Yellow leaves always indicate a nutrient problem, but the cause of the nutrient problem isn't always lack of nutrients. Too much or too little water, too cold, too hot etc. pH being too far off, onset of disease, many factors. When you see it on many plants suspect a broad environmental factor capable of affecting them all.

Peppers and basil are true warm weather plants and they do not tolerate cool weather (have your nights been below 50F?) at all well.

In all likelihood if it is cool weather they will turn around soon.

That's a good answer, thanks. I didn't know that, but it makes sense that any of those things might make the plant unable to use nutrients even if they're there. And the pepper plants were on my deck until I transplanted them, so they were exposed to the same cool nights. Yes, it's undoubtedly been below at least 55F at night. I was impatient and went by the calendar instead of by the temperature. It's taking too long to warm up, it's getting frustrating!

In addition to those potential causes that justaguy2 posted add an over abundance of some nutrients which can adversly affect a plants ability to uptake other, necesary nutrients. This is why a periodic good, reliable soil test is necessary, to be sure your soil nutrients are in balance.

Kimmsr, thanks. that's also good to know.

I disagree that yellow leaves always indicate a nutrient issue. Overwatering is often the most common cause of yellowing leaves. Cool weather may mean less water uptake by the heat-loving plants, so check if the soil seems too wet.

I disagree that yellow leaves always indicate a nutrient issue. Overwatering is often the most common cause of yellowing leaves.

Over watering is a cause of nutrient uptake problems. You may have missed it, but this was said:

Yellow leaves always indicate a nutrient problem, but the cause of the nutrient problem isn't always lack of nutrients. Too much or too little water,

As justaguy2 stated, yellow plant leaves always indicate a nutrient deficiency. The why of that nutrient deficiency can be any of a number of causes, including too much, or too little soil moisture, both of which can cause a plant to not uptake necessary nutrients creating that deficiency.
So overwatering can cause a nutrient deficiency by creating conditions that prevent your plant from getting those necessary nutrients

To follow up, as soon as the weather turned HOT (it went from 65 to 95 deg in 24 hours!) the basil greened right up and started looking better. The peppers still look a little yellow, and a few of the calendula too, tho the latter is growing like crazy!

kimmsr, you've often advocated for soil testing and I've been lazy about it, thinking that since my vegetables always look healthy and grow well there's no need to bother. I now realize it does make good sense to test it. My issue with it is that the $50 test kits you buy never seem conclusive to me. The water in the tubes is always some vague color I can't interpret. I hesitate to send soil out to the Extension because different areas of my garden have gotten different treatment over the years (some have leaf mold, others compost, others rock phosphate or greensand, others not much of anything) and I don't know if any one bed is representative of the whole thing. I don't want to send out 7 different samples....

Take a sample from each of the areas and mix them together and then send in one sample (or two if you think the soils are different enough to justify it). The instructions for taking the sample should say to take the sample from a certain depth rather than from the surface. While there will still be differences below the surface, it is the top few inches that will be most different due to being treated differently.

I think you will find the result of a soil test done this way useful. Regardless of what you have added (within reason) if your soil started with a low level of a particular nutrient, it probably is still on the low side and if your soil started with a high/excessive amount of a nutrient it probably still is high. These are the things you want to know (among others) so you don't add things that are going to do more harm than good.

Depending on what you grow where you may want to sample each bed seperately for testing. This does not have to be done with each bed every year, and anyone that tries to tell you that is just plain wrong. I have different planting beds tested in different years so there is at least one soil test in the lab each year, and at $12.00 per sample that is not too bad a price. If I have several beds that will be growing plants that have very similar needs I will mix the soil in each of those beds and have just one test done, but I would not mix a soil sample from a vegetable bed and the lawn.
There are some here that advocate owning your own soil test kit so you can test your soil every week and that is neither necessary or desireable because the changes you will see will take much longer than that to occur. The horticulturist at your CES office would tell you, too, that a test once every 5 years is more than enough unless you were a commmercial grower and even then for an organic farmer that may well be enough.

"if your soil started with a low level of a particular nutrient, it probably is still on the low side and if your soil started with a high/excessive amount of a nutrient it probably still is high"

1) Could you explain this? I've been gardening here for 16 years. Does this mean the nutrients haven't reached into the soil beyond the first couple of inches? Does this layer ever change due to the things you add? If not, I don't get how testing it would determine what to add. What has become of the nutrients I've added? Are they being used up each year by the plants? Don't many vegetable plants pull up nutrients from deeper down?

This is how I've treated the soil: The first 13 years I gardened in this spot I turned over or, later, just forked to loosen, the soil. Most of those years I added 1/2 to 1" compost to the top.

2) (Is this enough compost?)

For three years now I've just added compost and shredded leaves or leaf mold to the top and not dug. Some years I've added greensand and rock phosphate. I figured, it's natural, slow-releasing stuff, it can only help.

3) Could too much RP or Greensand be unbalancing the soil?

The information regarding yellowing leaves was very helpful.
Does this information apply to leaves of harvested kale or collards
where most of the batch is fine but about 5% of the leaves turn yellow
after you have washed them and put them in the frig?

It is not true that yellowing leaves always indicate nutrient deficiencies. There are several bacterial and fungal diseases that also cause leaf yellowing and chlorosis. So does lack of light,

http://extension.illinois.edu/focus/index.cfm?problem=chlorosis

http://www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/leaf-scorch

http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/bacterial-leaf-scorch