Ag Instructor Vic Martin: Pale Green and Purple Plants

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Great Bend Tribune
Published June 6, 2021

As of June 1st the Drought Monitor reports no drought in Kansas and only abnormally dry conditions in small patches in NE and SE Kansas.  Warmer, seasonable temperatures combines with good soil moisture should allow wheat to mature pretty normally unless there is leaf disease pressure.  The six to ten-day outlook (June 8 to 12) indicates above normal temperatures and below normal precipitation.  The eight to fourteen day outlook (June 10 to June 16) indicates more of the same.  Good news for field work, wheat maturation, crops already in the ground and for crops needing planting.  Also good news for producers trying to cut alfalfa.

In many fields the corn, soybeans, and even some early planted grain sorghum are up but don’t look quite right.  Some entire fields are a pale green with purplish leaves in spots.  In some fields the pale color is confined to certain areas.  Alfalfa overall looks normal and wheat with the exception of certain parts of fields also looks a normal green. 

The obvious answer where the pale color is confined to certain areas of fields for summer row crops and wheat is standing water that inhibited root function.  Without extended periods of standing water, wheat looks normal.  First, it’s a cool season grass so cooler wetter weather is fine.  Second, it was well-established prior to this cool, wet period.  For soybeans, a crop that relies on rhizobium bacteria to supply nitrogen (N), the plants look fairly normal and maybe a bit paler than normal.  For soybeans, the bacteria won’t fully infect and start supplying nitrogen for a while yet.  The bean plants rely on available soil nitrogen early in the season and with the more normal temperatures and sunshine, they should be fine since their period of rapid growth is still weeks away.  But what about pale green fields for corn and sorghum? 

There are several factors at work here for these summer grass crops:

  • Roots systems for these crops are still relatively undeveloped, aren’t exploring a large volume of soil yet, and are confined to close to the soil surface, especially with wet soil conditions.  Nitrogen will percolate downward with moisture so it’s probable the much of the N is a bit deeper in the soil than the roots are currently.  The cloudy, cool, wet conditions slowed root growth, and leaf growth, which compounded the conditions.  As the root system develops it should grow to the N and should be fine except in the parts of fields with standing water for extended periods of time.  The last several days of warm, sunnier weather will help greatly.
  • The purpling is a different matter.  It is caused by a phosphorus (P) deficiency.  While it is possible there are P deficient fields in the area, it’s not likely as producers have done a good job of P fertility in their soils and most band starter P at planting.  The culprit here for corn and sorghum is twofold.  First. P moves to plants roots by diffusion, a slow process, which is why producers provide concentrated bands of P near the seed at planting to aid uptake.  Cool soil conditions slow the diffusion rate and root growth.  Unless the soil is P deficient, the plants should grow out of it with warm temperatures.  This temporary condition if common early on, especially under no-till and shouldn’t negatively affect yields.

Overall, except where flooded or hailed out, while behind a bit summer row crops should be fine.