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Australian Macadamia Society Podcast: Episode 1

Last year’s tree water relations workshops with Dr. Dan Manson from Bundaberg and Theunis Smit from South Africa were a big success, giving growers a selection of innovative new tips and tools to manage water use in orchards. Our Industry Development Manager Leoni Kojetin spoke with Dan and Theunis in 2 new AMS podcasts, where they share their thoughts on these hot topics. Well worth a listen!

How much water do macadamias use daily in Summer and Winter? What is the best use of the water you have (how much and when should you irrigate)? What monitoring tools are essential for understanding tree stress? Theunis and Dan cover these questions and more in this podcast.

EPISODE 1: Macadamia water use

Leoni Kojetin (Australian Macadamia Society). Welcome, everyone. This is Leoni Kojetin from the AMS. I'm with two macadamia water-use experts, Dr Theunis Smit from South Africa and Dr Dan Manson from Australia. Today we're talking about how macadamias use water, and both of you have done a lot of research and have a lot of experience in this area. While each variety is different, in general, how much water do macadamias use in winter and in summer?

Dr Dan Manson. There's some variability between the macadamia varieties and how much water they might use on any given day in winter or summer. An average mature bearing macadamia tree above the age of, say, 10 years, on an eight by four-metre spacing, might be using between 30 and 40 litres per day in winter. Peak daily water use in the middle of summer, in late December and January, might get up to around 60 to 70 litres per day.

Leoni. So, 60 litres a day over seven days, is 420 litres a week. If I put that down on a Monday morning when I arrive in the orchard, is that a good use of the water?

Dan. It may be, depending on your soil type. But generally, with the well-draining soils and the new plantations in South-East Queensland, that much water does not generally hang around in the root system of a macadamia for the entire week. 

It's similar to trying to give yourself all your drinking water for the week. If I gave, say, 25 or 30 litres to you on a Monday, and said that you needed to drink all the week's water that day, and that you couldn't drink for the next five or six days, you might imagine you might be thirsty after about four or five days. It is the same with a macadamia tree. It can only take up a certain amount water of every day, so giving it the whole week's water in one big dose may not be the best approach. It might be better to split that up into smaller applications throughout the week.

Leoni. In an ideal situation, how should I irrigate that?

Dan. Ideally, you'd give the tree a little drink of water on a daily basis, preferably pre-dawn or as close to sunrise, as we've been finding that the best time to apply water is just before the sun comes up. Giving the tree a small drink of water, say 50 or 60 litres a day, every day, seems to be the best approach on some of the more common soil types in the Bundaberg region.

Leoni. Theunis, I have more than that. I've got a thousand litres per tree per week. Why can I not put it all on?

Dr Theunis Smit. There would be some difficulties in putting on all that water. Over-watering trees can have many problems, including leaching of nutrients and increased incidence of diseases such as Phytophthora.

Adding more water is not necessarily the best option as it's similar to you drinking a lot of water. Adding to Dan's metaphor, if you drink more water, you'll leach out a lot of the vitamins in your body. A lot of that, similarly, is happening in macadamia, so you might not just get the same efficiency out of your system by over-watering macadamia trees.

Leoni. Conversely, if I have a limited amount of water, for example two megalitres a hectare, what are the key phenological cycles that I have to be covering with that water?

Dan. I think primarily what you want to be doing if you've only got a limited amount of water is avoiding severe drought events, which might come at any time during the year. When a tree suffers from severe drought, the impact of that can last 12 or 24 months, or maybe three years from the damage from that severe drought event. 

The most intelligent way to use that water is to apply it whenever your tree is under most severe drought stress. If you have this as a primary goal, i.e., avoiding severe drought events, the other key times you want to be putting water on the tree are around flowering and early nut set, as well as during nut growth, which usually happens in December.

