Table 2.
Indicative spaces and distances between components of the soil relevant to root and rhizosphere function
| μm | Feature | Significance |
|---|---|---|
| ≤ 0·2 | Pores between clay particles (or in remnant cell walls) | Roots unable to access water in these spaces* |
| Some bacteria this diameter, including filamentous bacteria | ||
| 1–2 | Pores within soil microaggregates | Bacterial diameters |
| Fungal hyphae and root hairs can flatten to this diameter | ||
| 2–30 | Thickness of mucilaginous film retaining bacteria on field-grown roots† | Matrix (film) of gel-like compounds from roots and organisms that is water stable and may hold bacteria close to the root and soil |
| 5–10 | Distances within bulk soil microaggregates | Some large bacteria |
| Hydrated fungal hypha | ||
| Turgid root hairs | ||
| 1–170 (mean 90) | Distances between bacteria on field-grown roots within clusters† | Quorum signals between Gram-negative bacteria can diffuse quickly over this distance§ |
| 10–30 | Pores between soil microaggregates (mesopores) | Pores that can retain water against gravity for a day or two, i.e. largest water-filled pores at ‘field capacity’* |
| Nematodes | ||
| 6–500 | Pores between soil aggregates in the rhizosheath; see Fig. 1A‡ | |
| 7–250 (mean 50) | Distances between neighbouring hairs close to root in rhizosheaths‡ | Protozoa range between 2 and 1000 µm |
| 40–300 | Distances between neighbouring hairs in outer rhizosheaths‡ | |
| 30–2000 | Pores in soil created by roots or macrofauna or cracks | Water drains and flows rapidly in these pores |
| from wetting or drying or freezing (macropores) | Successive generations of roots colonize these pores (biopores) (see Fig. 2A) | |
| Organisms can move readily in these spaces | ||
| Diameters of roots, depending on type and species | ||
| Diameters of insects | ||
| 400–1000 | Distance between root surface and outer edge of rhizosheath in cereals‡ | Soil more tightly bound to the root in dry soil compared with wet;¶ depends on species where barley is 1·6-fold wider than wheat |
| Root hair lengths |
‡
Direct measurements using image analysis software (AnalySIS, Soft Imaging Systems, GmbH, Münster, Germany) of distances within the rhizosheaths of field-grown wheat and barley roots such as those shown in Fig. 1A that were frozen in the field in liquid nitrogen and observed with a cryo-scanning electron microscope (see Watt et al., 2005, for methods).