Cover crops in apple orchards

Q What are the benefits of cover crops for the soil?

A Daan Brink: We see a very good physical improvement in soils of orchards where we have mulched for a long time. Carbon is the foundation of soil physics – any increase in soil carbon improves the physical condition of the soil.

Most chemical reactions are in the rhizosphere (the thin soil layer in direct contact with roots). The more cover crops interact with soil, the more the elements in the soil become available to our trees.

It's not a fast process – it's not like applying nitrogen to the soil or spraying it on a tree. But in the long term, we definitely see better nutrient uptake.

Hendrik Pohl: We had two concerns when we started with compost and mulch. One was the provision of enough nitrogen and the other was an over-provision of potassium, because if you bring in organic material from another source you always bring a lot of potassium with it.

With cover crops, we can start to circulate the potassium in the system rather than adding more. And if we move to cover crops below the tree, we can start to bind some nitrogen from the ecosystem.

We did some trials with medicks where we measured the nitrogen that we could fix within one production cycle. And that can add up to 30 – 40 kilograms of pure nitrogen that we don't have to bring in as chemical fertiliser.

Q What are the ecological benefits of cover crops?

A Matthew Addison: There are two obvious benefits for apple and pear orchards. One is that diverse cover crops allow for biological control of mites. We know it works but we don't know quite how – there's currently industry-funded research on this.

The other is the control of woolly apple aphid through the use of mulch. Hendrik (Pohl) uses compost and mulch and woolly apple aphid is very difficult to find on his farm.

Hendrik Pohl: We've been mulching on the bankie (ridges) since 2003 on our farms in the Koue Bokkeveld, and we haven't done any woolly-apple-aphid or nematode control since then.

Q What are other benefits of cover crops?

A Keith Bradley: I think compost is a vital part of the picture and won't be replaced. But bringing in mulch is very expensive and involves a lot of transport. It also means that you're depriving someone else of the mulch. So it's possibly not sustainable.

The drive to cover crops has been about long-term sustainability through growing the mulch within the orchard.

Ian Cunningham: Coping with climate change and heatwaves. Keith (Bradley) took some measurements on a hot day this year. There was a 10 – 15 °C temperature difference between the cover crop and barren soil. So cover crops combined with pulse irrigation and increasing the humidity in the orchard offer a way to buffer and mitigate the heat.

Hendrik Pohl: Another benefit of cover crops is the stabilisation of the orchard floor, especially during winter. That allows quicker mechanised actions such as spraying.

Q How do the costs compare to the benefits?

A Ian Cunningham: If you consider that the production cost per hectare for apples is R200 000 – R220 000 and our average cover-crop seed mix costs less than R1 000/ha – so half a percent of production costs – that's not much to spend on improving your soil. Improving the carbon content and biology over time just makes us much more sustainable.

In the four to five years that we've had cover crops, we've averaged 20 – 30 cubes of dry-matter production per hectare. Mulch costs R20 – R30 a cube to buy and R150 – R180 to transport, so there's already a cost benefit.

Hendrik Pohl: Everything comes with a cost. We have a saying in our company: better before cost.

The biggest cost saving will be not to bring in mulch from outside – to produce the biomass in situ – and to harvest the benefits of plant protection and feeding in the long run. I don't think this is a benefit that you can measure in the short term.

Q How are growers establishing cover crops in their orchards?

A Keith Bradley: Early autumn planting is definitely beneficial in our area (Elgin-Grabouw-Vyeboom-Villiersdorp). If you plant in the first two weeks of April and get good establishment then the cover crop suppresses weeds.

One of the practical aspects is how to plant cover crops every year. In a new development, you can use a power harrow or disc and sow cover crops but it's impractical to disc or power harrow your orchards when you're still busy harvesting – and a lot of us are moving to later cultivars.

We've started with no-till planting, and it's been phenomenal. We only have a few machines running but I think the uptake on that is going to be huge.

The next thing we're busy with is hydroseeding (spraying a slurry of seed and mulch) because one of the challenges is getting seed germination on your bankie. We're running a trial – the products are available in SA. Unfortunately, there isn't machinery for orchards but we're busy developing that.

Hendrik Pohl: In a young orchard, it's much more productive to plant your cover crops after putting on mulch than to plant on a clean bankie where the weeds then take over. In a previously established orchard, the most difficult thing when you start to introduce cover crops is to get rid of the weeds.

Drip irrigation in the Koue Bokkeveld is a big no. Our summers are just too hot and dry. If you want to establish a cover crop in late summer, you need full-surface irrigation.

Ian Cunningham: In our area (Elgin) I have planted sorghum as a summer cover crop that did well, but we had a wet spring. However, over the winter months, we get more than enough rain and our winter cover crops are fantastic.

Q What are good and bad cover – crop options or mixes?

A Daan Brink: There's a massive difference between Grabouw and Ceres. (HP and I) were talking about plants that grow in my region – he tried it and it's not growing. I've been in the Greyton area where we sowed on the sandy areas next to the river and we had awesome growth. And we tried it on a rocky area across the river, and we had nothing.

The question is what do you want to achieve? From there you're going to start looking around in your area at what works. What is your neighbour doing?

I try to establish loads of different cover crops in different areas to start having an idea of what works in what area.

Matthew Addison: Cover crops are incredibly site-specific sometimes. I had trials in the Warm and Koue Bokkeveld and they were chalk-and-cheese different.

One thing that was obvious over five years was that you shouldn't ever disregard the succession in your orchard. Planting cover crops helps their establishment, but the species change over time.

My biggest fear is that we introduce something weedy. We have to be careful. But we need diversity in our orchards, and we must start looking very hard at indigenous species. The deeper the rooting system the better – we need deep-rooted indigenous grasses and broadleaves in our orchards.

Hendrik Pohl: I think a good mix is a mix with diversity. You need grass, broadleaves, legumes, and brassicas. I don't know which ones are the most beneficial – the one that grows, I think.

With respect to succession, we planted the exact same mix of medicks, crimson and subterranean clover on two sites on the bankie. In the Warm Bokkeveld, we got a monoculture of medicks and in the Koue Bokkeveld, we got a monoculture of crimson clover.

It's a different soil but it's also a different space within the succession of that soil. You see change over time even though you don't add seed to the mix. And you see the succession through the season as well.

Ian Cunningham: Diversity is the backbone. One thing I've learnt is that you should try and change the cereal every year. I've also included radish in my seed mix lately because it drills down and breaks up the soil.

Q What answers do we still need?

A Matthew Addison: The most contentious issue is the integration of animals. It's difficult but it is something to consider. I think it's got a place between tree removal and replanting. There are people doing it and it looks good.

Another issue is that cover crops use water. Yes, they use water, but they improve soil permeability and they put carbon back. What we need is a long-term, full-cycle measurement of water use in orchards with cover crops.

The other tricky one is measuring what cover crops are doing – there's no single measure for soil health.

Keith Bradley: I think the benefits of mite control need quantifying.

Q What is your take-home for growers?

A Ian Cunningham: You can start with one or two hectares, but just start. There are contractors out there that can sow for you.

Matthew Addison: Go try it – it doesn't even have to be in an orchard, it can be on the orchard margin as well, and cover crops are really good for bees.

Daan Brink: I'm getting some flowers in this season to see if we can attract more bees.

So go and decide what you want to achieve and then work from there – there are enough people you can talk to.

Hendrik Pohl: Make peace with the fact that your orchard's going to look different than what we are used to.

Keith Bradley: I agree with Hendrik (Pohl). Gone are the days of the well-manicured golf-course-looking orchard, so get that out of your mind.