Are the plants intended to be grown year-round indoors, or put outside after frost? We ask because 10 weeks ago was very early to have started seed for summer vegetables, as typically even the crops needing the earliest start get sown later this month.

Have the plants been fertilized, and if so, how? Chlorosis, or a yellowing of leaves while the veins stay greener, indicates a deficiency of either iron, manganese, magnesium, or perhaps zinc. A fertilizer formulation that contains micronutrients (all of those four nutrients, plus some others) should prevent that, unless the acidity level (pH) of the potting mix is notably outside of the pH range it should be. Occasionally, mixes using, for example, coconut coir, have measured quite acidic, or have enough salt contamination that plant roots become stressed or damaged. A pH tester (probe or paper strip kit) might reveal if the pH level is grossly outside of the ideal range, though such testers don't provide as fine a measurement as lab testing. (In this case, lab testing is probably not practical or worth the expense. Standard soil testing is not useful for soilless media like potting mix, so it would need to be a different lab test.)

If the potting mix isn't nutrient-deficient and the pH is okay, the culprit tends to be related to root health. They may be getting over-watered or under-watered, as both circumstances can lead to poor root function or dieback. If you happen to have any fungus gnat issues, over-watering would be more suspect. Drying leaf margins could have a range of causes, including over- or under-watering, root damage or dieback, and insufficient ambient humidity.

Is the potting mix a commercially-sold potting mix, or does it contain some composted manure? The latter sometimes contains still-active herbicide residues (they can survive the animal's digestion) that can affect the roots of seedlings grown in that medium. Usually, we see different symptoms in those cases, like foliage distortion and stunting, but it's a factor that could be ruled-out if you didn't add composted manure to the mix.

Over-fertilization can also stress or damage roots, so depending on what your fertilization regime has been, reducing the dose or frequency might be helpful. Sometimes, excesses of some nutrients (collectively called "salts")  interfere with the roots' ability to absorb other nutrients, or they "burn" sensitive root tips, causing them to die back. Since a compromised root system isn't functioning well, that's why foliage can develop deficiencies.

Miri

Thank you for your very thorough and quick response. I know that peppers There are two reasons I started the pepper seedlings so early (I know UME planting calendar says start in March) – 1) I was growing them for a demonstration/exhibit about how different seedlings look as they grow (workshop in Feb) and 2) I have found over many years that peppers are very slow to produce, often not setting much fruit until August and into Sept and October. I was going to keep them indoors, under grow lights, and then plant them in a container/grow bag in about April and carry them outside (and back inside or under cover if a frost was predicted). It was kind of an experiment. I’m still starting seeds at the correct time too.

To answer your other questions – I started seeds in plastic cell inserts using ½ potting soil (Foxfarm Happy Frog) in the bottom and 1/2 coir seed starter on the top of each cell. I have checked the Ph of the cells with a meter – it’s consistently either 6.8 or 6.9. I also tested the coir and its Ph was 7.1 Foxfarm Happy Frog potting soil has – per the internet – a Ph of 6.5.

Fertilizer – Since true leaves formed, I’ve been using (diluted to 50%) Miracle Gro Quick Start liquid fertilizer NPK 4-12-4 weekly. It doesn’t say if it contains any other nutrients. I did use diluted fish emulsion a couple times in the beginning but switched to miracle gro (not a great fragrance in the office) Also, recently, I added (again 50% strength) Kelp fertilizer 0-0-14 dissolved in water thinking the symptoms might be a potassium deficiency. Maybe that is why the newer leaves are not yellowing? Maybe not.

Your observation about watering and fungus gnats (yep got em, drenching with BtI Knock Out Gnats helped reduce those) might be the reason. I typically bottom water. Maybe I need to let it dry out more.

I don’t know how I would know if the potting mix is deficient in iron, magnesium, or other nutrients. I can use diluted Epsom salts for magnesium if you think that might help but without testing the media (impractical, as you pointed out), that would be just guessing. Do you recommend I start using a different fertilizer that has more micronutrients? Would fish emulsion (5-1-1) be better?

Thanks you.

Ok, thank you for the additional information. If two different soil/media types are layered, that can impact how well water and nutrients are distributed throughout the root zone. For future plantings, it is best to mix any potting media ingredients together thoroughly. Bottom-watering might be keeping the lower half of the mix too consistently damp. It also doesn't help flush out surplus nutrient "salts" like top watering does.

As for fertilizer, try switching to a formulation whose N-P-K ratio is closer to 3-1-2 (say, 12-4-8 or whatever comes close) and also contains micronutrients. The label will list any micronutrients included, like manganese, magnesium, etc. (Often it's all of them or none of them, and the fertilizer you're currently using doesn't appear to have any.) Diluting it to a half-rate might be a bit weak and not supply enough nitrogen, but you can see how the plants respond after switching and adjust as needed; it may take a couple weeks or so to start having a visible effect. Not all nutrients are mobile in the plant, meaning that areas of deficiency yellowing may or may not be able to green-up again after missing nutrients are replenished. As long as the youngest growth remains healthy-looking and the oldest leaves don't yellow and shed prematurely, you can probably assume what you're using for supplemental nutrients is sufficient.

The pH is a bit high (6.5 would be better), but in this situation, you don't need to try to change it. Some types of fertilizer (certain forms of nitrogen) can gradually acidify media over time anyway. Fish emulsion might contain some micronutrients, but probably not all of them. You're right that it would be hard to tell without lab testing if a potting mix material is lacking in certain nutrients. Unless the company producing the mix "charged" the material with a starter amount of added nutrients, they probably don't contain much on their own, so plants would rely more on what is provided via fertilizer, particularly over time as any nutrient amounts that were present at the start of growth are used up or flushed out of the pot from successive waterings.

Miri