Nutrient burn or heat or overwatering?

[NorCAKnowYourGrow]

I'm guessing nutrient burn....but maybe this is heat. Was hot as hell most of this week... I've had more of these leaves this week than ever. Fed them last week Friday and then again today. So maybe too many nutrients?

98% of each of the plants look amazing with an occasional fan leaf that looks like these. Otherwise they seem good. I was concerned I overwatered two days ago in the heat so let them dry out a bit yesterday. Maybe this is overwatering?

It's cooler now and no plans to turn back into the seventh level of hell so if this is heat we're probably good now.

Thoughts?

DETAILS:
Outdoor grow
Fabric black pots
Humboldt's Secret Nutrients: Today I used recommended amounts of Base A/B, Golden Tree, Plant Enzymes and Flower Stacker on those that are in flower. The plants with the most yellow/curled leaves are in the VERY early stages of flowering so they did not get Flower Stacker.
Variety of strains: most affected - the sativas (which are the ones in the EARLY stages of flowering)
Soil: 420 and Sloat organic

I apply nutes directly to soil - trying to avoid the leaves but sometimes I get some drippage as I use a watering can.

 

[TysonOG]

Phosphorus deficiency, could be low nutes or incorrect root pH

 

[NorCAKnowYourGrow]

Phosphorus deficiency, could be low nutes or incorrect root pH

Hmmmm....I've used nutrients weekly or even bi-weekly. Perhaps P can get locked out?

Will check pH of soil tomorrow. Thanks.

 

[TysonOG]

Check the soil pH and go from there. Its definitly a phosphorous deficiency, whether caused by pH or nute levels. Especially if you're seeing this on one strain, I'd say you just need to increase the amt of bloom nutes for that strain.

 

[NorCAKnowYourGrow]

I checked pH - between 6.0-6.5 (Rapid Soil so it isn't exact but it was definitely not above 6.5).

I also tested NPK.

N: Sufficient, almost surplus
P: Sufficient, almost surplus
K: Adequate

So it appears I need a bit more potash....don't have anything that is JUST potash, however.

 

[Bush Doctor 77]

I checked pH - between 6.0-6.5 (Rapid Soil so it isn't exact but it was definitely not above 6.5).

I also tested NPK.

N: Sufficient, almost surplus
P: Sufficient, almost surplus
K: Adequate

So it appears I need a bit more potash....don't have anything that is JUST potash, however.

Wood ash(Potash). Be careful and start with a very week solution.