Question: At what growth stage do (corn) roots stop going deeper to find moisture?
Answer: If there’s no obstruction to the corn roots – no compaction and no horizontal layer – corn roots will grow and move down until they reach the water table. The roots will continue to grow as long as they have access to water and oxygen. If the water table drops, the roots will follow the water. Typically, during rapid growth stage, a corn plant will put on 1" to 1.5" of root growth a day, while your water table drops in only a few centimeters per day.
If there is compaction or a horizontal layer, the roots will typically grow horizontally once they hit that layer until they can find a crack in the soil that allows them to grow vertically again. The common problem at that point is if the water table has dropped very far, the roots will not be able to keep up with it even though they’re growing vertically again. If temperatures get too high, roots begin to die due to the heat and lack of water. In a drought situation the roots typically die from the top down and not the bottom up.
If you have compaction or density layers in your fields, you have to get them out of there. That’s one reason why we talk quite a bit about vertical tillage. Anther option, if you don’t have irrigation and your fields drain off water quickly, is to entertain the idea of installing gated tile to slow down how quickly your water table drops. That will take some planning and help from someone who specializes in this type of work, but it could be a good investment for future corn crops.