I have a 400 sq ft organic garden. The soil is all composted grasses, leaves, house scraps, etc.(8 in raised beds)over black clay. Tomatoes are planted every year-Bush early girl and celebrity. I live in zone 7 just outside Dallas, Texas. The tomatoe plants start great but as the fruit begins to form the lower leaves and stems begin to turn yellow and brown off. The fruit does well until the yellowing is over whelming. I have tried foliar sprays, corn meal, calicum tablets, etc. While they are turning yellow the plant continues to grow vigorously until just the new growth is green. Any suggestion? Also, what is the best way to store seeds and how long should they still be viable? Wanting to plant sweet potatoes and new potatoes. Any experiences or suggestion?

You need to mulch your tomatoes with straw or plastic-something to keep soil from splashing up when it rains. I say this as it sounds like you are getting hit with one or more soil born diseases and a mulch will effectively stop this from happening. Compost is not an effective mulch for this purpose.

As for storing potatoes for seed-put the spuds in a cool dark dry place to store (like a basement) until about 1 month before they are to be planted. than bring them into the light (they will have most likely have sytarted to sprout-that's just fine) without damaging the sprouts and let them green up (this will stop the sprouts from growing any more) for a couple of weeks to sa month. Than cut them into small pieces making sure there is at least one eye per piece and planrt within 48 hours.

Seed storage depends on the seed-some can store for a decade but others such as onions store for about 9 months. Basically keep them cool and dry. There are several good books on the subject.

Try rotating crops. Planting tomatoes in the same place year after year is dangerous. You can develop bacterial infestation and disease in the soil that is hard to rid of.Like fusarium wilt and others. Also watch for spider mites. Tiny little critters almost impossible to see. They get on the plants around the leaves and suck the juice right out of the plant. Look close, they are small.