It takes an ecosystem to grow a tree. By Anna Mouton.
We humans often describe a clean floor as one you can eat off. But a clean orchard floor starves soil organisms. Cover crops can boost diversity both above and below ground, as well as offering many of the same benefits as mulch. An ongoing study led by Matthew Addison, crop-protection programme manager at Hortgro Science, examines the impact of cover-crop diversity in orchards. We spoke to Addison to learn more about the results so far.
Setting the stage
“The initial motivation for cover-crop diversity had to do with the biological control of mites in orchards,” says Addison. It turned out that the key to controlling pests such as red spider mites is to encourage a population of predators. Food and shelter for predators are provided by broadleaved plants that form part of diverse cover crops in the work rows. Predators can also hunt and hide in plant litter and mulches.
Once the idea of diverse cover crops caught on, growers became aware of other advantages, says Addison. He became involved in a study on cover-crop management in organic stone-fruit orchards in the Western Cape. The research team was led by Klaus Birkhofer of the Brandenburg University of Technology in Germany. Organic orchards in this study had significantly different ecosystems to conventional orchards, but it wasn’t clear which practices offered the best outcomes.
More research was needed. “My interest was in whether diverse cover crops are productive, and whether they allow for really beneficial ecosystem services in orchards,” says Addison. He partnered with farming group ZZ2 to test different cover crops, and to figure out the best ways for measuring their performance.
ZZ2 have been leaders in implementing systems to improve soil health, and Addison was able to draw on the experience of Hendrik Pohl, production manager at Bokveldskloof. The cover-crop trials were conducted on two ZZ2 sites in Ceres — a Cape Rose orchard in the Warm Bokkeveld, and a Bigbucks orchard in the Koue Bokkeveld.
Cover-crop performance in orchards
Cover crops were first sown at the Warm Bokkeveld site in 2017, and at the Koue Bokkeveld site in 2018. The plants were resown in 2020 using a seed drill. Four treatments — see table 1 — were established on each site and compared to a control with no planting in the work row.
“We go from simple to complex in four steps in the work row,” explains Addison. “In the tree row, we’ve planted what are effectively cover crops under the trees in two of the treatments.” Each treatment occupies four orchard rows of 120 metres.
Table 1: Cover-crop trial treatments and control
|Grass1 + triticale
|Compost + straw mulch
|Grass1 + triticale + medicks
|Compost + wood chip mulch
|Grass1 + triticale + medicks + clover2
|Compost + medicks
|Grass1 + triticale + medicks + mix3
|Compost + medicks + clover2 + other4
|Compost + straw mulch
1Tall fescue, creeping red fescue, and perennial rye grass.
3Buckwheat, tillage radish, chicory, and oats.
4White mustard, and nasturtiums.
So far, the performance of all the treatments is satisfactory, although they all experience weed competition. Weed suppression is a potential benefit of cover crops, but only if the cover crops can outgrow the weeds.
Dry mass production of the different groups of cover-crop plants was measured at the end of 2018. The Koue Bokkeveld site yielded substantially more dry matter than the Warm Bokkeveld site. Overall, the more diverse the cover crop, the more dry mass was produced. In subsequent years, the cover crops were rolled, in both the work and tree rows. This allows the plant biomass to be incorporated into the soil.
One aim of the trial is to find a non-destructive way to measure the growth and production of cover crops over time. Addison and Ansuli Theron, the master’s student on the project, captured digital photographs monthly, and evaluated these to determine the height and volume of the plants.
Soil samples were collected for physical, chemical, and biological analyses; nematode assessment, including a diversity index; fungal diversity index; and soil fauna and litter decomposition. Preliminary results show a trend toward increased soil carbon and soil aggregate stability from 2018 to 2019 in the treatments. Microbial activity as measured by soil respiration rate has also been increased.
The status of the various cover-crop species as hosts for plant-eating nematodes was investigated by Dr Rinus Knoetze of the Nematology Laboratory at the Agricultural Research Council. Most of the cover crops were found to be poor hosts, but buckwheat turned out to be a very good host.
Comparison of soil food webs in the Koue Bokkeveld shows that these became more complex, with more species and more interactions between species, from 2018 to 2019. This was true for the treatments and the control, and reflects the restoration of the system after the orchard was cultivated prior to planting in 2017.
In general, food webs appear to be more stable and complex in the treatments than in the controls. Bacterial and fungal diversity, and the number of collembola, were also higher in treatments with greater plant diversity than in treatments with less.
“It takes time for the system to stabilise and become diverse, and there are strong links between what happens under the tree, and what happens in the row,” says Addison.
For more on the cover-crop trials, visit our YouTube channel and search for the Cover-crop Trials playlist.
Small-scale cover-crop trials
Addison and Theron tried out several additional cover-crop candidates in small-scale assessments conducted in orchards in the Warm and Koue Bokkeveld. Teff, white mustard, peas, lucerne, strawberry clover, crimson clover, and serradella were tested. Each was sown in three plots of 3 x 2 metres located in the work row.
The first sowing was in May 2018. Teff and serradella failed to germinate. Lucerne, strawberry clover, and crimson clover established and grew moderately well at the Koue Bokkeveld site. White mustard and pea survived throughout winter at the Warm Bokkeveld site. In general, germination and seedling survival were poor at the Warm Bokkeveld site, most likely due to poor drainage on heavy clay soils.
A similar experimental design was used to trial several plants that could serve as floral crops in the tree row. Floral crops are beneficial because they attract pollinators and predators. Felicia, vygies, chamomile, coriander, dill, thyme, origanum, and alyssum were sown in May 2018. Neither thyme or origanum germinated, and all species performed badly at the Warm Bokkeveld site.
The floral crops fared better at the Koue Bokkeveld site. Chamomile and alyssum flowered strongly, and when the plots were checked in 2019, alyssum had established. Both chamomile and alyssum are low growing, making them ideal for establishment under trees.
Overall, the establishment of cover crops at the Warm Bokkeveld site was challenging, due to heavy clay soils and strong weed competition. This highlights the importance of site differences in cover-crop management, and why growers will have to experiment to see what works best in their own orchards.
Addison believes that the demand for more sustainable and resilient food production will continue to increase, and that cover crops will be central to helping South African growers to achieve this.
“I’m convinced that our markets are going to start differentiating on how we’re producing our food,” says Addison. “And if you look at the basic ecological research, diverse soils are more sustainable and productive when compared to soils with low diversity.”