Common reasons for yellowing of leaves in outdoor plants include waterlogging or dehydration, excess lime in the soil, and deficiencies of iron, manganese, zinc or other nutrients. The case-specific cause depends on the types of plant and conditions of the plants' environment.
Waterlogging and dryness hamper the ability of plants to absorb nutrients from the soil. The presence of excess lime in the soil also causes yellowing of leaves by deterring the uptake of minerals by plants. Along with the sun and water, iron, manganese, zinc and other minor elements are essential nutrients for plants. Deficiencies of these nutrients cause problems for plants, including yellowing of leaves.