RE: Why the Best Time Ever to be a Gardener is NOW
It could be that you need to rotate crops more. A very general rule of thumb is
Root - Fruit - Green - Bean rotation each year. Each successive type of plant needs a different set of nutrients, but each also provides nutrients for the next. This is a nice little saying as a guideline, but it falls short of perfection. Still, it's better than the same crop in the same soil year after year. So you might grow lettuce or spinach next time, then some beans, then some potatoes or other tuber, then back to tomatoes. If you're not composting what's left of your plant after harvest, then make it a habit to chop and drop the remains right where they grew (cut them up into small pieces to mulch them). They'll add more nutrients to your soil and provide litter that protects the soil as well.
Look for complementary plantings too. What grows well with your tomatoes, that you like?
Solid advice. Most of my garden is trees, btw, so I only have a small patch of land for tomatoes, peppers, etc. I like watermelons too, but I haven't planted them in years.
In the winter I'm leaving it to "rotate" to whatever grows naturally (weeds, flowers, etc).
For the time being rotation should not be an issue (3rd year intact - it's left to grow wild weeds, I cut them down once per year and then let them to rot on top of the ground. It creates a nice dark-colored soil, where next years weeds and flowers are growing with even more vigor.
If I am to plant tomatoes next year I may do an experiment. I'll plant some there and some in another patch of land and see what happens.