Agriculture Reference
In-Depth Information
adecreaseddiurnalchangeinsoiltemperature.Lowerdaytimesoiltemperatures
decrease the evaporation rate and favor earthworm activity.
The distinction between the function of mulches and composts is discussed
in “Cover Crops and Mulches” and “Manures and Composts” in chapter 5.
Drainage, Leaching, and Salinity Control
What Causes Poor Drainage?
hebestvineyardsoilsareinvariablyfreelydrained.heyareoftendeepsoils(e.g.,
seeigures1.10and1.11),butshallowsoilsonlimestonecanalsobefreedraining,
providedthelimestoneisporous(asintheCoonawarraregion,SouthAustralia)
orfracturedandissured(asintheCôted'OrregionandSt.Emilionappellation,
France).
Animpermeablesubsurfacehorizon,whichisoftenapoorlystructuredand
denseBhorizon,impedesdrainage.hesodicclaysubsoilsofsomeduplexsoilsin
southernAustraliaareanexample,asaresomeoftheduplexsoilsintheWestern
CapeProvinceregion,SouthAfrica,andthePiedmontregionofeasternUnited
States.Inwinter,whenrainfallexceeds
ET
for long periods, water accumulates at
thetopoftheBhorizontoforma“perched”watertable(seeigureB1.3.1).he
soilabovetheBhorizonbecomeswaterlogged.BecausethesolubilityofO
2
in
water is low, O
2
diffusion through the water-filled pore space is very slow com-
paredwithitsdifusioninair.Consequently,rootsandsoilmicroorganismsare
starved of O
2
and cannot respire normally: the soil becomes anaerobic. Although
somemicroorganisms,suchasthoseresponsiblefordenitriication(thereduction
ofNO
3
−
ions),canswitchfromaerobictoanaerobicrespirationandcontinue
to function, vine roots cannot function under such conditions, and the plant's
nutrient uptake and growth suffer. Box 1.3, chapter 1, discusses other biochemical
redox reactions occurring in waterlogged soils.
Although well-drained soil should not develop a perched water table, the
riseofgroundwatertowithin1to2mofthesoilsurface(see“ForcesActing
onWater,”thischapter)cancreateproblems.Becausewaterisrisingfromdeep
intheproilewherethereislittleorganicmatter,thecarbonsubstratesrequired
for anaerobic microbial respiration are in short supply, so undesirable redox reac-
tions are less problematic. However, too much water supplied to the deeper roots
encourages excess vigor. Figure 4.22 shows a deep, sandy soil at the bottom of a
slopeintheSt.Emilionappellationwherethisproblemoccurs—similarlyinsome
of the heavier textured soils of the Pomerol appellation, Bordeaux region, France.
Furthermore, if the groundwater is saline or salts have accumulated in the soil as
a result of poor irrigation management, capillary rise of salts can seriously affect
vinehealth,cropyield,andfruitquality.
