Strange rusty spots, calcium maybe?

[Acid]

Hey guys,

So first of all, the setup:

* 24°C / 55% Humidity, 24/0
* Autoflower, early flower, going into full flower, around day 49 since sprouting
* 300W MarsHydro light @ around 700PPFD
* Airflow, oscillating fans, exhaust, everything working fine
* BioBizz AllMix substrate (peat with organics)
* BioBizz Nutrients
* 2 Gallons of substrate
* Watering @ 6.3-6.8PH, around 3L of water with 2ml/L of Bloom, 1ml/L of TopMax, 1ml/L Heaven (I feel like she has enough nitrogen, so no Bio Grow in the last watering)

I water this plant whenever it loses around 80% of its weight in water, until the substrate is fully soaked again, with minimal to no runoff. That's what this substrate needs, I can't let it dry out fully, it needs some residual water in it to work perfectly. Not saying I'm doing this perfectly, obviously I DO have a problem here, but the watering schedule itself is fine, I finally know what I'm doing in that case with exactly this substrate, after months of dying plants.

I haven't used CalMag lately, as I use tapwater with an EC of 0.3 already, and I feel like this may have caused it, but I'm not 100% sure. I don't know what exactly I'm looking at here. There are spots everywhere on some leaves, dark brown, almost red maybe. Some leaf tips get this strange dark burn on their side.

Any idea what's going on here?

 

[Phytoplankton]

The leaves overall are a dark green, with the browning on the margins I would lean more towards a P or K issue than calmag.

 

[SmokingWings]

I water this plant whenever it loses around 80% of its weight in water, until the substrate is fully soaked again, with minimal to no runoff. That's what this substrate needs, I can't let it dry out fully, it needs some residual water in it to work perfectly. Not saying I'm doing this perfectly, obviously I DO have a problem here, but the watering schedule itself is fine, I finally know what I'm doing in that case with exactly this substrate, after months of dying plants.

The plant has gone into flowering and most likely it is needing more in the way of nutrients than it can get on time. My thought is that upping the fertilizer schedule to flowering if one is available or adding any nutrient that seems to be low or missing.

I was thinking that some increase in Potassium (K) would help but as @Phytoplankton mentions maybe boost the Phosphorous (P) and Potassium together. Many times a Calcium deficiency starts to show with bronze or copper colored spots and it does not look like there are any on your leaves, just the brown or burnt looking spots. But it would not hurt to add a bit more Calcium

Your watering method sounds like you are using something along the "wet and dry cycle" that many soil growers have adopted. And, it looks like it is working well for your plant.

 

[Sativa1970]

Get a good look at the back side of the scared leaves with any magnification you can. With the dark spots next to the veins on the leaf I would rule out pests first. Spider mites are damn near invisible to the naked eye but those spots would have me taking a closer look. Rule out the easy diagnoses first.

 

[7Dust2]

Add calmag to water . 5 mL per gallon twice during cycles. Veg and flower. Plants eat that stuff like crazy. 5 mL a gallon last about month in soil.

 

[Azimuth]

The leaves overall are a dark green, with the browning on the margins I would lean more towards a P or K issue than calmag.

I'd agree here. Calcium deficiency would look more like random rust spots in the middle of the leaf. The brown patches are also consistent with early flower as both P and K become more prominent.

 

[Acid]

So here's what I did today - A good flush, even though it's organic. Shouldn't really flush that. Luckily I have a bag of soil microbes to get it all back again.

Big flush with ~20% of the normal nutrient intake added, until the whole pot was overly soaked. The roots are well established, with big woody above-ground roots already everywhere, so I don't worry about too much water. Just to make sure, I'd rather have a neutral playground, rather than adding just more.

Then I did a normal feeding as usual, but with 9ml per gallon of bloom nutrients, rather than the 6ml per gallon I usually do. Going by the BioBizz schedule and the fact that I'm around week 6-7, 3ml/L should've been normal anyways. Should've maybe followed that guide more precisely. The damage is way worse currently, I hope this'll stop the spread though.

Pests - I do own a magnifying glass, and there's literally nothing anymore in my tent. I had some gnats here and there, but they got destroyed with BTI dust which I top-dressed onto every pot. Since then, there's just straight up nothing anymore. I checked the substrate, main stem, leaves, especially the ones with damage, but there's nothing, luckily.

 

[Sativa1970]

Good to here you got the pests under control early and that can be ruled out. When you did the flush how much run off did you run out the bottom of the pot? Did you add ca/mag to the flush water?