Agriculture Reference
In-Depth Information
Summary Points
1. Grapevines are vigorous perennial plants that are potentially deep
rooted,dependingonthedepthofsoilanditsstructure.Soilstructure
is an expression of how particles of clay, silt, and sand combine to form
aggregates and of the nature of the pore spaces, or porosity, created within
and between aggregates. Aggregate type and stability, aeration, drainage,
water storage, and soil strength are all important factors affecting vine
growth that depend on a soil's structure.
2. A predominance of exchangeable Ca
2+
ions on the clay particles ensures
soil aggregates are stable and able to resist the disruptive forces caused
by raindrop impact and cultivation. Aggregate stability is enhanced in
topsoils by well-humified organic matter, which interacts with the clay
particles, and by gums and mucilages secreted by plant roots, bacteria,
and fungi. Aggregate stability in subsoils depends more on the presence
of Fe and Al oxides and is much less reliant on organic compounds.
3. Aggregation is potentially unstable in sodic soils in which exchangeable
Na
+
ionsoccupymorethan6%ofthesoil'scationexchangecapacity
(
CEC
).AcombinationoftoomuchexchangeableNa
+
and a dilute
concentration of salts in the soil solution predisposes clay to disperse or
deflocculate and disrupt the structure of small aggregates. Gypsum is
recommended for amelioration of sodic soils, and deep ripping may be
requiredtotreatsubsoilsodicitywithgypsum.
4. Asoil'sporespaceisoccupiedbywaterandair.Whenfullywet,thesoilis
saturated, and, as it drains, air gradually displaces water from the widest
pores,followedbyincreasinglynarrowpores.Normally,withintwodays
a soil drains to its upper limit of available water, which is called the field
capacity(
FC
)orreillpoint.helowerlimitofavailablewater—the
permanentwiltingpoint(
PWP
)—isreachedwhenthevinescanno
longer extract water at a rate that matches their transpirational demand
(theywiltanddonotrecoverturgoratnight).hewaterheldbetween
thesetwolimitsistheavailablewatercapacity(
AWC
),whichisafunction
of both a soil's structure and texture, and is usually expressed in mm water
per meter depth of soil.
5. Suctionforcesandgravitydrivewatermovementintoandthroughasoil.
The suction arising from surface tension forces is called matric suction;
the concentration of dissolved salts determines the osmotic suction.
Gravity is the dominant force when the soil is wet, especially when there
are large cracks in which water can flow rapidly downward. In a nonsaline
soil, the hydraulic head is the sum of the matric and gravitational
components of the soil water suction.