Theunis. I would agree. It's difficult to make those decisions if you don't have monitoring tools of some sort in your orchards. There is some good equipment out there and some highly skilled people who can help you install it in your orchards. And you need to monitor your equipment, otherwise, you won't be able to see if the trees are under severe stress or not.

Leoni. What root zone area am I really targeting with irrigation?

Theunis. I would say in macadamias it will depend, again, on cultivar and soil type. In general, we find that macadamias have a lot of feeder roots in the top 300 to 400 centimetres of the soil, sometimes even less. And you really want to keep those topsoil layers fairly healthy and happy. There are other roots deeper down and if you have the water available, you want to water a little bit deeper down to keep the taproots also happy. Otherwise, you'll have to wait for rain to keep that bottom part of the soil happy.

Leoni. But I can't do this intuitively, rather I would need to use some sort of monitoring tools?

Theunis. Yes, definitely. I think there are some good ones. There are good soil moisture monitoring tools out there, for instance capacitance probes and tensiometers, which are easy to use, but they only give you half the picture. 

Dan can give you some more information on those sap flow meters, which he's been using successfully in Australia.

Dan. Soil moisture monitoring is your primary monitoring tool just to ensure that you're not watering too deeply and to ensure adequate penetration into the soil profile is occurring. Once you become an expert on your monitoring your soil, then you can take it to the next level by monitoring the plant. 

There's a new range of plant physiological monitoring tools, including sap flow meters, which you can install on farm and have information streaming to your smartphone to enable you to see the current stress status of your crop on a live basis. As a first stop, however, I think we should all be monitoring the soil to make sure that the water is getting to adequate depth, and it's getting to that adequate depth on a frequent basis.

Leoni. If I understand my water use and I understand the soil profile, can I use that water to manipulate the phenological cycle to encourage or to pull up a flush?

Dan. In most other crops, they've found that mild water stress during a certain part of the phenological cycle might be advantageous for increasing or maintaining high yields. 

The research is really in its early stages for this approach in macadamias. We're finding that the crop can be quite sensitive to drought stress throughout the production cycle. If there was going to be a period where it might be advantageous to stress the tree, that may be when there aren't nuts or flowers on the tree, which also coincides with the autumn flush. So, there might be an ability to apply mild stress during late autumn, after your crop's finished its production cycle, to reduce the size of the tree so that they don't get too crowded in the plantation situation.

Leoni. Theunis, is there an ideal irrigation system? It has to be sprinkling, or it has to be drip? Or are there different principles to orchard management with irrigation?

Theunis. I think there's not an ideal system for everyone. All farms do not have the same irrigation system. It will depend on a lot of factors, including soil type/s and the amount of water available. 

Most of the time we've seen that it's highly dependent on the type of management that you want to apply on a specific system. Both of those systems, micros and drippers, have distinct advantages and disadvantages, and it will depend greatly on what you're trying to achieve in a specific orchard.

Leoni. We know we're in drought conditions in both Queensland and New South Wales. Are there any ideas that you can give growers for how they could best use the limited water that they might have in those conditions?

Dan. If you've got limited water, I think it's best not to give too large an application of water, maybe 20 or 30 litres on a given day when it's quite hot or dry, or it's a specific time of the year when you think the crop may really need that water. This amount each day might get your orchard out of trouble and allow you to maintain a decent yield from year to year, given these dry conditions.

Theunis. I'd agree. I think keeping them out of the extremes of stress is very important. And if you've got small plantations, which are not highly crowded, and even if they are crowded, I'd suggest that you do look at your soil conditions and see if you can keep the soil as cool as possible. Keep the little bit of moisture that's in there with some mulch or some organic matter. 

In smaller orchards, especially, you can go a long way by just adding a bit of organic matter. Bare earth is never a good idea in any type of orchard. And I think you can stretch your water a way with that.

Leoni. Thanks so much for your time today. It was really interesting. And for growers that have any more questions, they can contact the AMS for some more ideas on irrigation efficiency. Thank you.