Practical advice for growers aiming to get the most out of cover crops in orchards. By Anna Mouton
Tillage – preparing soil for sowing – is as old as agriculture itself. It helps seed germination and plant growth by loosening the soil and removing weeds. This sounds like a good idea when sowing cover crops in orchards. But there is a downside, explains Matthew Addison, crop-protection programme manager at Hortgro Science.
Addison rotovated the soil in existing orchards as part of cover-crop trials. “It disrupted the whole system,” he says. “The soil carbon just flat-lined.”
Results from Addison’s cover-crop trials suggest that populations of soil micro-organisms take three to four years to stabilise after tillage. “When you plough up the soil, you disrupt the topsoil, and you’re going to lose soil,” cautions Addison. “And you’re delaying the process of getting healthier soils. Whereas, if you don’t plough it, you’re preserving the soil.”
Pome-fruit grower Craig Johnson of Nidderdale Farms acknowledges that tilling can benefit seed germination. “When we created a seedbed, we had better results. But keep in mind, how many times do you have to drive a tractor down the row to do it? Every action, every time you drive down that row, has a cost implication.”
Besides the cost, more tractor hours also equate to more diesel burned and more carbon emissions. And there are logistical issues. Johnson points out that the best time for planting cover crops overlaps with harvesting – and harvesting naturally takes priority.
“Time is money,” says Johnson, “so the more quickly and effectively I can plant my cover crops, the better.”
Read MoreNew cover crops in old orchards
Sowing cover crops in new orchards is by and large not too difficult. The challenge is to establish a cover-crop in an older orchard, especially when the soil in the work row has been compacted by years of tractor traffic. Tillage in the first year of planting is usually unavoidable.
Johnson describes the process he follows before initially sowing a cover-crop into an established orchard. “First we mow the existing grass as low as possible. Then we ghrop [cultivate] it a couple of times to loosen the soil. We run a ripper through to minimise the compaction in problem areas, especially on the tracks where the tractor drives up and down the row. Then we run a disc just to level out the rows.”
From the second year onward, Johnson does no tillage before sowing cover crops. “We just seed with a no-till machine, and we’ve had good results.”
No-till planters have been around for decades in grain-production systems. These machines cut a furrow in the soil, drop in seeds, and close the furrow again. According to Johnson, there is a limited offering available locally, and the focus tends to be on cereal crops. After investigating the options, he eventually partnered with Piket Implements, a South African company, to develop a no-till planter for orchards.
“Getting the right machine to do the job is important,” stresses Johnson. The planter makes eight to 10 parallel furrows, and needs to carry 200 – 250 kg of seed per planter disc unit, as a total weight of up to 2 500 kg is required to achieve the right soil penetration. However, the machines also need to be narrow enough to function in an orchard without becoming top-heavy and unstable.
“The rule of thumb is to plant cover crops from mid-April to mid-May in the Western Cape.”
Johnson has switched from a gravity-fed to an air-delivery planter. He believes that the air-delivery planter is better for dealing with a mix of seed sizes, and enables him to control which seeds are sown in which furrows. He sows coarser seeds in the middle of the work rows, and finer seeds of creeping cover crops like medicks on the edges of the ridges.
“The bankie is the most challenging part to seed,” says Johnson. “I have a planter that can plant on the bankie in young orchards, but in established orchards you have scaffolds that get in the way. We plant clovers and medicks on the outside so that they can eventually creep up onto the bankie.”
Caring for cover crops
Timing is critical when sowing cover crops. Johnson recommends that fruit growers take their cue from grain producers. “The rule of thumb is to plant from mid-April to mid-May in the Western Cape – mid-May is becoming late. The earlier, the better, but it’s also area specific.” He recommends sowing when soil temperatures are 18°C or above, and soil moisture is sufficient for germination.
After sowing, Johnson doesn’t specifically irrigate the cover crops, as they receive enough moisture from the soil and the micro-sprinkler drift. He fertilises once when seedlings have four leaves. Fertiliser applied to cover crops can also affect trees, so Johnson advises growers to consult their fertiliser representative.
Once the cover crops are growing, Johnson tries to delay mowing as much as possible, to allow maximum development of biomass. “We typically start mowing in mid-August, when we have to because we need to take other actions in the orchard,” he says. Meanwhile, prunings from the trees can go into the work row, and be converted to mulch when the cover-crop is mowed. Johnson has purpose-built a machine that throws the mulch onto the ridges.
Mowing on the ridges is more challenging, so Johnson may resort to chemical control with a scorcher. He prefers to have cover under the trees, but has to balance the benefits of living vegetation with those of organic matter incorporated into the soil. He also needs to keep the plants on the ridges from growing up into the trees, as this creates easy access for pests such as weevils.
Species selection
“There’s a whole debate around how annuals and perennials interact, and do we need both?” says Addison. “Annuals can be a pain, because sometimes they take, and sometimes they don’t, and they get outcompeted quite quickly.”
Many annuals have evolved to take advantage of disturbed soils – they usually feature in the first stage of succession in an ecosystem. But, as Addison points out, orchards are not in this stage, and the cover-crop community in an orchard will always be dynamic. “But you want plants in there that are predictable,” he says.
Johnson uses a mixture of a grain, a legume, and a broad-leaved plant for his cover crops. Broad-leaves such as tillage radishes have long taproots that open up the soil and improve water penetration. Legumes contribute nitrogen, and grains or grasses generate the volume of dry matter that is necessary for effective mulching.
“We still need to work on the ratios of the plants a bit, and on the seeding densities,” says Johnson. He tries to allow plants to flower before mowing them for the second time, in the hope that they will reseed themselves.
Addison is positive that cover crops will eventually become more self-sustaining. “I think we’re going to have very sophisticated minimal-till seed drills so that we can just enrich or supplement the cover-crop diversity.”
Seeing the effect of cover crops in his own orchards has convinced Johnson of their value, even though he is still experimenting to find the optimal management practices for his farm. “The cost per hectare justifies doing it,” he asserts. “How you put the seed into the ground is up to you, but we don’t want to disturb the soil. It takes time, but just keep going and eventually cover crops pay for themselves.”
