what makes vegetable leaves turn yellow
If its during the spring and/or summer it may be because of a lack of nutrients.. but specifically in winter or in places of low light the leaves turn yellow to adapt to the lack of light. Chlorophll in plants makes it green but in fact there are many types of chlorophll and the lack of light brings them out.
If the leaves are drying and yellow, it could be a lack of water, but usually yellowed leaves mean just the opposite, too much water.
the lack of H2O and nutreants to the cells in the leaf... in simple terms... its dehydrated and hungry and dying
A lack of nitrogen ions in the soil will cause the leaves to turn yellow