Agriculture Reference
In-Depth Information
Figure 4.12
Separatewaterdropsremainonthehydrophobicsurfaceofadrysandysoil(left
side).hesamesoilontheright(darkersoil)hasbeenkeptmoistandacceptswaterreadily.
in this chapter. The pores drained of water at
FC
are often called macropores,
whereas those that retain water are called micropores.
The matric suction corresponding to
FC
is 10 kPa. However, vine roots can
extract water at suctions up to 1500 kPa, when the vines lose turgor and wilt.
he
PWP
is reached if vines do not recover turgor at night but remain wilted
the following day. The choice of 1500 kPa for
PWP
is arbitrary, because wilting
dependsonacomplexinteractionbetweenthevariety(alsowhetheronownroots
orrootstock),theweatherconditions,andhowfastwatercanlowthroughthe
soil at large suctions. Tensiometers and gypsum blocks are used to measure soil
water suction, as explained in table 4.2. The installation of these instruments is
discussedlaterin“MonitoringSoilWater.”
SoilWaterRetentionandPlantAvailableWater
As the diameter of pores holding water decreases, there is a proportional increase
in matric suction, so that grapevines can extract water more easily from wide pores
holding water than narrow pores. The relationship between the amount of water
held and its matric suction is called the soil water retention characteristic, which is
an important soil property.
Figure 4.13 gives an example of water retention curves for a well-structured
clayloamatmatricsuctionsupto600kPaforthetop10cm(A)andfor40to
60cmofthesubsoil(B).Fromexperience,winegrowershavefoundthatsoilwater
heldbetween10and60kPasuctionisreadilyavailable(calledreadilyavailable
