Is this burn and salt build up?

At the very bottom lower leaves it is showing what looks like some burn and maybe build up but the rest of the plant is nice and green except that the leaves are starting to kind of fold in and get brittle this happened with my last run of this strain critical kush I thought it was maybe heat but its definetly not that I was wondering cause nutrient burn cause this the leaves are green and perfect looking just ridged and starting to fold on my last run at the end of harvest the leaves were so brittle if you barely bent them the leaf would break. my humidity is low maybe around 25-30% but im thinking this is nute problem I watered with 700ppm last water runoff was 1000 so maybe a slight salt build up she is on day 30-31 using AN nutrient line 1000 watt hps 100% coco coir would maybe giving her 500ppm for a few waterings slow and stop this problem. Any help would be appreciated I am trying to get this strain down so I have no problems and can work on bigger yields.

looks pretty normal to me,them lower leafs will die off as newer leafs form,in mt limited experience anyway
i only grow in soil and my plants do it too,i dont feed nutes to them unless they asking for it,the rest of your plant looks fine,i wouldnt worry to much about dieing lower leafs

yea but u see in the third pic how the mid leaf is cup in slightly last run they cupped in like a c shape or more

That type of "cupping"/"brittleness" is caused by under-watering and/or heat stress! The bottom leaves yellowing appear to be lack of light penetration. They could have another cpl of issues going on,,BUT, nothing I would worry about know as they look just fine except for the onset of under-watering...

I'll ask these things.
#1: How much do you water and how often?
#2: What size pot?
#3: What is the room temp?

NOTE: Smart pots can let to much water/feed run through with out getting all the soil in the pot "wet".

I suggest.

Take the pot OFF the "drain through" catch pan when you water. Water with your mix and let the pot soak it back up (It should do it fairly fast BUT, leave it like that for 5 min and then put it back on the drain through. The soil in the pot is NOT being given enough time to soak up the proper amount). Now (everyday at lights on) you stick your finger in the soil, up to the first knuckle. Feel dry? = time to water.
Next watering, water with "water" and do the same thing.

Is that a five gallon smart pot? 1500 - 2000ml at a time! (you may not have to water every day)

RAISE the humidity to 40 - 50%!!!! This will help as your humidity is drying out the plant a tad fast.

Your plant , otherwise looks GREAT!

its coco coir I water every other day 3 gal pot im still not sure that its heat stress cause on my last run I had a few 2 gal plants off to the side where it was easily 75 degrees definetly not about 80 I had the canopy at about 82 degress and these still cuped like this even off to the side where it was much cooler than directly under the light I read somewhere online from Jorge Cervantes that overfertilization can cause this but ive honestly never seen it. Ive always seen it in the tips

Yup I'll go with you, that it's not heat stress.....BUT!

I'm still at under watering as you have a 2 things going that tend to lead things that way.

Air pots and the way you water. Coco needs to get wet, just like soil.

Water in the style I suggested above, and RAISE that humidity!........I'll bet your problem goes away.

I don't see enough of any indication that you're over fertilizing to a point that you need to take action on that front.

Yea definetly need to up the humidity that could be the problem as leaves might do that to retain moisture once im in a better controlled environment I will hopefully have less problems