Agriculture Reference
In-Depth Information
Box 4.9 Instruments for Measuring Soil Water Content
In dry-grown vineyards, soil water content
θ
should be measured at two to
threedepths,downtothebottomoftherootzone.Inirrigatedvineyards,the
recommended practice is to manage the water content within the top 50 cm of soil,
so this defines the depth of interest for monitoring water content.
Capacitance and TDR probes rely on measuring the change in the apparent
dielectricconstantofsoilwithachangeinwatercontent(waterhasaveryhigh
dielectricconstantcomparedwithsoilsolidsandair).Acapacitanceprobecan
reside permanently in a narrow-bore PVC tube installed in the soil. A network
ofsuchprobesinstalledinavineyardblockcanbeconnectedbyWiFitoadata
logger and readings recorded regularly. Alternatively, a portable instrument can be
lowered to different depths in a network of PVC tubes and read directly. Correct
installation of the PVC tube is important to avoid air gaps between the tube and
surrounding soil; otherwise, false readings are obtained. The annulus of soil that is
sensed around the tube wall is small.
ForaTDR(sometimescalledahetaprobe),anelectromagneticpulseis
transmitted down parallel steel wave-guides, up to 30 cm long, inserted into the
soil. The time taken for the reflected pulse to return to a receiver is proportional
to the soil's apparent dielectric constant and hence varies with the water content.
TDR probes may be installed permanently at different depths and logged.
Alternatively, portable probes with the pulse generator and receiver built into the
probe head can be used for measurements of surface soil water content. The TDR
measures an average
θ
in a narrow cylinder of soil surrounding the wave guides.
Modified TDR probes are available to measure bulk
EC
upto6dS/m,aswellas
water content.
Nicholas(2004),Prichardetal.(2004),andCharlesworth(2000)givefurther
details of these instruments, which are also available on commercial websites.
Box 4.10 describes the calculation of
SWD
from measurements of soil
water content or
ET
and its use in irrigation scheduling. “Managing Natural
SoilVariabilityinaVineyard”inchapter6discussesirrigationplanningtotake
accountofsoilvariability;therelationshipbetweenwaterstressandwinequalityis
discussedin“ManagingSoilWater”inthesamechapter.
PlantMeasurementsofWaterStatus
A common plant-based measurement of water status focuses on leaf or stem
turgor, which reflects the suction experienced by water within a vine. The suc-
tion is expressed as a water potential, a negative variable that is measured as
eitherastemwaterpotentialoraleafwaterpotential(
LW P
).For
LW P
, a young
mature leaf in the upper canopy is enclosed briefly in a polythene bag before
being plucked and placed in a pressure chamber with the cut end of the petiole
