Sound a little like "root rot" to me.

Recognize root rot on your tomato plants by any shrinkage of the roots and lower stems and the appearance of reddish-brown spots. Theses spots grow until they encompass the stem or root. The root system's size gradually reduces, the leaves turn yellow and brown and they fall off. Eventually, the tomato plant dies.

The first step to preventing tomato root rot is preparing the soil properly. Tomato plants don't like wet feet. Standing water around the tomato plant's root system creates the ideal breeding ground for root-rot fungi. Improve your waterlogged soil's drainage by adding sand or organic matter. Also, keep the pH between 6.0 and 7.0. Root rot loves acidic soils.

Watering - Tomatoes do not like to sit in water and therefore should only be watered 1 to 2 inches once a week. One thorough soaking is preferred over frequent, light waterings so that the roots have plenty of time to dry out, and thus prevent root rot. Once the tomato plant is established, add mulch to the base of the plant so water doesn't evaporate too quickly.

I think that in the BED B some bio-chemical reactions are going on, which is composting, decomposition, rotting, decaying...
So the roots of the plant getting to the middle of such a situation, may become a victim. We can call this root rotting ? or , my other theory is that in that environment there exist air(more than necessary), to activate decomposition. Yet another point: we know that decomposition/composting will take Nitrogen away . So if the roots of plant happens to be there, are not going to get much Nitrogen. A pale green/yellow can be one of the indicators on insufficient Nitrogen.

Charlie:

I will take a closer look at my plants to check for evidence of root rot, but I don't recall seeing any reddish-brown spots. Regarding drainage, I believe that Plot B actually has better drainage than Plot A. I'm sure the pH is different between the two plots, maybe the pH value of Plot B is contributing to the problem.

seysonn:

I see your point about the composting process interfering with the plants. And, yes, there is more air in Plot B than Plot A judging by how soft the ground in Plot B is vs Plot A. Still strange that the composting process did not affect my plants in past seasons, only this year. As for the nitrogen, wouldn't the amendments of tomato tone and bonemeal supply nitrogen?

Should I leave the plants and hope they survive? Should I replace with new plants, and if so, will the new plants experience the same problem?

This leads me to believe the soil in Plot B is causing the problem. Could this be true?

Easily. It has nothing to do with the tomatoes other than that is what is growing there. The effects would be the same regardless of the crop planted.

The problem is the vastly different soil composition in the beds and the drainage problems, nutrient retention problems, pH imbalances, decomposition issues, and soil structure problems those differences create.

The fact that they performed equally well last year was likely the result of happenstance/coincidence more than anything else. Given the info provided on composition there is little to no chance that Bed B could perform well in following years. It is both mineral and nutrient deficient and will have severe water drainage/retention issues.

lasagne gardening method by piling on coffee grounds, leaves, and grass clippings atop a bed of cardboard

Lasagna beds are also supposed to have layers of soil incorporated into them when built and require regular additions of compost throughout the season and at season's end and again prior to planting. And that compost needs to be very diverse in ingredients and as pH balanced as possible (leaves are very acidic) for it to work. It was never intended to be a build and forget method. Rather it requires continuous building to work.

One can learn a great deal about the effects of different soil compositions as well as the problems associated with lasagna beds that aren't properly maintained and amended over on the Soil & Compost forum here.

Dave

Dave:

I had no idea that a lasagne garden needed regular upkeep. When planting tomatoes, I noticed many more and larger worms in Plot B vs Plot A, and the soil seemed darker and lighter. Based on this, I presumed it was actually better soil than what I had in Plot A. Apparently, not. I will do some more research in the Soil/Compost forum to determine how to revitalize this plot. Somewhat related question -- even if the soil was deficient of nutrients, shouldn't my newly planted tomatoes have been okay (at least in the short-term) since they could use the nutrients from the amendments I added to each hole?

I did inspect my tomato plants closer today. I do not see any brown spots, but after clearing some leaves from the base, I see that many of them in Plot B are "swimming" in water. Since they were planted on May 15th, we have had 3 inches of rainfall. And, the plants in Plot A are nowhere near as wet. So, the drainage appears to be much worse in Plot B than Plot A this year. Seems like a combination of drainage problems and soil differences are the culprits.

It is the drainage issues causing the yellowing this early. The nutrient issues take longer to develop.

You didn't indicate that you incorporated ANY soil into the lasagna bed and it needs it badly. Alternating layers of soil was a big part of the original lasagna bed recipe but the whole concept has become so misunderstood and misrepresented over the years that current claims of "it's a lasagna bed" have little in common with the original instructions. But even the perfect lasagna beds still requires ongoing amendments to be effective.

But even more importantly is the diversity of components issue. Leaves, even leaf mold, isn't compost. Compost has ratios of carbons to nitrogen and the better the ratios, the more diverse the ingredients, the better the quality. But even the best can be mineral deficient without soil.

Dave

At this point, should I:

a) Pull existing tomato plants, make mounds to help with drainage, add nutrients to the soil, and plant new tomatoes, or

b) Leave existing tomato plants, topdress with nutrients, and hope for drier weather and plant recovery, or

c) Do something else. If so, what?

How big is the bed? How heavily is it planted (is there room to work around the plants without disturbing them)? Do you have access to a large amount of compost and soil to add to the bed - enough to add several inches of each to the entire bed)?

How big and how long have the plants been in the bed - can they easily be transplanted? Do you have access to replacement plants? Do you know how to clone cuttings off the existing plants?

Lots of factors to consider in deciding what you can do but just adding nutrients won't solve the main problems..

Dave

Answers:

Bed is 10' x 10', 6 tomato plants, so yes there is room to work around them. I do not have access to soil/compost.

Plants are maybe 10" tall, and have been in the ground for two weeks. I have access to replacement plants, and don't know how to clone cuttings.

Replacing is not a big monetary hit, just want to make ensure that the replacement plants do not suffer the same fate as the originals.