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Australian Macadamia Society Podcast: Episode 2

Last year’s tree water relations workshops with Dr. Dan Manson from Bundaberg and Dr. 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!

Dan and Theunis talk about the interrelationship between nutrient management and irrigation management. How are growers currently using nutrients to manage their crop? Why is it critical to know your orchard’s soil type and to understand how nutrients are moving in your soil? 

EPISODE 2: Interrelationship between nutrient management and irrigation management

Leoni Kojetin (Australian Macadamia Society). This is our second AMS podcast of the series, and we're with Dr Dan Manson from Australia and Dr Theunis Smit from South Africa. We've been talking about water use in macadamias and today we're going to be chatting about nutrient management and how irrigation and water management can be used as tool on farm. 

Dan and Theunis, in the consultancy work that you do with growers, how do you think they are using nutrients to manage their crop?

Dr Dan Manson. A topic that commonly comes up is the interrelation between irrigation management and nutrient management, and often they go hand in hand. An irrigation system offers a vehicle for the application of liquid fertilisers (through fertigation). Micro sprinkler irrigation systems also provide an option of being able to wash in granular fertilisers that have been spread through the orchard. So, there are a couple of different approaches to nutrient management, and they can be often tightly linked with your irrigation scheduling and application method.

Leoni. But it's a minefield for growers. How do they manage that? Or where do they even start with that, Theunis?

Dr Theunis Smit. I would definitely say that, as with everything, you need to measure, you need to know what type of soil you're dealing with, you need to know what you're applying, and you need to know how those nutrients react with the soil. 

If you are not sure you can always consult one of your local agronomists, but it's critical that you know how nutrients move in the soil and what type of soil you've got on that farm. Certain elements are held tightly by the soil, and even though you wash a fertiliser in with water, a lot of those elements will not be available to the plant. So, it's pretty critical that growers know what they are dealing with before they start applying nutrients through that system. 

I would say a good suggestion would be just to get a generalised nutrient analysis done of your soil, but also then look at some textural analysis, soil depth, and try to get a good feel of what your irrigation system can do and how deep it washes certain nutrients.

Leoni. Do you think that growers understand the soil types that they're irrigating for, or managing, on all the blocks that they have?

Theunis. I don't think so, and I think, in general, soils vary a lot across farms. Very few of the farms that we have across the world have uniform soils for hectares and hectares. So, very few farmers I've advised in the past really have a clear indication of what the return is in different parts of the orchard with different soils and where a lot of these limitations to soil profiles are. 

I think if I had a rough guess, I would say about 15 to 20% of farmers actually know what type of soil they are dealing with on their farm, and it is a key limitation to a lot of these growers and for them to understand what they are doing.

Leoni. Dan, how do you think growers can navigate all the sales hype? Because there are a lot of products out there that they could use. Salespeople always throw "plant available" in there. How is it that growers can understand what nutrients are available to the plants depending on the soil type they're on?

Dan. The interaction between soil type and nutrient availability is pretty strong. Sand may not hold on the nutrients as well as, say, a heavy clay, but generally I think the feedback between soil and leaf nutrient levels is a good starting point. It is important to get a base idea of what soil nutrients are limited and what there is an excess of. As a primary tool, I think we should be using leaf nutrient diagnostics to manage nutrient applications. So, it's more important what the plant is actually taking up rather than what's in the soil.

Nitrogen, phosphorus and potassium are the major elements that the macadamia requires for the production. So, what we want to understand is whether the nutrients we are applying to our soils are making it into the plant when we've got some guidelines for optimum ranges of those nutrients in the leaves.

The basic requirement is to focus on these major elements - nitrogen, phosphorus and potassium - and only add the macro and trace elements through the soil when you are seeing deficiencies emerge in the leaves. One of the most important things I can say is that the application of complete fertilisers to some soil types might not result in the uptake of all those nutrients, and the available forms of those nutrients is critical, especially for phosphorus.

Macadamia, a member of the Proteaceae family, is a specialist at extracting phosphorous from the soil. It has specialised roots called cluster roots, or Proteaceae roots, which can extract phosphorous from usually highly immobile sources in soil and rock.

