For the past three years, all of my summer vegetable plants are rotting from the bottom up, including tomatoes, zucchini, cucumbers, green peppers and more. I went from grow boxes to shrugs to container boxes all with new soil, and I added some calcium. Any idea what is causing this?

How moist is your soil? What's your watering schedule? Why did you change the containers?

The raised grow boxes were old and as I am older, too, I needed something raised. I thought there might be something with the soil, so I went to another container and used new soil. Our summers are variable for rain, so I water based on that. If no rain, I water at dusk each day when I get home from work. If we have had a lot of rain, I don't. I have been gardening for many years and have sprayed for pests, but I have not had this problem in the past.

Many times if you water in the evening the soil will stay moist during the night and that invites pathogens to multiply. Root rot and damping off can result from too much moisture overnight. Is there a way to water before work? Better to water during the day to allow the soil to dry out a little. Be sure there are drain holes in the containers. Are you using soilless mix as the growing medium? Regular soil can compact and the water will pool or run off to the side. As the nutrients are depleted and washed out of the container, more nutrients need to be added every ten days or so.

That's all I can think of right now.

Are the actual plants rotting or just the fruit?

If it's the fruit you've exactly described blossom end rot and it's either due to overwatering or underwatering, it's rare that the soil actually lacks calcium that severely.

I treated mine last year with a handful of tums crushed up thoroughly, then mixed into a gallon sprayer and sprayed all over the foliage. That gets calcium to the plant better when the watering isn't perfect. Also, of course, adjust your watering. I find people overwater a lot more than they underwater.

Plants can survive wilting repeatedly, they can "survive" overwatering, but it ruins all of the fruit. I err on the side of underwatering.

I had exactly the same problem when I moved here: it's blossom end rot (BER).
Which is most likely caused by a combination of calcium deficiency and, going by your description, overwatering. Zucchinis and cucumbers may also suffer from lack of pollination (more below).

I've learned to add a bag of slacked lime when manuring in late Winter. It has helped immensely, especially considering the cost. I suggest you do that as well. Leftover lime can be used to have deep red hydrangeas.

However you should also change your watering patterns. Plainly said, you are giving your vegetable patch way too much water.
Tomatoes don't like being overwatered like you do, at all. Besides BER it makes many varieties crack their fruits, which in turn may invite assorted pests and molds. Tomatoes should be watered once or twice a week (depending on rainfall pattern and temperatures) and the watering should be deep. There are several ways to achieve this, for example by digging a bowl-sized depression every two plants, filling it water and letting it drain.
Peppers like water, but they like even more letting the soil dry up between waterings. Daily waterings should be avoided even during droughts.
Finally we get to the squash family (zucchini, cucumbers, courgettes etc). These plants love water, but in our climates will need to be watered daily only during very hot dry spells of weather. They require plenty (plenty) of organic matter to grow well.
However these plants also need to be pollinated, and pollinated by insects. If female flowers suffer from lack of pollination they will develop squashes that fail to develop properly.

You can try to attract pollinating insects by planting attractive plants in your vegetable patch. Zinnias are great, but Russian sage (Perovskia atripicifolia) is the most amazing bee and bumblebee magnet I've seen in my life. Both plants will grow in any soil with minimal care but as they both love poor soil with little water are best planted outside the beds.
Failing everything else the female flowers can be hand-pollinated with a small clean brush, but it's a very time-consuming endeavor.

Hope this wall of text helped you out a small bit.

Thank you all for the great ideas. As I mentioned, I have been an avid gardener for over 40 years, but this has only happened in the past three. I haven't changed my watering habits over that time. The plants look beautiful with plentiful flowering on all early in the season. The problem starts near the roots with the stems deteriorating and the leaves crumbling off as it moves up. I'm able to get a few nice tomatoes and peppers and cucumbers with little to no blossom end rot before it gets high enough to reach them. Zucchini is a loss! I'll try changing the watering and incorporate as much of this as I can. If it occurs again this year, I'll take some pictures! Thanks again to you all!

That sounds like a wilt disease or blight if it's affecting the foliage, not the fruit. Especially that it's recurring in the exact same way year after year. The bacteria, fungus, or virus harbors in the soil over the winter and will likely not go away for many years.

My only recommendation if it's not blight (which I assume you would know, since it's brown spots all over the foliage), would be try a calcium carbonate or calcium chloride foliar feed.

Something around a tablespoon, maybe 2 teaspoons (2/3 of a TBsp) if you want to be extra careful, of calcium chloride dissolved into a gallon of water used in a pump sprayer or something like that.

For the calcium carbonate I just crushed up a good handful of tums thoroughly, dissolved them in like a gallon of water, and fed the leaves with that for a few weeks. Not every day, maybe once a week or so.

I was worried about something in the soil which is why I changed to containers with all new soil with the same disappointing results. It does affect the fruit once it goes up the stem and leaves. I had mixed some calcium into the soil when I planted this past year because I have had a bit of blossom end rot in the past. Several folks have recommended th3ccalcium, so I'll try to apply it during the course of the season instead of just mixing it in when planting. Thanks again.