Calcium is an essential nutrient for cannabis plants, providing structural support and aiding in stress resistance, particularly heat. A calcium deficiency in cannabis can be identified by the presence of brown or bronze splotches or spots on the leaves. While yellowing is commonly associated with calcium deficiency, it may not always occur. Under LED lights, the splotches may even appear purple. Calcium deficiencies are often caused by low pH at the roots or by using soft or filtered water.