What is causing my garden vegies leaves to turn yellow, tomatoes cubumbers etc ?

Too much water
Too little water
Overcrowding
Lack of the right nutrients for each type of plant

The most common is too little water.

This is a good question to google outright, 'why vegetable plants yellowing,' because there are so many helpful gardening sites with answers already written.

Did you have a soil test done? We have no idea as to what type of soil, altitude, sunlight etc.