Agriculture Reference
In-Depth Information
(vi) Grazing by rhizophagous herbivores. Root consumption by nematodes has been
reported to affect the specific root length of wheat plants which is doubled in their
absence (Rovira, 1978) (Table III.7);
(vii) Depth. The specific root length of wheat plants increases markedly from the
upper few centimetres to 1 m depth (Drew and Saker, 1980).
This may be interpreted in terms of the age of the roots which decreases with depth.
3.1.2.2
Horizontal and vertical distribution of roots in the soil profile
Plants develop different strategies in the lateral exploitation of soils. In tropical forests,
Kahn (1984) observed four phases in the horizontal development of the root system:
i)- formation of a tap root axis and production of a few lateral rootlets; ii)- production of
large lateral roots from the initial tap root axis; iii)- development of these secondary axes
which moves the exploitation zone of distal roots away from the axis; and iv)- production
of new large lateral roots from the tap root (or from the trunk, or the bases of the major
lateral axes); these roots will exploit the soil close to the axis which is no longer exploited.
During the last two phases, the tap root system develops through the elongation of
the primary axis and the formation of new axes with positive geotropism from the large
lateral roots (Figure III.17).
In most plants the root system seems to develop at random. Nonetheless, roots tend
to maintain a minimum distance from each other in order to avoid crossing and to
optimise the extraction of water and nutrients available in the volume of soil that they
exploit. This is especially true for perennial species in arid environments (
e.g.,
Fernandez
et
al
., 1988) (Figure III.18). In a maize plantation, the half mean distance between roots was at
a minimum in the upper 0-6 cm of soil (0.5 cm) and increased towards deeper horizons to a
maximum value of 2.86 cm (Tardieu, 1988). In compacted areas, much higher values of up
to 6.3 cm were observed in the deepest (71-78.5 cm) horizon. In arid environments, the
average distances of fine roots of perennial shrubs ranged from 1.62 to 2.34 cm with higher
values in surface than in moister deep horizons (Figure III.19).
