How to measure soil PH in smart pots

I’m currently growing 4 plants in 7 gallon smart pots. Each are about 75% FFOF and 25% Perlite. Lighting and temperature/humidity is perfect. The first few weeks they were growing great and at a great pace. In the last couple weeks they’ve slowed dramatically and I’m seeing yellow leaves starting at the bottom and working upwards, with some leaves showing brown spots. I switched them to 12/12 cycle about a week ago which may have been a mistake since the slowed growth was present before I made the switch. I’ve been watering with Distilled water using Cal Mag and FF trio at about 1/2 to 3/4 strength every other watering (Camag with every watering) I had been using a droplet with color card to test PH and it seemed to be around 6.5. In the last couple weeks after seeing slowed growth I invested in a nice PH pen and I’ve been watering at about 6.3 PH but I am still seeing these issues. I want to accurately test my soil PH to rule that out as an issue but need help on how to do that with a fabric pot. Being that when I water, a lot of the water will seep out of the sides and run down into the trays which I would think would result in an inaccurate ph of the water run off. I’m wanting to figure out how you guys would recommend accurately testing my soil PH or what my issue is. Thanks!!

You’ll need to water more slowly so that the water filters through the medium, instead of down the sides. I have the same issue with my fabric pots.

And what PH should I water with? Will that effect the results of the runoff PH of the soil?

Ph down or baking soda. Will work. Vinegar will work, but not recommended long term.

If you can get the water to run through the pot and can collect some, ideally you want a runoff pH of around 6 give or take. No need to take drastic measures unless your runoff is really high or really low.

The yellowing is on lower leaves so it is just the old leaves being sacrificed. You might want to up the grow big to get a little more nitrogen. Are you following the feeding chart? Might want to go full strength on feedings at this point.

Also, once you start flowering you should consider supplementing the Fox Farm Trio to boost the buds. Fox Farms has the solubles which are designed to work with the trio to do just that, or you could consider a product from another company to boost PK, but using just the trio will not give you the big fat buds you want.

I’d also like to add that when feeding don’t have OCD and try to pH to an exact number. Different nutrients get absorbed better at different pHs so you should purposely vary the pH of the feedings from 6.2-6.8.