This video of time-lapse root growth from the Alabama Agricultural Experiment Station and the Soil and Water Conservation Research Division of the USDA was done in the 1950s or ’60s. It’s a looong video, but it’s pretty neat.
The experiment was performed using seedling roots of cotton and soybean plants. They were grown through soil behind a glass pane and photographed at regular intervals.
I watched the whole thing, but from our perspective, things really heat up close to the end, around 13:30.
“Compaction of the subsoil is one of the conditions which can limit the development of a large, efficient root system. Roots may be stopped entirely or may have their rate of growth drastically reduced.”