I think the emphasis should be on getting a baseline with your soil analysis, but I think more frequently than that, we should be looking at the nutrients that are making it up into the leaves. The macadamia has an amazing ability to extract nutrients that are at very low levels in the soil into healthy levels in the plant. So, we should be managing nutrients, not so much to fertilise the soil and get a complete set of nutrients that are available in the soil, rather to make sure there's some of every element in the soil and then checking how well the trees are pulling that out of your particular soil profile.

Theunis. You can easily over-apply nutrients and still not see them being taken up by the plant though. Even on sandy soils, which are not holding onto a lot of nutrients, if your irrigation management is not good, a lot of those nutrients will be leached out of the profile, and again, it will show up in your leaf analysis. You need a clear understanding of how water interacts with those chemicals on a certain soil type; and applying more nutrients and more water is not always the key to getting better yields or getting healthier trees.

Dan. No, and understanding how the water is moving through your soil profile gives you a good understanding of where your nutrients are going. So, coming back to the texture analysis, I think the texture of the subsoil and the depth to subsoil is probably one of the most critical elements of the analysis of your orchards. It gives you an increased understanding of how you should be scheduling and how often you should be putting nutrients on so that you're not pouring water and/or nutrients down through a deep sandy profile, so that only 30 or 40% of your applied water and nutrients are available. If you can apply that more frequently on well-drained soil profile, we may be able to put on less water and less nutrients and get better results.

Leoni. Another difficult area to understand is that we have a lot of foliar feeds that we could apply. What are some of the limitations to foliar application, Theunis?

Theunis. I think foliars are a type of a bandage that you put on the trees. If you see a deficiency, it's pretty easy to rectify it with a foliar feed, but it's definitely not the sustainable way to go. Certain elements such as calcium, even though it gets taken up in some scenarios by the leaf, never gets reallocated to the rest of the tree, so it's just sitting in that leaf. If you have a generalised shortage of stuff like calcium, you're not going to rectify it with a foliar feed. I think that's where growers really need to try and figure out how it best works for them. In general, the uptake of foliar fertilisers is minimal, but maybe it's a good supplement. It's a little bit of a hot topic, though.

As well, while foliars can be a good supplement to your normal fertiliser program, by no means can you only run on foliar fertilisers. The tree needs to take up a lot of critical elements through the roots, and they have to be translocated in that tree through the roots. That translocation process from a foliar spray into the rest of the tree is always a tricky one to get to. When spending a lot of money on foliar fertilisers, you must ask yourself whether it is worth your while or whether you should be spending it on the soil and trying to understand your soil better and how the plants can take up the nutrients from the soil.

Dan. That's a good point, Theunis. We over-emphasise sometimes the ability for the tree to pick up nutrients and the economics of spraying foliar fertilisers in a mature orchard sometimes don’t work, but there are a few, limited cases where some of these trace elements and certain soil types may not be taken up very well. 

There are often problems with zinc, with studies showing that it's sometimes difficult to get zinc into the roots. So, there might be limited cases where you have trace elements that are available in the soil, but the plant is not being able to take it up, just to top them up just before flowering coming into production season. But, as Theunis is saying, the best approach for getting your major elements into the tree on a long-term basis.

Leoni. Now, I'm sure you've both been on an orchard, given a recommendation for nutrients and come back and seen the growers applied double of what you've said, just as an insurance policy. What are some of the pitfalls of that?

Theunis. Hopefully, a lot of growers don't do it, but if they are applying double, there's an increased risk that you might run into causing a salt burn or nutrient burn in some of those trees, especially if you've got poorly drained soils and you've got nutrients sitting there.

One of the common cases is if you've already got a poorly managed irrigation system, and you don't get a sufficient washing of those nutrients into the soil, they just sit in the top layer, concentrate, and burn off a lot of roots. And that's just a physical limitation that you're getting into. 

One of the critical things is these trees, according to what I have seen over the past two years, are good at taking up nutrients. And if you are going to double up on certain elements, and I think Dan can add to this, for instance by adding a lot more nitrogen, the trees seem to just grow more and more vigorously. Increased vigour, at the end of the day, is going to cause a decrease in crop because trees will favour vegetative growth over reproductive growth, and that's just something we've been seeing in general.

