Corn Rooting Depth Will Impact Irrigation Scheduling

Richard Taylor, Extension Agronomist;

As you should expect, the strange weather pattern of a cool to cold spring followed by excessive moisture in many fields has limited the rooting depth and pattern for corn. While with a grower the other day checking on the timing for the next irrigation, we dug beside a number of corn plants checking the rooting depth. Corn roots were abundant in the top half to three quarters of a foot of soil although there were many fewer roots reaching the middle of the corn row than I would have expected. This also may have been due to the excessive rainfall following the nitrogen (N) sidedressing application which would have moved the applied N both downward and outward from the center dribble zone. Of course, fertigation contributed to the limited horizontal spread of corn roots since it provided N over the entire soil surface.

The surprising observation we made was the lack of significant rooting in the 12 to 24 inch soil layer. Although we could find a few roots going that deep, the number and density of roots seemed far below our expectations. I believe that the explanation for this lack of deep rooting is a combination of the very slow warm up of the soil and the excessive moisture many fields experienced whereby much of the applied N fertilizer was either denitrified and lost to the crop or was leached far below the rooting zone. I attended a national agronomy meeting of crop physiologist one year and although I don’t remember the exact temperature mentioned the group of scientists pointed out that corn rooting depth follows almost exactly the soil temperature curve as it warms in the late spring. The point of the meeting was to explain corn rooting under a no-till system versus a conventional tillage system. It was obvious from their data presentations that corn roots grew downward only as fast as the critical temperature for corn root growth moved down through the soil. Obviously in a no-till system, the more moist soil warmed more slowly than in the drier conventional soil and therefore the rooting depth for corn at any given stage of development was greater in a conventional system than in the no-till system until that critical temperature was reached at the maximum rooting depth for corn in a given soil type.

What’s this mean for our irrigated corn growers? To me, the shallower and possibly less dense root system of corn means that growers will need to monitor field soil moisture levels very closely to prevent corn from undergoing some yield limiting drought stress. Adequate moisture in the top 6 to 8 inches of soil will be critical to keeping the crop from being stressed, since the root system could be restricted mostly to this surface soil layer. With the scattered heavy downpour thunderstorms that have been moving across Delaware, the really difficult part will be maintaining that surface soil moisture without having more standing water injury to the corn crop. For many growers, the bottoms or low areas in fields have already drowned out and expectations for yield from these areas is at or near zero so making an error by irrigating too often likely will have less impact than applying too little water.

A last comment about shallow rooting in corn relates to standability following black layer. I’m a bit concerned that the shallow root system will mean that corn this fall will not stand as well as we expect. If we have warning of tropical storm development with a possible hit in our region, growers should try to harvest those fields hit hardest by the excessive rainfall and cold soil conditions this past spring and early summer. Growers also should consider beginning harvest as soon as harvest moisture falls into a range that they and the elevators can handle.