By Dave Franzen, Extension Soil Specialist
Some plant leaf symptoms, such as potassium deficiency — nearly always showing leaf necrosis at leaf edges — are decent beginning diagnostic indicators. However, purple corn is not.
Phosphorus deficiency in corn is famously shown as purple corn. However, purple corn may not have anything to do with soil phosphorus.
I have had more calls this year than usual regarding fallow syndrome after beets, and after fallow. The first consideration a grower should make is whether corn is a good choice for a crop after beets, or after fallow.
If a grower decides to charge ahead, they should be prepared to beef up the planter to apply a 2x2 band of 18 gallons per acre of 10-34-0 or equivalent, or 120 pounds per acre 11-52-0 or equivalent as a starter.
In-row starter fertilizer rates will not be able to overcome the reduction in soil mycorrhizae numbers following fallow caused by anything, or following a crop or cover crop in the Chenopodiaceae family or Cruciferae family (lambsquarters and mustard — forgive me, soil scientists have little opportunity to show off their plant taxonomic education).
This means that corn after sugarbeet (lambsquarter family) and any mustard, canola, any cover crop beet species, radish, field radish, tillage radish, radish or reddish radish will succumb to fallow syndrome.
Also, drowned-out acres left idle and not planted to a cover crop that includes something other than the non-mycorrhizae-supporting species mentioned above, or their relatives, will also have a fallow syndrome problem.
If the problem is evident in this year’s corn, there is little to do about it. It is possible to make a foliar application of a few gallons per acre of 10-34-0 cut with water (for greater flowability, not burn potential), but I doubt that the corn will ever be what it could have been had it been planted elsewhere.
Other possible sources of purple corn problems are anything that restricts rooting; spring compaction, higher salt or sodium levels, cold soils (not a problem now), insects feeding on roots, or root rots.
To determine whether the problem is phosphorus, dig (don’t pull) plants out of the ground and look at the roots. If they look fine, soil test a good and bad area, and take plant samples from the good and bad area and send the paired sample to the lab. While digging, you also have the opportunity to determine what the rooting zone is like.
If the root zone is hard, massive and difficult to dig, that may be part of the problem. If you see insects or insect larva around the root and feeding, that may be part of the problem. Don’t rely on plant color alone to tell you what the real problem is, because purple corn is a symptom of many things.