Hi!  Thank you for your question.   My first question is have you had a soil test for your garden?  A soil test identifies the pH of the soil and current nutrients in the soil. pH is very important so the nutrients that are there are available to the plant.    If you are rotating your location of the peas - you should not plant peas where there have been any beans (green, lima, dry, etc.) for a minimum of three years as they are all in the legume family.   Root rots will cause the leaves to turn yellow.  Especially from the bottom of the plant up.  We would need to see the plant (with roots) to look for disease.   If we have hot weather and no rain when the peas are mature and ready to be picked you will sometimes see the bottom leaves turn yellow.