There are two reasons corn turns purple. First, it is natural in some hybrid corns, especially those bred for purple ornamental ears.

Second, any cultural or environmental factors that limit root growth will aggravate phosphorus deficiency symptoms. Examples of such conditions include: cool temperatures, soil that is too wet or too dry, compacted soil, herbicide damage, insect damage, and root pruning by side-dressing knives or cultivators. Acidic soil can also intensify phosphorus deficiency symptoms. Low soil pH severely limits phosphorus availability to plants, which may cause deficiency symptoms even where high soil test phosphorus levels exist. Soil pH less than 5.5 typically reduces the availability of phosphorus in the soil solution by 30 percent or more. Acidic soil also reduces root growth, which is critical to phosphorus uptake.

For more explanations of phosphorus deficiency see Corn. Soil testing is critical for correction of this problem.