Agriculture Reference
In-Depth Information
Soil water content
Soil strength
too high
Non-limiting water range,
NLWR
-comparable to the
AWC
Aeration
adequate
Decline in soil structure, increase in bulk density
Soil strength too high
NLWR
Aeration adequate
Soil water content
Figure 4.8
A diagram illustrating how the non-limiting water range for vines decreases as
subsoilstructuredeteriorates.Soilwatercontentincreasesfromlefttoright.(Adaptedfrom
Letey,1985,andMurray,2010.)
The action of the suction force on water is more complex. As discussed pre-
viously, an integral part of a soil's structure is the network of connected pores of
diferentsizesandshapes.henarrowerasoilpore,thestrongerthesurfaceten-
sion force drawing water into that pore. Hence, water in contact with dry soil is
preferentially sucked into the narrowest pores, and if the supply is maintained,
water progressively fills larger and larger pores until all the pores are filled and
thesoilissaturated.helastdropofwatertoenterthelargestporeisheldatzero
suction.
The combined action of suction and gravity can be illustrated by the
behaviorofaspongeplacedinashallowdishofwater(igure4.9).Wateris
sucked into the sponge until it wets to a certain height—this is referred to as
capillary rise. The upper part of the sponge does not become wet because the
suction drawing water up is canceled out by gravity pulling water down to the
“free” water level in the open dish. The net result of these two forces—suc-
tion and gravity—determines the hydraulic head of the water. Exactly the same
situation occurs in a vineyard soil. In the absence of surface evaporation, the
soilwaterisatequilibriumwhenthesuctionforceonthewaterisequalbut
opposite to the gravitational force at each point in the soil profile. The suction
force resulting from surface tension at air-water-solid interfaces is called the
matric suction.
At some level, usually at depth in the profile, the soil may be saturated and we
find groundwater. The top of this groundwater is called the water table, where the
matricsuctioniszero.hisisillustratedinigure4.10,whichshowsthehydraulic
headproileinadryingsoilwithawatertableat90cmdepth.Relativetoatmo-
spheric pressure, water below the water table has a positive hydrostatic pressure
that increases with depth. Hydraulic head differences arise because of differences
in groundwater depth and land height, with the result that groundwater flows in
thedirectionofminimizingtheseheaddiferences.
