Cover Crop Trials – Beneficial preliminary results for deciduous fruit Growers

Why cover crops?

Addison finds his argument for cover crops in ecological theory, meaning that plant diversity will result in sustainability (a stable ecosystem over time). And equally important, resilience. How to prove the value of cover crops in an agri ecosystem is not easy, concedes Addison. “In order to measure soil health and soil ecology you first need to answer what is good and what is bad?”

With all this in mind, he set out with Ansuli Theron, a Masters student/research assistant, to measure and manage. One of their first goals was to measure production by harvesting cover crops in 0.5 m2, to determine plant growth. The most diverse treatment produced an impressive (in equivalent) 10 tonnes of dry mass per hectare.

Secondly, they assessed the diversity and growth of the plants by measuring it photographically and physically. “We wanted to see what was growing and how tall it was.” They used a measuring board and combined that data with image analysis, to establish how high and what percentage of coverage they had, as well as count the plant diversity.

In terms of soils, the team carried out Solvita tests, this included aggregate stability, measures of biological activity, soil nutrients, and soil carbon counts.

From a biology perspective, the team regularly collected soil samples at 20 m intervals at pre-determined sampling areas, from tree and work rows. The samples were assessed by Nemlab for nematodes, including plant-parasitic nematodes “that we need to keep an eye on, as there is a relationship between some cover crops and plant-parasitic nematodes”. The analysis gave us a nematode diversity index that is proving a stable measure on how balanced your system is, he says, “I can report that we had fantastic results.

“From this data, we got a nematode diversity figure and could see how stable the nematode community is. Over the last four years, one could see the nematode community has stabilised over time and moved into an area where it is not over-enriched and it is diverse, and that is exactly what we are aiming at.

“We recently also started doing tests that measure bacterial and fungal diversity via Sporatec. On top of that, bacterial enzymes in soils will also be investigated, in future.”

The team now has a four and five year record that shows that biological activity stabilises over time. According to Addison, the value is that the two trial sites show that they differ from treatment to treatment. “They show that we can manipulate the system. Secondly, cultivation and planting were very disruptive, and it took time to stabilise and is still dynamic. Regenerative agriculture has proved that one should avoid disturbing soils, which is difficult when establishing apple orchards.

But we could start early, plough and establish diverse cover crops and then plant the trees. Or can we retain stable soils while replanting? Think apple replant disease – is it a symptom of unbalanced soils?”

Addison is in entomology heaven. But what are the benefits for growers?

Cover crops are not just a green fad. They should be considered by any grower who is serious about soil health and sustainable production practices.

Some of the main findings to date

Addison says that the research results to date are complex and intriguing. “At the top of my list are the nematode diversity results. These results are a truly reliable measure and it reflects stability in the system that is developing over time and is still mobile four years later, meaning it doesn’t just re-establish,” he says.

“Secondly the carbon increase in the soils, specifically in the work row, is also impressive. I was expecting more action with orchard pests, but we had very little. There was a bollworm problem in one year, but it was not confined to this block or this farm, it was this area.”

Then there are the “new” guys on the block: Collembola, which are small primitive insects that live on and below soils and play an important role in orchard soil ecology. “The diversity of Collembola is new to us, and it’s been tricky, since it’s a first time effort to study it in orchards, but it can tell us a lot. So yes, we are excited about adding this study to the project.”

The team has also seen the value of the use of some clovers in cover crops. “Clover helps to put nitrogen into the soil, which means you save substantially on nitrogen inputs, and the trees speak for themselves – setting a good crop.”

“In general, we have seen better available plant nutrients with cover crops. Remember, you are drawing nutrients into your cover crops; they get mown or rolled and release the nutrients. The soil is just better,” says Addison.

Lastly, they have also seen how cover crop flowers influenced bee activity during blossom periods. “We did a preliminary trial in the Warm Bokkeveld that showed if you do have flowers in your cover crop, it tends to retain bees on the blossoms in those rows, i.e. your pollination should be more efficient with flowers than without flowers. Bee activity in those trials showed it very, very clearly.”

The benefits for growers can be summarised as the more diversity you have the better your biological control is going to work.

“In this orchard, there is non-detectable woolly-aphid and we don’t have mite problems. That is in itself huge. But our primary goal is to improve soils over time. If you look at results to date, both soil organic matter and stable carbons have increased each year.”

