Root-soil contact of field-grown winter wheat

Roots following existing macropores and voids normally have only partial root-soil contact. Roots penetrating the soil matrix (creating new macropores) initially have complete root-soil contact. Partial root-soil contact may decrease the roots effectivity in taking up water and nutrients. If all roots have complete root-soil contact, however, aeration may be problematic unless roots have a high air-filled porosity. For field-grown winter wheat in the Netherlands, root-soil contact was quantified from horizontally oriented thin sections at three depths, 15, 25 and 45 cm, resp. One day prior to sampling, surface-connected pores were stained by infiltrating a methylene blue solution. Roots were observed microscopically using polarized light, and their diameter, roundness (indicating orientation) and degree of soil contact were measured with an image analyser. No relation was found between root-soil contact and root diameter or roundness. At 45 cm depth root-soil contact was less. For two fields, differing in soil, organic matter content and current crop management, a different frequency distribution of root-soil contact was found in the plough layer. The percentage of roots with 100% root-soil contact was 65 and 37, that with 0% root-soil contact 5 and 14, respectively. For roots with partial root-soil contact the average degree of contact was approx equal to 60% in both cases. Average root soil contact for the plough layer of the two fields was 84 and 66%, resp. Roots without direct contact with the soil were growing mostly in surface-connected (blue stained) macropores. There was no difference in blue staining of the macropores with roots with 1-49% root-soil contact and those of the whole sample. Roots with 50-99% root soil contact occurred mostly in relatively small, non-stained pores