Agriculture Reference
In-Depth Information
later tree thinning or topping. At close spacing on the square, thinning is done
by removing the trees in every second diagonal row (Bower, 1981b). In Peru,
distances of 6 m × 4 m and 6 m × 3 m are being used, while in Chile 6 m ×
2 m, 6 m × 3 m and even 3 m × 3 m are being tried. The narrow plant spacing
needs careful formation and maintenance pruning to avoid overcrowding.
Intermediate plants will have to be removed as the canopy starts to overlap.
High-density schemes allow for higher initial yields, but management,
especially pruning, has to be more precise.
Placement of pollinizer trees in the orchard so that an adequate ratio
can be maintained after thinning may need to be considered in some spacing
schemes. The old guideline was one pollinizer for every nine trees. Large
blocks of a single cultivar have shown good production, presumably by self-
pollination, but interplanting pollinizer cultivars undoubtedly increases fruit
set for some cultivars that may not have pollen or are a poor pollen source.
'Ettinger' is an excellent pollen source for 'Hass' and there is a higher survival
of fruit to maturity.
Irrigation
The avocado can tolerate neither water stress nor excess moisture, especially
when drainage is inadequate. Water stress reduces yields, fruit size and
tree vigour. The soil around the trees should be moist but not wet. Irrigation
frequency depends upon fi eld condition, soil drainage and tree density, as well
as canopy size and prevailing weather conditions and past irrigation records.
Evapotranspiration data and tensiometer readings averaged over a period of
days provide a more accurate means of determining irrigation timing. The
application rate is calculated, taking into account the evapotranspiration
rate since the last irrigation, rainfall, percentage canopy coverage of the
ground and ei ciency of water use. Young, non-bearing trees require light,
more frequent irrigation. Older trees can withstand longer intervals between
irrigations but never to a point of water stress. Only 50% of the tree's
requirements should be given in the middle of the cool season and spring,
in order to favour fl owering rather than vegetative growth. When fruit set is
completed, irrigation reverts to normal amounts during fruit development.
High irrigation rates are necessary during fl owering and may be necessary as
the fruits approach maturity and if the weather is hot and dry (Fig. 7.3). Care
has to be taken not to let the soil dry too much at the time of fl owering and
fruit set, since a marked increase of water in the soil can produce a signifi cant
drop of small fruit. Irrigation is necessary down to at least 60 cm. Total
water applied per year is estimated at about 35-50 ha-cm for mature trees
(Gustafson, 1976).
On level land, less-ei cient furrow irrigation or sprinklers can be used,
with micro-irrigation systems (drip or micro-sprinklers) being more ei cient in
