THE AVOCADO IRRIGATION CONUNDRUM

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THE AVOCADO IRRIGATION CONUNDRUM

• Simon Newett
• Dept of Agriculture, Fisheries and Forestry,
  Queensland

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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

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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

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Moisture stress excessive fruit drop
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Water stress ringneck (incomplete fruit
abscission)
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Water stress death of seed coat small fruit
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Water stress in first 11 weeks after fruitset
poor shelf life due to low calcium levels
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Water stress poor quality fruitVascular
browning
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• inefficient at extracting moisture because
• they dont 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

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No root hairs
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Shallow feeder root system
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Shallow feeder root system
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Shallow feeder root system
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• 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)

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Drowned avocado tree
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Waterlogging fruit fall uneven ripening
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Healthy vs. Phytophthora infected roots
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Healthy vs. root rot infected tree
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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.

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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

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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
• dont 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

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Soil moisture monitoring
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The concept of Readily Available Water (RAW)
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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
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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
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Mulching avocado trees
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Mulching avocado trees
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Overhead misters
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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

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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
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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.

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Acknowledgements
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