Question
why do the leaves on my vegtable plants turn yellow and fall off.  

Answer
Greg, yellow leaves are generally an indication of poor drainage. The roots become water logged and cannot absorb the nutrients necessary for green foliage and good production.

My recommendation is to supplement the soil by adding organic matter. These amendments include peat moss, aged manure, and compost. If you do not have these readily available, you can purchase them by the yard at most nurseries. These will improve the tilth of the soil, which in turn will correct the drainage problem. They will also provide nitrogen and phosphorus to the soil, which have most likely have been depleted due to the excessive water in the soil.

I hope this answered your question. Please write again if I can ever be of assistance.

Regards,

Mike