EC is helpful for making sure that your nutrient mix is correct when compared to what the manufacturer posts for the EC of a properly mix set of nutes. Once it goes in the pool, it's, at best, a weak proxy for what's in the res.
N, P, K, and Mg are taken up very quickly and they have little impact on EC. Ca and another chemical are the big dogs in terms of EC but they take days to be taken up.
Another confounder for growing in hydro is that you've got a small res. Per above, 3 gallons of res/square foot is a good sized res. My res is in an 8 sq ft tent and it holds 28 gallons. I do three res changes pet 110 day grow and pH is very predictable. I've used the RO top off method and, for this grow, am adding back nutrient solution. There's simply no need to throw out nutrients because the Earth has spun X times on its axis. That's mythology that costs a lot of time and help nute manufacturers make more money.