Yellow coloring on vegetable leaves can indicate a wide range of problems, including too much water or too little water. Leaves can also turn yellow when the plant is not getting enough nitrogen. Other common causes of yellowing plant leaves include extreme temperatures, leaf damage, root damage, disease and over-fertilization.
Trial and error is the only way to identify the definite cause of yellowing leaves. Target the most obvious potential causes and eliminate each possibility systematically. To speed the process, seek the advice of other gardeners or greenhouse employees in the same area to learn about common local soil deficiencies or environmental conditions.