FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 462

This process is known as 'biological drilling' and its effectiveness varies greatly between
plant species. The tap root systems of woody perennial species ameliorate soil porosity
more effectively than the fibrous root systems of grasses (Hulugalle and Lal, 1986).
However, certain perennial grasses ( e.g ., deep rooted African species) and herbaceous
legumes ( Medicago sativa ) may penetrate 2-3 m into the soil and produce extensive
networks of soil macropores (Meek et al ., 1991; Fisher et al ., 1994). This process is of
utmost importance in the restoration of degraded soils.
Direct observation in the field shows that once root channels are freed following root
death and decomposition, they may be used preferentially by new roots (Dexter, 1986)
leading to an improvement in subsequent crop yields (Elkins, 1985). This enhanced
capacity for deep root penetration will only benefit the plant if the nutrients and water in
these pores are readily available (Cresswell and Kirkegaard, 1995).
Aggregation: effects of root enmeshment and SOM addition
Bartoli et al. (1993) showed that the surface fractal dimension Ds (as determined by
mercury porosimetry) is better than porosity at discriminating between the root zone and
non-root zone. This result indicates that the soil heterogeneity created by roots influences
both porosity and aggregation.
Roots may enhance soil aggregation by two complementary mechanisms.
Firstly, the release of mucilages by the root and the rhizosphere microflora 'glue' small
soil particles into micro-aggregates (Gouzou et al ., 1993), and secondly,
these small aggregates become entangled by fine roots and the hyphae of mycorrhizal
fungi to combine as compound or macro-aggregates, greater than (Tisdall and
Oades, 1982) (Section I.1.3.3). This model is supported by the results of Thomas
et al. (1986) who demonstrated positive relationships between root abundance and
aggregate size and abundance. Gouzou et al. (1993) also found that inoculation of
Bacillus polymyxa into the rhizosphere of wheat increased the amount of soil adhering
to roots by 59 % and the amount of aggregates in the size class 0.2 to 2 mm by 100 %.
Miller and Jastrow (1990) observed a correlation between the overall mean diameter
of water-stable aggregates and such biological parameters as root length and the
development of the hyphae of mycorrhizal fungi. However, the magnitude of this
effect is likely to differ between soils, with plant species and with the associated
mycorrhizal fungi. This is due to differences in the architecture and diameter of roots, in
the type of mycorrhizal association formed, the hyphal density of the mycorrhizal and
other fungi present and the amount and quality of exudates that they produce.
Aggregation in the rhizosphere may also result indirectly through the accumulation
of the faecal pellets of earthworms and other invertebrates that feed in the rhizosphere.
In pots containing the less than 2 mm fraction of a sandy A horizon soil (alfisol) from
the Côte d'Ivoire, macro-aggregate formation (>2 mm) after three months was limited
to ca. 10 % of the soil mass in the presence of the tropical grass Panicum maximum ,
whereas 80 % of the soil mass was aggregated in treatments to which endogeic
earthworms had also been added (Blanchart, 1992).