To add nitrogen to your tomato plants, you can use fertilizers specifically formulated for nitrogen supplementation, such as compost, manure, or synthetic fertilizers with a high nitrogen content. Apply the fertilizer to the soil around the base of the tomato plant, making sure not to directly touch the stem or leaves. Water the plant thoroughly after applying fertilizer to help it absorb the nutrients. Repeat this process every few weeks throughout the growing season to maintain a steady supply of nitrogen for your tomato plants. Additionally, consider planting legumes as companion plants for your tomatoes, as legumes have the ability to fix nitrogen in the soil. This can naturally enhance the nitrogen levels available to your tomato plants. Regularly monitor your plants for signs of nitrogen deficiency, such as yellowing leaves, and adjust your fertilization routine accordingly.