Crop growth is dramatically stunted and very obviously slows down. Older leaves yellow and – characteristically in brassica crops – there is a red discoloration starting at the margin of older leaves, this can indicate a nitrogen deficiency . Take care not to confuse this red discoloration with that of phosphate.
If the oldest leaves lose their healthy green leaf color, it can be a magnesium deficiency. As the deficiency becomes more severe, the area between the veins of the leaves yellows while the veins stay green. Severe deficiencies are seen as mottled, spotted or curled leaves, leading to a scorched or burned appearance.
Both magnesium and potassium deficiencies may appear similar, starting with a chlorosis at the leaf margin. However, while potassium deficiency rapidly develops into necrosis and leads to leaves with a sharp borderline between healthy and necrotic tissue with the inner leaf maintaining a green color. Magnesium deficiency in contrast starts as interveinal yellowing and the whole leaf shows chlorosis before necrotic spots start to develop.