The loss of green color in tomato seedlings, known as chlorosis, is a common sign of distress. Chlorophyll, the pigment responsible for the plant’s green appearance, is either degrading or its production is inhibited. This yellowing is a symptom indicating an underlying imbalance in the plant’s environment or nutrition. Rapid identification of the specific cause is necessary because seedlings have fewer resources to withstand prolonged stress.
Water and Drainage Issues
Excessive moisture in the soil is the most frequent cause of chlorosis in indoor-grown tomato seedlings. Saturated soil displaces essential air pockets, causing root suffocation. This lack of oxygen severely compromises the plant’s ability to respire and absorb water and nutrients, leading to yellowing.
Yellowing from overwatering typically appears on the older, lower leaves first, which often feel soft or slightly droopy. If the container lacks adequate drainage holes or the soil mix is heavily compacted, water retention creates an anaerobic zone that prevents healthy root development. A proper seedling mix should include materials like perlite or vermiculite to ensure sufficient porosity and airflow around the roots.
Poor drainage and overly dense soil exacerbate the problem, making it impossible for the roots to access the oxygen they require. Even if the soil contains adequate nutrients, the compromised root system cannot transport them effectively to the foliage. To prevent this, growers should check the moisture level by inserting a finger about one inch deep into the soil.
The substrate should feel dry at this depth before more water is applied. Confirming that all containers have unobstructed drainage holes is paramount, and containers must not sit in standing water. Allowing containers to sit in standing water rapidly leads to the saturation and subsequent suffocation of the developing root structure.
Nutrient Deficiencies
The chemical composition of the growing medium directly influences leaf color. Seedlings initially rely on limited nutrients stored in the seed or starter mix. Once the plant develops its first set of true leaves, these reserves are often depleted, and a nutrient deficiency can quickly manifest as chlorosis.
Nitrogen (N) deficiency is common because nitrogen is a mobile nutrient. When soil nitrogen is insufficient, the plant moves it from older leaves to support new growth. This migration causes a general, uniform yellowing that starts predictably on the oldest, lowest leaves.
Magnesium (Mg) deficiency also results in distinct yellowing. Magnesium is a component of the chlorophyll molecule. A lack of magnesium causes interveinal chlorosis, where the tissue between the veins turns yellow while the veins remain green, also starting on the older, lower leaves.
Iron (Fe) deficiency presents differently because iron is an immobile nutrient. Since iron cannot be easily relocated, its deficiency appears as interveinal yellowing on the newest growth at the top of the plant. Growers should begin feeding seedlings with a highly diluted, balanced liquid fertilizer once the first true leaves have fully emerged.
Light and Temperature Stress
Insufficient illumination is a frequent cause of pale or yellow growth in indoor setups. When light intensity is too low, seedlings stretch excessively (etiolation) to find a light source, resulting in weak stems and pale leaves due to insufficient chlorophyll production. Providing supplemental light from an artificial source is necessary to ensure the required intensity.
Tomato seedlings require 14 to 16 hours of intense light daily, followed by darkness for metabolic processes. Placing the light source too close can cause light burn or bleaching, where tissue turns white or pale yellow due to excessive light damaging the chlorophyll structure. Standard grow lights should be positioned between 2 and 4 inches above the tallest leaves.
Temperature fluctuations and cold exposure can also induce chlorosis. Tomato seedlings thrive best when the ambient temperature remains consistently between 65°F and 75°F (18°C to 24°C). Exposure to consistently cold temperatures, particularly below 55°F (13°C), drastically slows the plant’s metabolic rate. This reduced activity inhibits nutrient uptake and translocation, causing general yellowing. Sudden temperature shifts or cold drafts can also shock the plants.
Pests and Diseases
Yellowing can signal the presence of biological agents, requiring a close visual inspection. Damping Off, caused by soil-borne fungi, typically collapses the stem at the soil line, but general yellowing can be an earlier symptom. This condition is favored by high humidity and poor air circulation.
Fungal or viral infections can cause distinct patterns of yellowing, such as irregular yellow spotting, mottling, or mosaic patterns. These patterns differ from the uniform yellowing caused by nutrient deficiencies and often require specific diagnosis.
Tiny insect pests, including spider mites and aphids, feed on the plant’s sap, leading to localized yellowing. Spider mites cause fine, stippled yellow or bronze spots as they damage individual plant cells, often congregating on the undersides of the foliage.
Increasing air circulation with a small oscillating fan helps prevent Damping Off fungi. Minor pest infestations can often be managed with a gentle spray of insecticidal soap or a highly diluted mild dish soap solution.