I have encountered a serious problem: a hardpan three to four inches deep in dozens of fields over the last two years that is robbing north Alabama growers of yield and profits in cotton, corn, soybeans and wheat.
The shallow hardpan has more potential to rob growers of yield due to a much more restricted root system compared to the deeper hardpans we often see in sandy soils. The good news is that shallow hardpans are also easier to correct than deeper hardpans requiring less horsepower and fuel since growers only need to run a subsoiler four to five inches, or just deep enough, to break the hardpan.
The poor root system due to the shallow hardpan makes plants more likely to lodge, especially taproot crops like cotton and soybeans. The restricted root system also limits the area the plant is able to take up nutrients. The plants are more susceptible to drought as the roots cannot get to subsoil moisture, and the hardpan makes the rainfall or irrigation more likely to runoff rather than penetrate the subsoil. The limited root system also magnifies problems such as nematodes.
Mike and Will Flynt called me out to look at a problem cotton field in the winter of 2016. The dryland field only yielded 660 pounds per acre, while a field just down the road made over 1,100 pounds per acre. Many north Alabama growers had record cotton yields in 2016 so the Flynts were especially concerned about correcting the problem.
A penetrometer detected a hardpan about three inches deep, and the roots were restricted to the top three inches. A very high reniform nematode population (1752 per 100cc of soil) magnified the problem. Reniform populations over 250 are considered high enough to affect yield and a nematicide or rotating to a non-host crop, such as corn, is recommended.
The Flynts decided to subsoil and rotate to corn in 2017. Both paid dividends as their corn in that field placed third in Alabama with 239.5 bushels per acre in the non-irrigated no-till/strip-till class of the National Corn Growers Association yield contest.
Poor Water Penetration
I asked the Flynts to not subsoil a portion of the field to see if they could see a difference in subsoiled versus non-subsoiled corn. There was a huge difference in plant height and in root development.
Mike noticed another significant difference. “We had the wettest summer in recent memory and the ditches around the field didn’t fill up. They filled up with one inch of rain in the past. The subsoiling also help prevent soil erosion by allowing the water to penetrate deeper rather than running off with our soil,” he said.
“We also tapped into a nutrient bank deeper in the soil. When we saw we were having a problem with our cotton, we tried throwing more fertilizer to the cotton and we were just backing up. You can’t fix a hardpan problem with more fertilizer,” Will said,
A hardpan restricts the area where roots forage for nutrients and may lead to nutrient deficiencies. I have noted stemphylium leafspot in several cotton fields with a hardpan. Stemphylium leafspot is actually a secondary problem brought on by potassium deficiency. Potassium adds strength to the leaf cells. The lack of potassium in the leaf cells makes them weak and susceptible to secondary fungal infections such as stemphylium.
Fungicides will not control stemphylium as potassium deficiency is the primary problem. The cottonseed companies continue to bring new higher yielding varieties on the market. These higher yielding varieties are demanding more potash, so growers need to monitor their soil potassium levels and plan their potash applications according.
Petiole testing could help avoid this problem since it will help detect potassium deficiencies up to two weeks in advance, especially as the crop moves towards peak bloom. Foliar potash may lessen the damage if potash deficiencies are caught early enough (before 4th week of bloom).
(Eddie McGriff is the Alabama Extension Regional agent in northeast Alabama.)