Agriculture Reference
In-Depth Information
though this varies with rice variety, soil fertility and other factors. The maximum
primary root length required to explore this volume of soil would be 26.7 cm,
which is within the range calculated with the model. Note that some roots with
a greater coverage of laterals could be shorter than this, exploring the soil at
shallower depths.
In summary, the model shows that a system of coarse, aerenchmymatous
primary roots with gas-impermeable walls conducting O
2
down to short, fine,
gas-permeable laterals provides the best compromise between the need for inter-
nal aeration and the need for the largest possible absorbing surface per unit root
mass. Though the model assumes a fairly simple picture of the root architecture
and the changes in gas-permeability across the roots, this is the basic system in
most current rice genotypes. The significance of rates of loss of O
2
to the soil of
the magnitude calculated is considered in Sections 6.4 and 6.5.
6.2.2 ROOT SURFACE REQUIRED FOR NUTRIENT ABSORPTION
Having explored in the last section the limits that the need for internal aeration
places on the size of the root system and its optimal architecture, we may now
consider what root surface is required for nutrient absorption in submerged soil.
I consider the case of nitrogen because it is the nutrient required in the greatest
amounts. In fertile moist soil, the main plant-available form of N is usually the
NO
3
−
ion, and because NO
3
−
is largely not adsorbed on soil surfaces and is all
in the soil solution, its rate of delivery to absorbing root surfaces does not limit
the rate of uptake. In submerged soil, however, the principle form of N is the
NH
4
+
ion which is adsorbed and therefore diffuses through the soil more slowly.
In quantifying the rate-limiting step in uptake and the root surface required we
therefore need to allow for the rate of transport through the soil and the rate of
transfer across the root surface.
The main transport processes involved are shown in Figure 6.7. In essence
these are the same as in a non-flooded soil: there is a dynamic equilibrium
between solutes in the soil solution and those sorbed on the immediately adjacent
Liquid
diffusion and mass flow
rapid
exchange
very slow diffusion
Solid
Figure 6.7
Solute transport processes near an absorbing root (Tinker and Nye, 2000).
Reproduced by permission of Oxford University Press
