Summary
Roots which grow down through a seed-bed and encounter a strong, untilled sub-soil beneath 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. A major source of such paths is vertical cracks in the sub-soil.
Model experiments were done with artificial impenetrable sub-soils containing parallel cracks with widths in the range 0.5–3 mm. Roots of pea and wheat were grown down through beds of aggregates to encounter the artificial sub-soil at random positions. The roots were deflected horizontally until they encountered the vertical cracks. The proportions of roots which entered the cracks were found to decrease strongly with decreasing crack width and increasing (more perpendicular) angle of incidence between the root and the crack.
The experimental results were combined with the results from computer simulation studies which gave the proportions of roots encountering cracks at various angles with both hexagonal and parallel crack patterns. This showed that parallel crack patterns should enable a greater proportion of roots to enter the cracks than hexagonal crack patterns. Monocotyledonous plants which have several seminal root axes per plant have a statistical advantage over dicotyledonous plants which have only one seminal axis per plant with regard to crack entry.
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Dexter, A.R. Model experiments on the behaviour of roots at the interface between a tilled seed-bed and a compacted sub-soil. Plant Soil 95, 135–147 (1986). https://doi.org/10.1007/BF02378859
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DOI: https://doi.org/10.1007/BF02378859