Mulch is used to benefit plants and soil, but sometimes “sour” or “acid” mulch can damage plant tissue and lower soil pH levels. Toxic mulch is caused by poor handling or storing that prevents mulch from breathing or being aerated. Symptoms of mulch toxicity include yellowing of the leaf margins, scorching or dropping of leaves and occasionally entire plant death. Although it may be several days before symptoms appear, spreading sour mulch can damage plants immediately.