Your plants are telling you something. Maybe not in English but if you observe them they are talking to you. That dark green color of leaves after you fertilize is a good indicator of high nitrogen fertilizer, good growth tells you plenty of nitrogen was present earlier in the season. Leaf scorch might tell you the plant is in the wrong spot or planted wrong. Yellow leaf color may be due to a lack of nutrients, improper watering, or a cold winter. Plants do talk. You just have to stop and listen. Learn how on this podcast of Desert Horticulture.