One reason for yellow foliage is overwatering. Overwatering will cause the root tips to rot. Rotting of the root tips will prevent the root from being able to uptake nutrients. A solution for this would be to cut back on watering. If the problem is not addressed, it will result in complete defoliation. Another reason for yellow foliage is lack of fertilizer. For Citrus to be productive it requires a constant source of Nitrogen. It is recommended to fertilize regularly according to the label directions with of a quality fertilizer. The ideal ratio for fertilizer is a higher amount of Nitrogen (N) relative to Phosphorus (P) and Potassium (K). The format of the ratio is Nitrogen (N): Phosphorus (P): Potassium (K) with the ratios being 3:1:1 or 4:1:1. For lemon trees specifically, it is normal to observe yellowing and loss of interior leaves. Lemon trees grow vigorously and benefit from heavy pruning. This will maintain a bushy look due to new branches forming where it was pruned