A brief history of table-grape irrigation in SA. By Anna Mouton
After climate, water is the most important limiting factor in fruit production, reads a 1951 article on irrigation of fruit trees and vines from the very first volume of The Deciduous Fruit Grower – the monthly magazine that became the SA Fruit Journal in 2002.
According to the article, many Western Cape growers in the 1950s still regarded rainfall as sufficient for fruit production. Nowadays, all table grapes are irrigated with the help of technology that our industry pioneers could never have imagined.Read More
Systems for soaking the soil
The 1950s article lists irrigation methods as flooding, furrows, basins and sprinklers, adding that growers can use any of these as long as soil erosion is prevented, and the full soil surface and profile are soaked. But starting in May 1951, The Deciduous Fruit Grower published a two-part series promoting sprinkler irrigation.
Initially, sprinkler systems were portable. Light-weight steel pipes – later replaced with aluminium pipes – connected by quick couplings were moved through vineyards. Some growers used perforated pipes that could be considered a primitive form of drip irrigation.
The next step was installing permanent sprinkler systems, says Dr Philip Myburgh, a retired senior researcher with the Agricultural Research Council (ARC) and author of Handbook for the Irrigation of Wine Grapes.
"Having permanent systems allowed much better control of irrigation as people weren't spending time moving pipes around," he says. "Weekends used to be chaos because the movers weren't available."
Early sprinklers covered a large area – lines were installed in every third or fourth row. Over time, these systems were replaced by micro-sprinklers, which have been the dominant method for table-grape irrigation for decades.
"Micro-irrigation started as full-surface applications with overlapping micro-sprinklers that gave a uniform distribution over the entire area," says Chris Malan, agronomy manager at Netafim. "Nowadays, that has changed to stand-alone micro-sprinklers and strip wetting, so about 30 cm of the work row remains dry."
Modern micro-sprinklers also tend to have a lower delivery rate, although research shows that most table-grape growers still over-irrigate – more on this later.
Developments in drip irrigation
In 1971 The Deciduous Fruit Grower reported that a large audience of farmers attended an illustrated lecture on drip irrigation by an Israeli specialist in the Stellenbosch town hall. More articles on drip irrigation followed, based on visits of South African researchers to Israel and Australia.
"In general, table-grape growers prefer micro-sprinklers to drip," notes Myburgh. Nonetheless, he says, the industry is gradually adopting drip as water becomes scarcer and costlier. Thanks to drip irrigation, even arid areas with limited water supplies have seen table-grape production expand.
Drip irrigation delivers water directly to the soil surface, avoiding the inevitable evaporative losses associated with sprinklers and micro-sprinklers. Slower water delivery also helps to prevent water losses due to deep drainage.
"Low-flow drip has been a fantastic tool in especially the sandy, rocky soils," comments Karen van der Westhuizen, soil scientist and irrigation team leader at Agrimotion.
The latest trend is toward even lower water delivery, using ultra-low-flow drip. This approach has been very successful in citrus production, but is still being evaluated in table grapes. "You can achieve greater accuracy by reducing flows," says Malan, "and you lose less water and fewer nutrients."
"Something that's changed in the past 20 years has been nutrition," adds Myburgh. "You can't separate it from irrigation – with micro-sprinklers and drippers, nutrients are applied through the irrigation system."
Growers used to correct the macronutrients in the soil during soil preparation. They would then analyse the nutritional status of their vines annually, supplementing as needed, usually only with nitrogen and potassium. Feeding nutrients through the irrigation system – fertigation – allows far more frequent application.
From sunflowers to software
"The tricky thing with irrigation scheduling is that it determines about 80% of the quality of your crop," says Van der Westhuizen, "but there's so much variation between and within different vineyards, and it's a decision that you need to make daily."
The Deciduous Fruit Grower already recognised the challenges of scheduling in 1951 and offered a solution: plant sunflowers. The sunflowers will reach wilting point before the perennial crops, thereby indicating when growers should irrigate.
Nowadays, capacitance probes have replaced sunflowers as the recommended method for measuring soil moisture. Myburgh and his former colleague Dr Carolyn Howell, senior researcher at the ARC, recently completed a project looking at scheduling irrigation based on the relationship between soil water content and plant water status.
"The ideal value for stem water potential in a table-grape vineyard would be about -0.8 megapascal," says Howell. "That's where you would reduce the amount of water you're using but not your yield." After harvest, vines can tolerate stem water potentials of -1.2 megapascal without adverse effects on future yields.
Myburgh and Howell calibrated a neutron soil-moisture probe against stem water potential in 10 experimental plots and set refill lines for each based on these values. The plots were irrigated accordingly for three seasons. Their experimental vines used 28% less water than the control, which was irrigated using standard industry scheduling.
"The results showed that the grapevines did very well with less water," says Howell. "We maintained yields, and there was no evidence of carry-over effects to the next season."
Currently, growers combine continuous soil-moisture measurement by capacitance probes with data from weather stations. "The weather station indicates the atmospheric demand so that you can adjust your irrigation accordingly," says Malan. "But I think in future we may rely more on remote sensing systems like FruitLook."
The idea that computers could help plan irrigation was first mooted in a 1971 issue of The Deciduous Fruit Grower in the context of large-scale water supply to farms.
What would 1970s table-grape growers have thought of getting continuous soil-moisture readings and satellite data delivered to a pocket-sized device? Let alone using software on the same device to control irrigation systems?
Irrigation in the 21st century
Modern irrigation systems start with soil surveys that inform the physical and chemical preparation of the soil, as well as which cultivars are best suited to the site, explains Malan. "Each soil type and cultivar will have different requirements regarding irrigation, fertilisation, and ripening period – this is why you need professional designers to install a decent irrigation system."
In SA irrigation designers are certified by SABI – the South African Irrigation Institute – founded in 1976. Another development has been the SABS (South African Bureau of Standards) approval of equipment and materials, protecting growers against poor quality.
As with all aspects of farming, shrinking profit margins equal shrinking margins for error. At the same time, consumers expect both better fruit quality and more sustainable agriculture. Table-grape buyers are asking questions about irrigation scheduling and water footprints.
"Market requirements don't drive decisions about irrigation systems – yet," says Malan. "But those issues will become more important."
"I think climate change will play a greater role in dictating the approach to irrigation," reflects Myburgh.
Recent droughts forced growers to reassess their water use, and on-going electricity shortages and loadshedding call for a re-evaluation of irrigation systems so that the crop water demand can be met in fewer hours.
However, the pressure to save water might be a blessing in disguise for many table-grape growers. "We proved over and over that growers can manage with less water," stresses Myburgh. "Whether you look at the past or the present, one of the biggest problems is over-irrigation."
A recent SATI-funded project measured stem water potential to calibrate soil moisture levels for irrigation scheduling
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