Agriculture Reference
In-Depth Information
3.1.2.1
Root:shoot ratios and specific root length
As indicated by (Bray, 1963), root:shoot ratios are characteristic of species and communi-
ties although they are sensitive to environmental influences and may vary markedly over
the life of a single plant. They generally increase with increasing nutrient, water or cold
stress and decrease with the amount of light or increasing biomass. High phosphorus
availability favours greater root production (Callot
et al.
1982; Strong and La Roi, 1985).
Grazing by nematodes also tends to increase the root:shoot ratio (Rovira, 1978).
Comparable patterns have been observed at the community level (Scholes, unp. data)
(Figure III.15) and temperate ecosystems tend to have much higher root:shoot ratios
than tropical.
Grasslands tend to have much higher root:shoot ratios than forests. Measured values
range from 1:5 to 10:1 in temperate ecosystems, and 1:100 to 1:2 in forests. Root:shoot
ratios tend to be lower in tropical ecosystems than in temperate or boreal situations since
above-ground biomass increases much more rapidly than below-ground. The root:shoot
ratio is five times greater on average in temperate than in tropical ecosystems.
Nonetheless, considerable variation has been noted, reflecting the local and regional
effects of different soil types, nutrient status and soil water regimes, together with
variations among plant taxa.
At a smaller regional or local scale, root development may be severely curtailed
by high soil strength and, where this occurs, roots may follow large communicating
pores or channels where these are present. Low porosity often has inimical effects
on root development, particularly when associated with high bulk density, fine texture,
high clay contents or compact structure since the size, distribution and continuity of
macropores directly influences oxygen supply to roots (van Noordwijk
et al.,
1993).
When grown on clay, loam and alluvial sand, the total root lengths of mature wheat
