Model experiments on the behaviour of roots at the interface between a tilled seed-bed and a compacted sub-soil

Summary

Roots which grow down through a seed-bed and encounter a strong, untilled sub-soil beneath may be unable to penetrate the sub-soil and may be deflected horizontally. They will continue to grow horizontally along the top of the sub-soil either until the seed-bed dries out and the roots wilt and cease elongating, or until they find some path of low resistance down through the sub-soil. Such paths are often cylindrical biopores such as earthworm tunnels or channels left after the decay of previous root systems.

Model experiments were done with artificial impenetrable sub-soils containing arrays of round holes of various diameters. Roots of pea and wheat were grown down through beds of aggregates to encounter the artificial sub-soils at random positions. The roots were deflected horizontally until they encountered the vertical holes. The proportions of roots which entered the holes were found to decrease with decreasing hole diameter.

Computer simulation studies were done to investigate some aspects of roots encountering impenetrable sub-soils containing random arrays of round holes. The distances that randomly-deflected roots would have to travel before encountering holes were studied as functions of hole diameter and hole density.

The experimental results were combined and compared with the results from the computer simulations. It was found that the numbers of roots encountering holes within certain distances in practice were not significantly different from those simulated on the basis of random chance. Therefore there was no evidence for the roots sensing and growing preferentially towards the holes (trematotropism) in the well-aerated system used in the experiments. However, limited evidence shows that the possibility of trematotropism cannot be ruled out for poorly-aerated systems.