Foliage discoloration and yellowing can occur on lemon trees due to a few environmental and soil conditions. According to Glen Wright, a specialist in citrus at the University of Arizona, yellowing lemon leaves can be a side effect of overwatering and trace mineral deficiencies, but sunburn may also be a factor in the condition. Environmental adjustments can often halt and prevent further damage or even reverse the discoloration, returning the leaves to their natural green hue.
Too much water or extremely poor drainage can waterlog the soil, suffocate and even rot the roots. This in turn causes leaves to yellow and even drop prematurely. Soil should be moist at all times to an inch or so down from the surface but not consistently soaking wet. Container grown lemon trees must have multiple drainage holes and not be forced to sit in pooling water.
Deficiencies of needed trace minerals in the planting soil such as iron, manganese and zinc can all cause leaf yellowing and leaf drop--a malady called chlorosis. When the trace minerals are sufficient in the soil but the soil is very alkaline, the lemon tree roots may not be able to absorb and process the minerals, resulting in leaf yellowing. Soil tests can determine the exact situation, and soil amendments can easily correct it by adding the minerals or by lowering the soil pH.
Excessively hot sun or high ambient temperatures can also cause leaf discoloration. This is exacerbated when soil moisture and ambient humidity is low or the tree roots are experiencing drought stress. Providing light filtered shade during the middle and hottest part of the day can reduce sunburn. Keeping the soil very moist (but not sopping wet) at all times will also help reduce stress on the plant and prevent the reaction of leaf discoloration.