Agriculture Reference
In-Depth Information
III.3
ROOTS
Roots are the principal biological components of the soil system since they comprise the
largest part of the living soil biomass and exercise a major control over such processes
as pedogenesis, soil organic matter, nutrient and water dynamics. In terms of their function
in the soil system, they act as true heterotrophic organisms since they obtain their energy
and carbon from the above-ground part of the plant and their nutrients and water from
the soil. Roots supply energy and return absorbed nutrients to the soil through the produc-
tion of below-ground root litter and, while living, through the production of exudates, a mix-
ture of readily-assimilable compounds and modifiers ( i.e. , growth factors, hormones and
allelopathic substances).
Their importance as pedogenetic agents is widely recognised and they have major roles in
soil reinforcement and in the maintenance of structure (see Chapter I.1.3.3). The spatial and
temporal patterns of their growth through soils are particularly important to the ways in
which they utilise soil water and nutrient resources.
Finally, roots have close relationships with the free-living microflora and fauna, and
in a wide range of natural and agricultural ecosystems, frequently form associations
with such symbionts as nitrogen-fixing bacteria, fungi (mycorrhizae) or actinobacteria
(actinorhizae).
3.1
The morphology of the root system.
3.1.1
DEFINITIONS
Root topology influences both the exploration and transport characteristics of the root
system. Differences in the efficiencies of seven theoretical systems, all with 16 tips, but
with different branching systems are illustrated in Figure III.13. System B is most
effective at exploiting a restricted soil volume and is most common in nutrient rich
soils; system A is less efficient in exploiting zones of high nutrient concentration, but
better adapted to exploration (Fitter, 1985). Systems C to F are intermediate in nature.
Root systems may initially be described in terms of their vertical branching patterns
(see e.g., Jenik, 1978; Kahn, 1984). The order and importance of their ramifications
may also be considered. Basically, two main types of root systems may be defined:
(i) A sinker or tap rooting system in which secondary roots originate from a primary
root which grows vertically into the soil;
(ii) A lateral, or fasciculated rooting system in which roots expand from the base of
the stem in all directions. Many intermediate patterns occur between and within species
and represent modifications of a genetically-determined design imposed by local envi-
ronmental conditions. The root system of the grape vine ( Vitis vinifera ), e.g., comprises
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