Cover crops support soil health by increasing soil carbon – organic matter – which stimulates microbial activity and improves soil structure. By Anna Mouton
The impact of cover crops on soil health was discussed at a workshop facilitated by the World Wide Fund for Nature (WWF) held at Oak Valley in Grabouw in August.
Ivan Jansen van Rensburg from Barenbrug described eight soil-health benefits of cover crops: sequestering carbon, supporting microbes, suppressing diseases, fixing nitrogen, suppressing weeds, preventing erosion, building structure, and improving aeration and infiltration.
Different cover crops have different strengths, so diversity is important. For example, radish roots muscle into the soil, breaking up compaction, enabling water movement. But Jansen van Rensburg pointed out that the finer roots of cereals are as useful as the large roots of radishes.
"A diversity of roots creates micro and macro pores in the soil for water and air," he said. "Deep-rooted cereals can also put carbon deeper into the soil where it is more stable than carbon located at the surface."
Legumes offer the bonus of nitrogen fixation – their dry matter typically contains 1.5 – 4.0% nitrogen. "This is relevant as the price of nitrogen fertilisers has skyrocketed recently," said Jansen van Rensburg.
However, he cautions that the amount of nitrogen supplied by legumes will depend on the variety and species of cover crop, as well as the amount of biomass it produces. "Growing conditions and how happy the plant is, as well as how successful the inoculation of rhizobium on the roots has been, will have a big influence on the amount of nitrogen fixed."
Read MoreCover-crop selection
The number of available cover crops is continually increasing, as breeders develop improved varieties. "It's easy to be confused by all the options," conceded Jansen van Rensburg.
He recommends that growers start by identifying the goal of the cover crop, then consider rainfall, irrigation, soil type, site-specific challenges, equipment, and their budget. When choosing legumes, for instance, vetch and medicks do well on loam and clay soils, but serradella and lupines are a better option on sandy soils.
Equipment can be a challenge as fruit producers are not geared for growing field crops. "But we're seeing more farmers investing in no-till planters and equipment for terminating cover crops," said Jansen van Rensburg, "and that's going to help a lot."
Although diversity is desirable, Jansen van Rensburg warns that mixtures with too many varieties can be difficult to establish and manage. Planting equipment may not cope with the seed size and seeding depth variability. Once seeds germinate, the different cover crops might not be compatible. Termination can also become a problem when some cover crops are already in seed while others are still actively growing.
"What I prefer is to rather make smaller mixtures," said Jansen van Rensburg, "then you know it's going to be easier to terminate, the species are compatible, and your equipment can handle it."
Cover-crop rotation is an alternative to very diverse mixtures and can help when the aim is to eradicate stubborn weeds like couch or bindweed. Grass-selective herbicides can be applied during a broadleaf rotation and vice versa. Herbicide use will become less and less over time, as new weeds are outcompeted while the established weeds are killed off.
"Keep in mind that upping your seeding rates generally suppresses weeds better," added Jansen van Rensburg, "but you might reduce the amount of biomass because of competition. You must decide what your main objective is – to produce biomass or to suppress weeds, a balance can also be achieved."
Succeed in the plant row
Cover crops in the tree or vine rows need a different approach to those in the work rows, according to Jansen van Rensburg. The plants need to be low-growing, self-sustaining, and weed-suppressing.
Medicks, subterranean clover, white clover, grazing vetch, turnips, and radish are all options, if the height of the cover crop is appropriate for the main crop. Aggressive twining crops such as grazing vetch or tall plants such as radishes can become weevil bridges when used unwisely.
Establishing cover crops requires more labour in the tree rows than the work rows – so perennial or self-seeding annual varieties are ideal. Medicks fit the bill as they readily reseed and generally become self-sustaining after two or three years of sowing. White clover is a perennial that is slower to establish than medicks, but will eventually dominate in a mixture with medicks if the growing conditions suit it.
These cover crops should be sown under irrigation early in autumn – no later than May – to get a head start on weeds before it starts to rain. Planting into a mulch will also help to suppress weeds. "You'll need to spot spray and pull weeds because the weed seedbed is still going to be there," said Jansen van Rensburg. "But we're trying to replace that seedbed with beneficial species over time."
Cover crop seedlings will need frequent but little watering, especially during early establishments when rainfall is insufficient. Vigorously spreading varieties planted under drippers will eventually cover the soil surface, forming a mulch that quashes weeds and retains moisture over summer. Unlike chips or straw, the cover crop requires no expensive, greenhouse-gas emitting transport.
Win in the work row
Both annual and perennial cover crops are used in work rows, but Jansen van Rensburg focussed on the latter. Low-maintenance perennials like dwarf fescues and white clover have been popular in the fruit industry for years.
"Low-maintenance cover crops are neat, and you don't have to cut them that often, so it saves labour and fuel," said Jansen van Rensburg. He recommends using pasture varieties of fescue and cocksfoot that grow taller, and can be cut for mulch more often.
Growing cover crops in the work row to provide mulch for the tree row – known as mow-and-blow – is more economical than buying in mulch. Different cover crops mine for different nutrients and legumes fix nitrogen, so diversity in the work row can translate into added goodness in the mulch. Jansen van Rensburg sees this as more optimal utilisation of the total surface area of the orchard or vineyard than having bare work rows.
"Just remember," said Jansen van Rensburg, "you want equipment that can chop the material as coarsely as possible, so it lasts longer as a mulch. If you chop it very finely, it just melts into the ground and decomposes much quicker."
Mow-and-blow does have drawbacks. Less biomass in the work rows equals more gaps for weeds, while unwanted cover-crop seeds can germinate in the tree rows. Careful timing of cuts can help reduce these problems. However, Jansen van Rensburg prefers rolling the cover crop in the work row and having a separate cover crop in the tree row.
Going forward, Jansen van Rensburg sees livestock integration becoming more popular. "It does come with challenges, but livestock can greatly benefit the microbial population in your soil."
The many benefits of cover crops ultimately translate into reduced inputs and greater profitability. "We can definitely decrease our carbon footprint, use less chemicals and fertilisers, and have soil with a higher carbon content that can hold water more effectively," concludes Jansen van Rensburg. "I think there's always a way that you can introduce cover crops into your system and get the benefits."
