Agriculture Reference
In-Depth Information
in a nonsaline soil of the same water content. Osmotic suction causes saline
groundwater to rise by capillarity from depths of 1 m in sandy soils and up
to 2 m in clay loams, which can create salinity problems for grapevines. The
management of soil salinity is discussed later in this chapter in “Drainage,
Leaching,andSalinityControl.”
Availability of Water to Vines
SoilIniltrationandWetting
Infiltration starts when water falls on a soil as rain or irrigation. Both matric suc-
tion and gravity pull water into the soil, with suction being dominant initially
when the soil is dry but negligible when it is thoroughly wet. If water is applied
rapidly, as with heavy rainfall, the capacity of the soil to accept it may soon be
exceeded, with the result that water collects on the surface and begins to run off.
Running water can cause erosion, especially on steep slopes when the soil is bare.
Box 4.5 discusses ways of measuring rainfall.
Asiniltrationproceeds,azoneofwetsoilextendsdownwardfromthe
surface. Figure 4.11 is a good example showing a “wetting front” clearly
separating wet soil above from drier soil below. Although suction remains
important for drawing water into pores at the wetting front, gravity is the
dominantforcedrivinglowthroughthebulkofthewetzoneabove.Box4.6
describes how to estimate the depth to which water has penetrated after rain
or irrigation.
Inclaysoilsthatcrackwhendry(seeigure4.1),iniltrationandwettingdo
not occur in such an orderly way. Rather water flows rapidly down the cracks so
that the soil columns between the cracks wet from the sides as well as the top.
This type of flow is called preferential or bypass flow, implying that the water
lows down preferred pathways between aggregates more quickly than within
aggregates.
Another situation causing irregular infiltration is when the surface soil
becomes water repellent (igure 4.12). Water repellence is most noticeable in
sandy soils that have become very dry. Organic matter coating the sand grains
creates a hydrophobic film, repelling water that may then be “funneled” down
old root channels and worm holes. The effect gradually disappears as the soil wets
up under persistent rain and can be ameliorated by mixing subsoil clay with the
topsoil.
RedistributionofWaterintheSoil
Watercontinuestolowevenafteriniltrationceases,butlowbecomesprogres-
sively slower as the largest pores and cracks empty of water, followed by the next
largest, and so on. The soil water content attained after two days drainage in a
well-structured soil defines the soil's
FC
,asdescribedin“WaterStorage”earlier
