THE AVOCADO IRRIGATION
CONUNDRUM
Simon Newett
Dept of Agriculture, Fisheries and Forestry,
Queensland
Origin of avocados - Central American highlands
•Central American highlands (e.g.
Michoacan State, Mexico)
•Deep, well drained volcanic soils
•Soil surface mulch of leaf litter
•Well structured soils, high in
organic matter
•Benign climate
– regular rainfall (about
1500mm/yr)
– absence of temperature
extremes
Avocado is still a ‘young’ crop:
1911: First release of a selected,
propagated variety
1940s: First attempt at rootstock
selection
The conundrum . . . . .
Avocados have a high water requirement and
are sensitive to water stress but they are …
• inefficient at extracting moisture from the soil
• sensitive to over watering
Moisture stress = excessive fruit drop
Water stress = ringneck (incomplete fruit abscission)
Water stress = death of seed coat = small fruit
Water stress in first 11 weeks after fruitset =
poor shelf life due to low calcium levels
Water stress = poor quality fruit
Vascular browning
• inefficient at extracting moisture because
– they don’t have root hairs
– have a very shallow feeder root system
(90% are in the top 15cm of soil)
– can extract little water below -20 kPa
No root hairs
Shallow feeder root system
Shallow feeder root system
Shallow feeder root system
• sensitive to over watering because …
– feeder roots have a high oxygen
requirement (trees die after 48 hours
inundation) and …
– roots are susceptible to Phytophthora root
rot (a “new encounter” disease for avocado)
Drowned avocado tree
Waterlogging = fruit fall & uneven ripening
Healthy vs. Phytophthora infected roots
Healthy vs. root rot infected tree
In appreciation of these facts …
“A more focussed and responsive
approach to moisture monitoring and
irrigation is required.
This is emerging as a key component in
achieving high yields of good quality
avocados in Australia.”
The solution
Plant only on well drained soil, ensure orchard drainage is
near perfect
Install an irrigation system that delivers water and fertiliser
evenly to every tree and does not leak when turned off
Install an accurate & reliable soil moisture monitoring
system plus a back up
Follow weather forecasts to be prepared for peak demand
Monitor soil moisture several times a day in warmer months
The solution (continued)
Use the concept of Readily Available Water (RAW) and aim
to keep soil moisture between the Full (-8kPa) and Refill
(-20kPa) points
Respond immediately to irrigation needs, but …
… don’t exceed Full Point
Irrigate several times a day if necessary
Use mulch to create a more hospitable root environment,
reduce evaporation, increase soil organic matter and
help combat root rot
Use overhead misters for cooling during heat waves
Soil moisture monitoring
Full point
Refill point
The concept of ‘Readily Available Water’ (RAW)
The concept of ‘Readily Available Water’ (RAW)
RAW is mm of water in the soil
readily available to the plants.
For avocados
= Full Point (-8kPa) - Refill Point
(-20kPa)
Soil
texture
Soil water
extractable between
-8 and -20kPa
(mm/cm)
(Mallee soil, 0.5% OM)
Sand
0.33
Sandy
loam
0.46
Sandy
clay
loam
0.39
Clay
loam
0.30
The concept of ‘Readily Available Water’ (RAW)
RAW is mm of water in the soil
readily available to the plants.
For avocados
= Full Point (-8kPa) - Refill Point
(-20kPa)
Example:
Sandy loam
Main root zone 15cm
15 x 0.46 = only 7mm
Literature suggests that RAW
can be increased by 3 to 4mm for
every 1% increase in soil organic
matter.
Soil
texture
Soil water
extractable between
-8 and -20kPa
(mm/cm)
(Mallee soil, 0.5% OM)
Sand
0.33
Sandy
loam
0.46
Sandy
clay
loam
0.39
Clay
loam
0.30
Mulching avocado trees
Mulching avocado trees
Overhead misters
Research needed
Establish ‘Crop factors’ for different times in the annual
growth cycle (thought to exceed 1.0 during flowering)
More accurately determine what water tension levels
result in ring neck, excess fruit shedding, death of
seed coat, and other water stress effects
Partial drying of the roots has been tried but results in
crop loss
Using water stress as a management tool only has
negative effects
Water stress ‘memory’: induces avocado to block
xylem with tyloses as a survival mechanism
Un-watered
Well-watered
The water contained a red fluorescent dye –
so red means water can flow through the vessels
Typical avocado irrigation system
Tree spacing 9 x 4m
Under-tree mini sprinklers (one to two per tree)
Systems deliver 80 to 120 L/tree/hour
About 60% of orchard floor is wetted
Precipitation rates about 2mm/hour in wetted area
Systems must be capable of meeting peak demand days
Capacitance probes (e.g. Enviroscans), gypsum block
and tensiometers are popular moisture monitoring
tools. Capacitance probes typically measure hourly.
Acknowledgements