Where to next with cover crops?

Addison agrees that there is still a lot of work that they need to do. “We have to include soil moisture in the study to understand the water holding capacity of soils with increased carbons in them. We will need to do water ingress studies on these soils after four years of cover cropping vs. the farm standard. We still have a huge amount to learn about how to get cover crops established successfully, and maintaining the diversity in the cover crop, we want broadleaf diversity and flowering plants.

“The integration of weed management into the system is critical, at the moment we rely on herbicides, they are becoming limited. We need to learn how to control our weeds in an integrated manner. We are also investigating the use of indigenous cover crops. Most of what we have used were exotics, simply because they were easier and cheaper to get hold of, but it shouldn’t be. We will look into getting indigenous cover crops.”

Addison is in constant consultation with the project’s advisory team and currently, the list has over 30 different proposals. “Cover crops and orchard soil ecology is very complex. It involves a lot of different disciplines, and to decide on the value of a single trial is very difficult,” he says.

He points to the trial block behind him,”the thinking of this trial is five years old. But I can tell you people are excited and there is a lot of action happening on farms from Elgin to Villiersdorp, all of it involves new thinking.” Regenerative thinking.

The cover crop trial layout

The layout of the trial consists of four treatments and a control. There are two components to each treatment, a work row and a tree row. In terms of the work row, initially, three-component grass mixes were applied. This was supplemented in treatment two with a subterranean clover. In treatment three the same grass mix was applied with subterranean clover and a medic. The fourth treatment was the same, but with added components such as buckwheat, yellow mustard and tillage radish as a diverse treatment.

Tree row treatment consisted initially of straw mulching, followed by wood chips in year two. In year three they planted a medic/subterranean clover mix and diversified that by adding additional species in treatment four. In the first year, they also planted 20 kg/ha of Triticale just to get the biomass going.

This was compared to a “business-as-usual” control block of how the farm normally does it. They used compost with each treatment in every season. Addison is happy to report that they managed to harvest 30 tonnes/ha with a very high pack-out.

Cultivar: BigBucks on M9 – Now in the fourth leaf.

The more diversity you have in cover crops, the better your biological control is going to work.

Why plant cover crops in tree rows and work rows?

Addison says: “When one starts dealing with mulch, there are 370 bales/ha being put out on this farm. One starts worrying about importing organic material from other regions in here and it is expensive. Hendrik Pohl, the farm manager here, had some experience planting medics under trees. He was very happy with them, and they had a whole lot of benefits to them, including nitrogen. So we had a go, and so far it has been successful. We still have an issue with weeds, with some of the cover crops not being competitive enough.

“The motivation is to have a green organic mulch that will die off in summer, self-seed and then come back in winter and exclude weeds. One thing it has illustrated is our continuous battle in keeping weeds under control.”

Management of cover crops

In-situ management of cover crops is a developing art, says Addison. Management is almost site-specific. What happens in one place doesn’t necessarily work in another. Originally, they mowed and then started using a roller, which was more successful. Another option they are developing is “mow and blow” – taking material out of the row onto the bankie. Mowing it with a rotary mower doesn’t work, as it breaks plant material too much.

Currently, the farm is experimenting with cover crop management on the bankie. Again, the roller worked best, because it allowed manipulating cover crops without terminating them, and manipulation before they seed. That ties into integrating weed management into cover crops.

Learnings comparing differential sites

“Never work on pears in the Warm Bokkeveld, especially on clay,” saysAddison. “Weed control in the Warm Bokkeveld was a nightmare and still is, it is really difficult. Pears are very different. Clay is really challenging in trying to keep cover crops alive through summer, in a hot climate.

“Which orchard is going to benefit more from cover crops? Definitely the clay. It will bring down soil temperatures, and it will start improving that soil.”

Is there growth competition between cover crops and trees?

The team has seen no growth competition between the trees and cover crops, but it would be difficult to measure it on thistrial design, says Addison. “A lot of the biological action with cover crops, be they under the tree or in the row, is after harvest and before bloom. It is a balance between cover crop benefits and cover crop competition with trees. This in itself is an interesting subject.”

Photos: Elise-marie Steenkamp, Hortgro / Carme Naudé, Hortgro / Matthew Addison