1. THE AVOCADO IRRIGATION CONUNDRUM
Simon Newett
Dept of Agriculture, Fisheries and Forestry, Queensland

2. 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

4. 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

5. Moisture stress = excessive fruit drop

6. Water stress = ringneck (incomplete fruit abscission)

7. Water stress = death of seed coat = small fruit

8. Water stress in first 11 weeks after fruitset = poor shelf life due to low calcium levels

9. Water stress = poor quality fruit Vascular browning

10. 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

11. No root hairs

12. Shallow feeder root system

13. Shallow feeder root system

14. Shallow feeder root system

15. 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)

16. Drowned avocado tree

17. Waterlogging = fruit fall & uneven ripening

18. Healthy vs. Phytophthora infected roots

19. Healthy vs. root rot infected tree

20. 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.”

21. 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

22. 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

23. Soil moisture monitoring
Full point
Refill point

24. The concept of ‘Readily Available Water’ (RAW)

25. 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

26. 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

27. Mulching avocado trees

28. Mulching avocado trees

29. Overhead misters

30. 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

31. The water contained a red fluorescent dye –
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

32. 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.

36. Acknowledgements