Why do my plants have yellow leaves?

Leaves that are light green or yellow rather than a healthy dark green may have a condition called chlorosis (yellowing). Compacted soils, poor drainage, and root damage can cause chlorosis. However, one of the most common causes of chlorosis is a deficiency in two important nutrients: iron and manganese. If soil levels of iron and manganese are sufficient, an soil alkalinity (pH above 7.0) may be causing the problem by preventing plants from taking up these nutrients.

Shallow-rooted plants frequently have iron and zinc deficiencies caused by excessive phosphorus. Iron deficiencies are characterized by yellowing between the leaf veins. Zinc deficiencies show a bleaching of the tissue. Both iron and zinc deficiencies occur in the youngest tissues and can occur at the same time, which makes it difficult to identify the actual problem.

Other nutrient deficiencies that could cause yellowing are sulfer or nitrogen. Sulfur deficiency is accompanied by a vibrant and uniform yellowing, while nitrogen deficiency symptoms display a lighter, bleached yellowing that is more pronounced on the lower leaves. Advanced symptoms occur on nearly all of the vegetative tissue, including the stems.

If leaves are abnormally yellow and you suspect chlorosis and/or nutrient deficiency, have your soil tested with a SoilKit if you have not already. A soil test will also tell you both the nutrient content and the pH of your soil, so you can know how to treat the issue.