A big problem in a lot of the South African production areas is over-application of phosphorus, which leads to a whole bunch of other problems. Phosphorus toxicity is one of them, but it also binds a lot of your key elements, such as iron, which then leads to a lot of other problems that you don't want to run into. 

So, most of the time those fertiliser programs are well thought through by your trusted agronomist or consultant, and they try to balance a lot of those elements in the soil.

Dan. I agree, Theunis. Unfortunately, macadamia leaf isn't worth that much money. In a mature orchard, if you're growing a lot of leaf, you can't sell that leaf for money; you can sell nuts for money, and you can actually control the balance between leaf production and nut production in a mature orchard if you manage your nitrogen correctly. 

You have to pay a lot of attention to the response of your different varieties to different nutrient applications, especially nitrogen. I don't think there's any blanket recipe for multiple varieties in one orchard where you can say, this is a nitrogen application. This is a good start to give an even amount of nitrogen to every tree. You need to understand that there's a very fine balance, sometimes, with some varieties, where if you get the nitrogen application perfect, then you can markedly increase your sustained yields in your orchards.

Theunis. I think that also has implications for combination fertilisers and certain times of the year, when you might not want phosphorus and potassium in the mix. There are certain times you don't want vegetative growth on the trees, and that's why it's highly advisable, even if those mixes are perfectly balanced for your specific need, to stay away from them, rather go to some more balanced mixes. This is especially the case with a lot of the mixes we see that have high levels of phosphorus in them, and as you said earlier, trees are good at taking up phosphorus, and if you're disturbing those balances, it might just cause a significant reduction in yield.

Leoni. And what gets more complicated is with the addition of organic material on the orchard floor. A lot of growers are doing that, it does give a whole lot of benefits, but how do they manage that as well in the nutrient release in that going into the plant availability?

Dan. Some of the manures in these composts are quite high in phosphorus and have come with nutrients. So, when you're doing your nutrient balance, you must pay attention to the nutrients that are coming in with the compost because, as we said before, macadamia trees are very good at extracting nutrients in any form from the soil profile, however that comes into the farm. 

A good starting point is to calculate for your yield how much phosphorus, or nitrogen, or potassium you might be taking off your farm when you harvest three or four tonnes of nuts, and that gives you a basic idea of the replacement requirements for sustaining those yields year in, year out. If you apply excess fertiliser or excess mulch compared to what you're pulling off of the farm, then you can drive the trees to become imbalanced and grow too much leaf, which leaves them to be much more susceptible to drought events and heatwave events into the future.

Leoni. Theunis, do you think that we can manage trees with nutrients to keep them buffered from drought events?

Theunis. Yes, I think you can. As we said at the start, it's that link between water and nutrients. If you are going to go and push a lot of growth into vegetation, or push a lot of nitrogen to try and stimulate a lot of vegetative growth, you are adding a lot more leaf area to that tree. If your irrigation system is only washing those nutrients into a small portion of the soil, your roots will be fairly limited, because they won't have to work as hard and won't be as extensive.

If you then get into a drought scenario where you've got a big canopy and a limited root volume, if that small amount of soil volume dries out, you run out of water, or you're sitting with a very big canopy to take care of with a limited amount of roots. And that's typically when you will see trees starting to shed a lot of leaves, especially if they are highly stressed, just to try and maintain that balance and ensure their survival.

It's a little bit difficult though because, as with the atmosphere, soils are quite vast, and we can't always control what type of elements are held, and these trees are good at extracting those elements in the soil, so it's something one needs to take care of straight through the production cycle. And, as Dan has mentioned, keep it sustainable. It's better to get four and a half tonnes over the next 10 years than just getting it one year and then nothing in the rest of the time.

Dan. We often fall into the pitfall of just observing the above-ground growth of the tree, but I think the most important thing we're finding in orchard management, is that the balance between root growth and crown growth is important in making resilient, highly productive trees. 

As Theunis is just saying, if you're putting the nutrients and water in a very small part of the root volume, you can grow a very big tree with a very small amount of roots, but those trees that you create may not be the most resilient or the most highly productive trees. By giving trees small amounts of water and nutrients, so they actually have to go and work and look for the water and nutrients, we can grow a much larger root system, which will lead to more resilience in drought and will tend to lead to a highly productive orchard.

Leoni. Thanks so much for your time today. It's been great hearing from you.