What is causing this leaf symptom?

These are seedlings that are now about a foot tall.

They are in one gallon plastic pots with roots organic soil.

Watered only when pot has dried down. Basically been watering every 4 days or so.

Watered with plain chlorine filtered water, no nutes.

These are outside under shade cloth, the temps are high at mid to upper 90s.

What is causing this?

Calcium deficiency

For starters at 1 foot tall they should be in a minimum 3gal pot. Some go to 2, I prefer jumping right to 3.

4 days for the pot to dry out is excessive IMO; and

You should be feeding them.

ok, do transplant into bigger pot and start giving them liquid nutes in addition to what they get from soil?

Could high heat be causing this also?

Visually that looks kind of like nutrient burn or a deficiency, is it only on the lower leaves? If it's starting from the bottom moving up that's probably mobile nutrients. If it's actually a deficiency that means is probably nitrogen, phosphorus, potassium, magnesium, or zinc. Looking at the chart I always use, I would wager it's maybe a potassium deficiency. Emphasis on the maybe. And if those are really a foot tall they definitely need to be up potted into something larger I think, it's hard to tell exactly how big that pot is just by looking at it. It looks like one of the tiny ones I use for starters, but I'm not sure how big those deck slats are.
https://www.loudclouds.co/wp-content/uploads/2016/07/marijuana-deficiency-chart-jorge-cervantes.jpg Is the largest copy I could find of that chart, highly recommend saving it some place handy.

AgentPothead said:

Visually that looks kind of like nutrient burn or a deficiency, is it only on the lower leaves? If it's starting from the bottom moving up that's probably mobile nutrients. If it's actually a deficiency that means is probably nitrogen, phosphorus, potassium, magnesium, or zinc. Looking at the chart I always use, I would wager it's maybe a potassium deficiency. Emphasis on the maybe. And if those are really a foot tall they definitely need to be up potted into something larger I think, it's hard to tell exactly how big that pot is just by looking at it. It looks like one of the tiny ones I use for starters, but I'm not sure how big those deck slats are.
https://www.loudclouds.co/wp-content/uploads/2016/07/marijuana-deficiency-chart-jorge-cervantes.jpg Is the largest copy I could find of that chart, highly recommend saving it some place handy.

Click to expand...

Looks to me like 1x4" (being a 1 gal pot is 7" across at the top

Browning edges can indicate too much water. A plant pushes water through to the edge of its leaves, and if given in excess, this can cause veins at the edges to burst, which leads to browning. Let the soil dry more between waterings.