One neighbor went on a short trip a couple of weeks ago. When he returned home his neighbor's field of corn out by the road was purple. Not just a purple leaf or even a purple plant here or there – it was 100% purple across the entire field!
Frost could be a possible cause, but it hadn't been that cold by any stretch of the imagination. A phosphorus deficiency in the soil will turn plants purple, but a deficiency is hardly that uniform or that severe, especially for phosphorus in Indiana. Too many years of 6-24-24 fertilizer in the old days built many phosphorus levels so high they still aren't down into the deficient stage.
The other thing that will do it is soil compaction, agronomists say. In this case the farmer knew his neighbor had planted the field on the wet side because he was anxious to get some corn planted back when it seemed to rain every other day in early- to mid-May.
He visited with his neighbor. As it turns out it was a hybrid that has the tendency to show some purpling anyway. The added stress of soil compaction turned the plants purple.
As mentioned, purple still means there is a phosphorus deficiency in many cases, but it can be because the plant can't get the phosphorus because it doesn't have a functioning root system, not that the phosphorus isn't in the soil.
In this case, the seedsman had assured the farmer with purple corn that it would green up as soon as the weather warmed up. Cool weather not conducive to good growth also can cause purpling, especially when corn is small.
At last report the field of corn that had turned purple was turning back to green again. The spokesman for the seed company says final yield won't be affected.
There is a caveat, however. If the purpling was induced by soil compaction and the weather turns dry, soil compaction may limit yield, not the fact that the corn was once purple.