(Image credit: Shutterstock) Yellow leaves on tomato plants signal something's wrong with their growing conditions. Unlike gradual aging where bottom leaves naturally yellow and drop, widespread yellowing across multiple leaves or rapid discoloration indicates a problem that needs fixing. Tomato plants need specific conditions to thrive: consistent moisture without waterlogging, nutrient-rich soil, full sun exposure, and protection from pests. When any of these factors fall out of balance, leaves turn yellow as the plant struggles to photosynthesize and grow properly. Article continues below 1. Nitrogen deficiency Nitrogen deficiency is the primary cause of yellow tomato leaves, especially when yellowing starts at the bottom of the plant and moves upward . Nitrogen is essential for chlorophyll production and if they don't get enough, leaves can't maintain their green color. Tomatoes are heavy feeders that deplete soil nitrogen quickly during the growing season.…