Leaves with ammonia injury temporarily wilt and may turn dark green or gray green. A cross section of the taproot shows that the center has turned yellow, red, or brown. Smaller roots may be dead.
Solutions
Ammonia injury occurs when fertilizers break down to form free ammonia in amounts toxic to lettuce. This is most common with aqua ammonia and urea fertilizers. Chicken manure can also be a problem. Cool, waterlogged, compacted soil exacerbates the situation. Keep ammonium forms of nitrogen out of the seed row. Avoid overfertilizing.
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