Why does an inadequate percolation limit root growth?

As water percolates through the root zone, air rushes to fill the open pore spaces that the water has vacated. This fresh air, newly introduced to the root zone, is essential to root growth and health. If percolation is inadequate the volume of air placed into the root zone is not sufficient for healthy roots. Layers of organic soil or compacted soil can limit percolation and air movement to the detriment of root growth. Under sever conditions these layers can cause water to virtually stop its downward movement, saturating the soil above the layer and forcing out the air necessary for growth.  more