Farmer's Weekly (South Africa)
Optimising macadamia water use drop by drop
Dr Nicky Taylor, senior lecturer in the Department of Plant and Soil Sciences at the University of Pretoria, and Dr Elrea Strydom, research and development manager at Macadamias South Africa, discuss the efficient irrigation of macadamia orchards.
The local macadamia industry has expanded rapidly in the past decade to an estimated 70 000ha, with a large proportion of this area under irrigation.
As it is still a relatively new crop in the country, very little research has been conducted on water use in macadamia orchards, which makes judicious irrigation scheduling quite challenging.
Macadamias South Africa, the Water Research Commission and the University of Pretoria have worked together over the past six years to answer important questions about how much water macadamias need, which phenological stages are most sensitive to water stress, and how to apply water most efficiently.
In the past, irrigation efficiency focused on water savings, but the current energy crisis has complicated matters, and growers must now also consider flow rates and pressure.
CONSERVATIVE WATER USERS
The volume of water lost from an orchard, via transpiration and evaporation from the soil and cover crops, depends on weather conditions, canopy size, tree spacing, pruning method, irrigation system, and how the orchard floor is managed.
Typically, transpiration increases in macadamias with an increase in canopy size, but it doesn’t always increase at the same rate as atmospheric demand. In other words, as it gets hotter and drier, the trees don’t use more and more water.
They can restrict transpiration to ensure the rate of water transport within them meets the volume of water lost through transpiration, as determined by atmospheric evaporative demand.
Applying more and more water when it gets hotter and drier can even restrict transpiration if oxygen in the soil is reduced due to waterlogging. This means that on a leaf-area basis, macadamia transpiration is lower than that of other fruit tree crops.
What we don’t know is whether different cultivars use different volumes of water. Researchers are trying to answer this question by studying the leaf anatomy of different macadamia cultivars from different production regions. This will determine if field trials are needed to compare the water requirements of different cultivars.
MICRO VS DRIP IRRIGATION
Current work focuses on optimising and comparing drip and micro-sprinkler irrigation systems in macadamia orchards, where the aim is to assist with decisions on when and how much to irrigate if you know the weather forecast for the week ahead.
Researchers and farmers also want to know if low-flow drip systems live up to the hype when it comes to water savings.
In summer rainfall regions, the most critical time for irrigation is during flowering and fruit set. In a study in Nelspruit, this was the time during which mild stress was detected in the trees, as conditions were hot and dry, with no rainfall received during this stage.
ON A LEAF-AREA BASIS, MACADAMIA TRANSPIRATION IS LOWER THAN THAT OF OTHER FRUIT TREE CROPS
Despite withholding water for fairly long periods of time during the other phenological stages (nut sizing and premature nut drop, shell hardening and oil accumulation) key indicators of plant stress did not suggest that trees were stressed for long, and neither did yield.
This suggests that by making the most of rainfall, significant irrigationwater savings can be made.
The ability to detect macadamia tree stress using remote sensing tools will also be tested, as this is becoming a very useful tool to manage irrigation over a large area.