Leaves may yellow and die if sufficient nutrients are not available to plant roots. Nitrogen, in particular, may be in short supply early in the growing season. The leaves of lettuce, cabbage, spinach, and other vegetable plants may appear overall light green or yellow as a result.
Iron chlorosis is a yellowing of plant leaves while the tissue close to the leaf veins remains green. Typically, it is caused by an iron deficiency, usually due to a high soil pH (pH above 7.0). An ideal soil pH for vegetables is 6.5 but plants will produce if the pH is in the 6.0 - 7.0 range.
Incorporate organic matter into the soil prior to planting outdoors. Use a starter solution on transplants following label directions. Test your soil pH and nutrient levels every three years. Apply a liquid fertilizer at the base of each plant or to foliage if nutrient deficiencies are observed.
Lack of nutrients in seedlings and vegetable transplants can cause leaves to turn purple
Nutrient deficiency in seedlings is a common problem in early spring. Root systems are immature and unable to uptake nutrients that are held in soil organic matter. Also, cool soil temperatures inhibit soil microorganism activity and processes that allow nutrients to be released and made available to plant roots. This occurs to some degree on all soils but is more prevalent in soils with low fertility and organic matter content.
The solution is to apply a balanced soluble fertilizer to the root zone (around the base of each plant) or to the leaves (be sure that to thoroughly spray leaf undersides). The best time for foliar applications is early morning or early evening when the stoma (leaf breathing pores) are open. Compost tea, seaweed extracts, fish emulsion, and store-bought soluble fertilizers are all suitable. So-called "starter fertilizers" have a higher percentage of phosphorus (the middle number in the N-P-K analysis) to encourage root growth. In all cases, these soluble fertilizers are mixed with water according to label directions. Dilute one part compost tea with three parts water.
Members of the cabbage family often show red to purple coloration on leaves and stems. This is sometimes exaggerated when plants are stressed, especially transplants that have remained in the same container for more than 5-6 weeks.