When does soil get too dry for young corn? Or, when will you know that the young corn seedlings will not make it?
Variations of these questions have been coming in as the lack of rain continues and the temperatures rise on young corn plants.
The quickest, and perhaps most practical answer, is to scout fields regularly and identify areas where plants are not recovering from drought stress. In other words, the best indicator of when the plants will not make it is when they have died.
A more technical answer is that a soil gets too dry when the ‘permanent wilting point’ is reached — the level of soil moisture at which plants will fail to recover(Taylor and Ashcroft, 1972, Physical Edaphology: Physics of Irrigated and Non-Irrigated Soils, Freeman and Co., San Francisco).
Most plants will display drought symptoms long before the permanent wilting point is reached.
Most silt loam soils will reach the permanent wilting point around 8 percent moisture content (weight/weight), while a clay loam will reach the permanent wilting point around 10 percent moisture content (weight/weight).
The permanent wilting point varies with temperature, compaction/structure/pore arrangement, and the resulting ability of the soil to conduct water.
There are sensors that can measure when soils get near the permanent wilting point. The challenge with a sensor is deciding where to put it. A person could put 20 sensors at 20 different depth increments and still not have the level of sensitivity that each plant in the field has.
So, practicality brings us back to field scouting. Observe the plants in the different soil types and identify when (or if) plants die. As long as the plants are living, they have a chance to be productive.
Small seedlings do not require a lot of water. The dry soil surface will reduce the amount of evaporation from the soil and the plants are small so transpiration is low as well. So, even in these dry conditions, water losses may be minimized.
In addition, some of these soils are very dry on the top but still have some deeper plant available water deeper. If the root system reaches that water, then there is a good chance for the plants to survive.
If that root system has not reached the moisture, then the seminal root system (roots coming from the seed itself) could be compromised and these plants will be the first to show drought symptoms.
If areas of the field express drought symptoms, the soil should be excavated around the plants both with and without symptoms to determine the location and behavior of the early seminal root system. From that, you can determine if compaction is inhibiting root exploration of deeper soil.
A young corn seedling has two root systems. When corn first germinates, it produces the seminal (or seed) root system. When the corn reaches V3, the nodal root system becomes active.
The nodal root system develops above the seed but below the soil surface. If this area is dry, then the nodal root system could be compromised and we might see "floppy" corn later in the season. That is something we can scout later in the season.
Perhaps the final thing to consider is that if the young corn plants survive the drought prior to the V4 to V5 growth stage, there is probably minimal long-term damage from the drought, meaning that we would expect very little yield loss.
At V6, tassel and ear development are starting, so drought stress from about V6 to tasseling will cause progressively more damage the closer the drought stress occurs to tasseling.
Again, field scouting is our best tool in-season to assess drought stress.
Special thanks to our corn colleagues around the country for contributing their thoughts and experiences to this post.
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