Soil-borne diseases, of which affect tomatoes, are usually held in the top 6 inches or so of soil. That's why I solarize my soil after tomato plants are removed. The benefit of contact with gumbo clay is that there are so many minerals and other trace elements there. The loose medium in which the tomatoes are planted gives root systems time and oxgyen in which to prosper. Then the mycorrhizae can go out into contaminated gumbo clay and harvest nutrients. Mycorr also produces an antibotic that thwarts many diseases. I hardly ever water, because I live in an area that received 72 inches of rain annually. That's the reason for raised beds. As far as feeding the soil, I don't. There is enough good stuff there already. Too much N-P-K destroys mycorrhizae. To the point, 1 tablespoon of pelleted fish meal under each tomato when they are set in the ground is the extent of fertilization. If you line your beds with plastic, you will defeat the purpose of mycorrhizal fungi, which sends hypae out 20-30 feet in every direction to gather nutrients. That's if you are considering using the fungus. Doreen