Yellowing leaves

[junecon]

My first grow of autoflowers and i had some issues along the way with heat and humidity as well as air circulation.
Growing 16 plants. x10 gelato fb x4 pineapple express and x2 magnum autos.
Growing in gold label special mix soil and watered the plants just plain water for 3 weeks and added a few feedings with rhizotonic.
the temprature in the start of the grow was very high at 32-39 degrees and humidity at 17%, i have now gotten the temprature down to 25-30 degrees and humidity with the humidifier up to around 30 %, i guess the out vent coal filter helped with temprature.
now i have got my self a ph meter and i realised i have been feeding my plants with plain water with a ph of 7.8 in my soil wich contains a ph of 6.1 for 3 weeks
i have not fed any nutrients the first weeks just given them one feed as of now nearing week 4. i keept the ph of feeeding and watering at 6.5 now.
The yellowing of the leaves seems to start from the bottom and i defoliated them of. i stopped giving them water for two days cause i thought maybe i was overwatering with the 0,5 l i gave them everyday, but i now think it might be a ph problem or just that the plants have used up the nutrients in the soil and need some adding to it.
what do you guys think, ph lockout, nutrients need cause the soil is 3-4 weeks. nitrogen deficency or septoria ?
if nutrients should be added how much should i give them each week without over fertilizing ?
i gave them coco canna a+b 2.5 ml of each for 1 liter water ph 6.5. i grow in 11 liters air pots.
and question, is it to late to lst plants in the week 4 ? the are pre flower i think, i see quite a few pistols and the plants are around 30-35 cm and some smaller.
all viewpoints will be appricated.

 

[Arthur]

Lot going on... soil I get 3 or 4 days between watering. I'd like to see full plant pics too.
how you tested the soil pH? Slurry test?

The pH issue would be where my attention was focused.

Have seen grows run fair with 7.0 watering, but understand, pH is on a scale like earthquakes, it's not linear, it's on a bell curve... 7.8 is way out...

What are you using for up and down? I use a pH up that is lime based, it goes in and sticks around buffering the soil up as feeds continue.

If I checked and hit a 6.2 in the soil, I'd be feeding at about 6.7, 6.8. This slowly adjusts the pH up over a couple feeds. But again, I'm using a limestone based pH up.

 

[junecon]

I have been watering the plants after reading the web about giving the plants water after the top layer of the soil feels dry to an inch down, my soil seems to dry out pretty fast, just after 24 hours. my temp inside the room now is 29.4 celcius and 25 % humidity.
i use ph down as shown in picture and haven't been using a ph up yet since i haven't had the need for it yet ( got no picture of the ph up bottle since the etiquet is faded ). The 6.1 ph is just the base of the soil as it is new, have not checked the ph of the soil itself in its current state, but i would guess the ph would have climbed to over 7 since i have been feeding 7.8 water for weeks. any tip to check the soil ph ?
Thanks for the answer. pictures are not optimal.