Here are some symptoms, causes and remedies for common seed-starting problems. But don't be discouraged after reading the list of woes—seeds have an innate desire to live, even when their caretakers are less than perfect. Give seeds room to grow, good light, clean soil, the proper amount of water and food, and you'll be rewarded with a bounty of sturdy, young plants.
Symptom 1: Seedlings collapse; rotten stems at soil level or roots
Possible cause: Damping off
Remedies: In future, clean recycled plastic seed trays and cell packs in a solution of nine parts water and one part bleach; rinse well. Use fresh, sterilized seeding mix. Don't overcrowd; grow in cooler temperatures once seeds germinate. (If only a few seedlings collapse, remove them and keep remaining seedlings dryish.)
Symptom 2: Leaves curl
Possible cause: Overfertilizing
Remedies: Reduce amount of feeding and/or replant seedlings in fresh mix.
Symptom 3: Lower leaves yellow
Possible causes: Overfertilizing; magnesium deficiency
Remedies: Adjust feeding schedule and/or repot in fresh mix.
Symptom 4: Long, weak stems; large spaces between leaves on stems
Possible causes: Insufficient light; growing at too high temperature; overcrowding
Remedies: Leave grow lights on longer or use them to give more light to window-sill-grown plants. Reduce temperature. Thin plants. Transplanting checks growth.
Symptom 5: Green mould on soil surface
Possible causes: Poor drainage, lack of air circulation; overfertilizing
Remedy: Scratch small amount of powdered charcoal into surface.
Symptom 6: Pale leaves
Possible cause: Nitrogen deficiency
Symptom 7: Underside of leaves reddish purple
Possible cause: Phosphorus deficiency
Symptom 8: Bronze or brown leaf edges
Possible causes: Potassium deficiency; overwatering
Remedies for symptoms 6, 7 and 8: Feed with fertilizer containing trace minerals; if growing in soilless mix, repot in soil-based mix or add compost to planting mix. Reduce watering for #8.