Agriculture Reference
In-Depth Information
on an orchard basis, the water status of trees and their constituent parts, and
the overall effects of irrigation and water status on tree performance.
Soil water availability
Knowledge of the factors controlling soil water availability is essential to the
understanding of fruit tree water use and irrigation requirements.
The amount of water available in the soil depends on the amount supplied
(by rainfall or irrigation), the amount lost by run-off from the surface or by
drainage to below the rooting zone, and the amount retained in the rooting
zone until taken up by the trees.
Run-off from the surface depends on slope and on soil texture and struc-
ture. The latter factors influence the speed with which surface-applied water
infiltrates into the soil. Data on infiltration rates into different soil types
are published in irrigation reference books and reviews (Anon.,
; Miller,
mm h
−
, into
). Infiltration into sands can be at rates of more than
mm h
−
. The compactive effect of rain or sprinkler drops
on a bare soil surface can result in a low porosity crust on the soil surface
which reduces infiltration (Miller,
clays at less than
) found that the
infiltration of water into herbicided alleyways in apple orchards was only
). Jackson
et al.
(
mm h
−
while that into the corresponding positions in grassed alleyways
.
mm h
−
. Hamer (
was
% of summer rainfall could
be accountable for in the centre of grassed alleyways but only
.
) found that
%inthe
corresponding, rooting zone, depth in herbicide-treated land. Mulching can
be used to ameliorate poor water penetration and retention under bare soil
conditions (Rom,
). Where the herbicide treatment
is initiated by killing a grass sward the soil is re-wetted by rainfall to a much
greater extent than where it is initiated on cultivated soil (Atkinson and White,
, quoted by Stiles,
).
Drainage to below the rooting zone also varies with soil type, the soil water
content at which the downward fluxof water becomes slow being called the
field capacity. This can be influenced by layering in the soil profile. In compar-
ison with a uniform soil, any profile layer with a different pore size distribution
will increase the water content above the layer whether this is of finer or coarser
soil. This is an important feature in some major apple production areas e.g.
Washington State, USA (Miller,
).
The wilting point is the soil water content below which plants growing
in that soil remain wilted even when transpiration is nearly eliminated. This
varies with soil type: soils with more than
% clay can be at the permanent
wilting point when they have
% of water on a volume/volume basis,
coarse-textured sands reach the same point at less than
-
% water content.
