Agriculture Reference
In-Depth Information
prevents or reduces energy-requiring nutrient uptake. As this is reduced to a
greater extent than shoot growth the mineral ion concentration in leaves is
reduced. It also reduces hydraulic conductivity of the roots, which can result in
leaves wilting as water uptake falls below transpirational loss, and can trigger
ABA (abscisic acid) production leading to a check to leaf growth and reduced
stomatal conductance. In orchard trees the effect of waterlogging is minimal
or not evident in the dormant season. Non-lethal waterlogging in the grow-
ing season has greater adverse effects on root than on shoot growth (Olien,
).
Oxygen diffusion to deep soil layers may also be impeded in very dense soils
and soil crusts and so limit root growth even in the absence of flooding.
Mechanical impedance
The rooting zone is often limited in depth by the shallowness of the soil layer.
At East Malling in Kent sandy loam soil overlies ragstone rock at depths from
approximately
) and although in
England it is recommended that the soil should have a depth of at least
cm to
.
m (Rogers and Head,
cm
forappleproduction(MAFF,
)theapplesoilsoftheWesternCapeinSouth
Africa are usually only
). Pans of different
types may also provide a barrier to root penetration, e.g. the caliche layer
of lime or lime-silica cemented hardpan found in some central Washington
soils (Dow,
-
cm in depth (Huysamer,
). Compacted
soil may also provide a mechanical barrier. Roots cannot penetrate soil if its
strength is greater than
) and frangipans in Ohio (Fernandez
et al.
,
kPa, such compaction arising either from natural
processes or use of heavy vehicles (Kotze,
). Root growth is often facilitated
by soil cracks, earthworm channels and old root channels. Auxt
et al.
(
)
found that use of a tractor-mounted soil auger to create a planting hole could
so compact the sides of the hole as to prevent its penetration by secondary and
tertiary roots.
A special case of mechanical impedance is that provided by the use of woven
nylon root restriction membranes to contain the root system within a limited
volume. The objective is to achieve a consequent reduction in shoot growth.
This is achieved partly by concomitant reduction in availability of water. Part
of the reduction in growth is, however, found even when irrigation maintains
high soil water levels within the restriction membrane and maintains leaf water
potential at a similar level to those of control trees with unrestricted roots. It
has been suggested that this component of the restriction of root growth is
due to effects on root production of plant growth substances. Roots appear to
respond to physical impedance, including that caused by root restriction, by
producing a transportable chemical signal (i.e. abscisic acid, ABA) which can
reduce shoot growth and leaf expansion. This signal, which is also produced
in response to soil drying, is discussed in Chapter
. Roots are also important
